BOOK III. PROPOSITION VII. THEOREM VII.

_That there is a power of gravity tending to all bodies, proportional

to the several quantities of matter which they contain._

That all the planets mutually gravitate one towards another, we have

proved before; as well as that the force of gravity towards every

one of them, considered apart, is reciprocally as the square of the

distance of places from the centre of the planet. And thence (by prop.

69, book I, and its corollaries) it follows, that the gravity tending

towards all the planets is proportional to the matter which they

contain.

Moreover, since all the parts of any planet A gravitate towards any

other planet B; and the gravity of every part is to the gravity of the

whole as the matter of the part to the matter of the whole; and (by law

3) to every action corresponds an equal reaction; therefore the planet

B will, on the other hand, gravitate towards all the parts of the

planet A; and its gravity towards any one part will be to the gravity

towards the whole as the matter of the part to the matter of the whole.

Q. E. D.

Cor. 1. Therefore the force of gravity towards any whole planet arises

from, and is compounded of, the forces of gravity towards all its

parts. Magnetic and electric attractions afford us examples of this;

for all attraction towards the whole arises from the attractions

towards the several parts. The thing may be easily understood in

gravity, if we consider a greater planet as formed of a number of

lesser planets meeting together in one globe, for _hence it would

appear_ that the force of the whole must arise from the forces of

the component parts. If it is objected that, according to this law, all

bodies with us must mutually gravitate one towards another, I answer,

that since the gravitation towards these bodies is to the gravitation

towards the whole earth as these bodies are to the whole earth, the

gravitation towards them must be far less than to fall under the

observation of our senses.

Cor. 2. The force of gravity towards the several particles of any body

is reciprocally as the square of the distance from the particles; as

appears from cor. 3, prop. 74, book I.

FOOTNOTES:

[Footnote 11: Translated from the _Philosophiæ Naturalis Principia

Mathematica_.]

X

BENJAMIN FRANKLIN

1706-1790

_Benjamin Franklin, representative of the rationalist tendencies

of the eighteenth century, was born in Boston, January 17, 1706.

His early life and political missions are intimately related in his

“Autobiography,” a classic in American literature. Apart from his

political services to the cause of American independence, he attained

distinction in the field of scientific researches and experiments. In

1746 he began the experiments in electricity which resulted in his

identification of electricity with lightning. He died in Philadelphia,

April 17, 1790._

THE IDENTITY OF LIGHTNING AND ELECTRICITY[12]

But points have a property, by which they draw on as well as throw

off the electrical fluid, at greater distances than blunt bodies can.

That is, as the pointed part of an electrified body will discharge the

atmosphere of that body, or communicate it farthest to another body,

so the point of an unelectrified body will draw off the electrical

atmosphere from an electrified body, farther than a blunter part of

the same unelectrified body will do. Thus, a pin held by the head,

and the point presented to an electrified body, will draw off its

atmosphere at a foot distance; where, if the head were presented

instead of the point, no such effect would follow. To understand

this, we may consider, that, if a person standing on the floor would

draw off the electrical atmosphere from an electrified body, an iron

crow and a blunt knitting-needle, held alternately in his hand, and

presented for that purpose, do not draw with different forces in

proportion to their different masses. For the man, and what he holds in

his hand, be it large or small, are connected with the common mass of

unelectrified matter; and the force with which he draws is the same in

both cases, it consisting in the different proportion of electricity

in the electrified body, and that common mass. But the force, with

which the electrified body retains its atmosphere by attracting it, is

proportioned to the surface over which the particles are placed; that

is, four square inches of that surface retain their atmosphere with

four times the force that one square inch retains its atmosphere. And,

as in plucking the hairs from the horse’s tail, a degree of strength

not sufficient to pull away a handful at once, could yet easily strip

it hair by hair, so a blunt body presented cannot draw off a number of

particles at once, but a pointed one, with no greater force, takes them

away easily, particle by particle.

These explanations of the power and operation of points, when they

first occurred to me, and while they first floated in my mind, appeared

perfectly satisfactory; but now I have written them, and considered

them more closely, I must own I have some doubts about them; yet, as I

have at present nothing better to offer in their stead, I do not cross

them out; for, even a bad solution read, and its faults discovered, has

often given rise to a good one, in the mind of an ingenious reader.

Nor is it of much importance to us to know the manner in which nature

executes her laws; it is enough if we know the laws themselves. It is

of real use to know that China left in the air unsupported, will fall

and break; but how it comes to fall, and why it breaks, are matters of

speculation. It is a pleasure indeed to know them, but we can preserve

our China without it.

Thus, in the present case, to know this power of points may possibly

be of some use to mankind, though we should never be able to explain

it. The following experiments, as well as those in my first paper, show

this power. I have a large prime conductor, made of several thin sheets

of clothier’s pasteboard, formed into a tube, near ten feet long and a

foot diameter. It is covered with Dutch embossed paper, almost totally

gilt. This large metallic surface supports a much greater electrical

atmosphere than a rod of iron of fifty times the weight would do. It

is suspended by silk lines, and when charged will strike, at near two

inches distance, a pretty hard stroke, so as to make one’s knuckles

ache. Let a person standing on the floor present the point of a needle,

at twelve or more inches distance from it, and while the needle is

so presented, the conductor cannot be charged, the point drawing off

the fire as fast as it is thrown on by the electrical globe. Let it

be charged, and then present the point at the same distance, and it

will suddenly be discharged. In the dark you may see the light on the

point, when the experiment is made. And if the person holding the point

stands upon wax, he will be electrified by receiving the fire at that

distance. Attempt to draw off the electricity with a blunt body, as

a bolt of iron round at the end, and smooth, (a silversmith’s iron

punch, inch thick, is what I use,) and you must bring it within the

distance of three inches before you can do it, and then it is done

with a stroke and crack. As the pasteboard tube hangs loose on silk

lines, when you approach it with the punch-iron, it likewise will move

towards the punch, being attracted while it is charged, but if, at the

same instant, a point be presented as before, it retires again, for the

point discharges it. Take a pair of large brass scales, of two or more

feet beam, the cords of the scales being silk. Suspend the beam by a

pack-thread from the ceiling, so that the bottom of the scales may be

about a foot from the floor; the scales will move round in a circle

by the untwisting of the pack-thread. Set the iron punch on the end

upon the floor, in such a place as that the scales may pass over it

in making their circle; then electrify one scale by applying the wire

of a charged phial to it. As they move round, you see that scale draw

nigher to the floor, and dip more when it comes over the punch; and, if

that be placed at a proper distance, the scale will snap and discharge

its fire into it. But, if a needle be stuck on the end of the punch,

its point upward, the scale, instead of drawing nigh to the punch, and

snapping, discharges its fire silently through the point, and rises

higher from the punch. Nay, even if the needle be placed upon the floor

near the punch, its point upward, the end of the punch, though so much

higher than the needle, will not attract the scale and receive its

fire, for the needle will get it and convey it away, before it comes

nigh enough for the punch to act. And this is constantly observable

in these experiments, that the greater quantity of electricity on the

pasteboard tube, the farther it strikes or discharges its fire, and the

point likewise will draw it off at a still greater distance.

Now if the fire of electricity and that of lightning be the same,

as I have endeavoured to show at large in a former paper, this

pasteboard tube and these scales may represent electrified clouds. If

a tube of only ten feet long will strike and discharge its fire on

the punch at two or three inches distance, an electrified cloud of

perhaps ten thousand acres may strike and discharge on the earth at a

proportionately greater distance. The horizontal motion of the scales

over the floor, may represent the motion of the clouds over the earth;

and the erect iron punch, a hill or high building; and then we see how

electrified clouds, passing over hills or high buildings at too great

a height to strike, may be attracted lower till within their striking

distance, And, lastly, if a needle fixed on the punch with its point

upright, or even on the floor below the punch, will draw the fire from

the scale silently at a much greater than the striking distance, and so

prevent its descending towards the punch; or if in its course it would

have come nigh enough to strike, yet being first deprived of its fire

it cannot, and the punch is thereby secured from the stroke; I say, if

these things are so, may not the knowledge of this power of points be

of use to mankind, in preserving houses, churches, ships, &c., from

the stroke of lightning, by directing us to fix, on the highest parts

of those edifices, upright rods of iron made sharp as a needle, and

gilt to prevent rusting, and from the foot of those rods a wire down

the outside of the building into the ground, or down round one of the

shrouds of a ship, and down her side till it reaches the water? Would

not these pointed rods probably draw the electrical fire silently out

of a cloud before it came nigh enough to strike, and thereby secure us

from that most sudden and terrible mischief?

To determine the question, whether the clouds that contain lightning

are electrified or not, I would propose an experiment to be tried where

it may be done conveniently. On the top of some high tower or steeple,

place a kind of sentry-box, ... big enough to contain a man and an

electrical stand. From the middle of the stand let an iron rod rise

and pass bending out of the door, and then upright twenty or thirty

feet, pointed very sharp at the end. If the electrical stand be kept

clean and dry, a man standing on it, when such clouds are passing low,

might be electrified and afford sparks, the rod drawing fire to him

from a cloud. If any danger to the man should be apprehended (though I

think there would be none), let him stand on the floor of his box, and

now and then bring near to the rod the loop of wire that has one end

fastened to the leads, he holding it by a wax handle, so the sparks, if

the rod is electrified, will strike from the rod to the wire, and not

affect him.

FOOTNOTES:

[Footnote 12: From Franklin’s correspondence with Peter Collinson, July

29, 1750. _Works of Benjamin Franklin_, Philadelphia, 1809, Vol.

III, pp. 45-49.]

XI

LINNAEUS

1707-1778

_Carl von Linné [Linnaeus] was born May 13, 1707, at Rashult in

Smaland, Sweden. At the age of four he showed a precocious interest

in plants, an interest which seriously interfered with his studies

when he went to school. When his father was about to remove him, a

friend urged that the boy be fitted for the profession of medicine.

Linnaeus entered the university at Lund in 1727, but in the following

year transferred to Upsala. In 1732, at the expense of the Academy

of Sciences, he explored Lapland. Later he made pilgrimages to many

of the most eminent professors of Europe, returning to Stockholm in

1738. After his marriage, in 1739, he was appointed professor at

Upsala, where he continued his work in botany and established it on a

rational basis. He died January 10, 1778, noted as one of the foremost

botanists of his time, having discovered sex in plants and given his

name to a famous botanical system of classification._

THE SEX OF PLANTS[13]

The organs common in general to all plants are: 1st. The root, with its

capillary vessels, extracting nourishment from the ground. 2nd. The

leaves, which may be called the limbs, and which, like the feet and

wings of animals, are organs of motion; for being themselves shaken by

the external air, they shake and exercise the plant. 3rd. The trunk,

containing the medullary substance, which is nourished by the bark, and

for the most part multiplied into several compound plants. 4th. The

fructification, which is the true body of the plant, set at liberty by

a metamorphosis, and consists only of the organs of generation; it is

often defended by a calyx, and furnished with petals, by means of which

it in a manner flutters in the air.

Many flowers have no calyx, as several of the lily tribe, the

Hippuris, etc., many want the corolla, as grasses, and the plants

called apetalous; but there are none more destitute of stamina and

pistilla, those important organs destined to the formation of fruit.

We therefore infer from experience that the stamina are the male

organs of generation, and the pistilla of the female; and as many

flowers are furnished with both at once, it follows that such flowers

are hermaphrodites. Nor is this so wonderful, as that there should be

any plants in which the different sexes are distinct individuals; for

plants being immovably fixed to one spot, cannot like animals, travel

in search of a mate. There exists, however, in some plants a real

difference of sex. From seeds of the same mother, some individuals

shall be produced, whose flowers exhibit stamina without pistilla, and

may therefore properly be called male; while the rest being furnished

with pistilla without stamina are therefore denominated females; and

so uniformly does this take place, that no vegetable was ever found to

produce female flowers without flowers furnished with stamina being

produced, either on the same individual or on another plant of the same

species, and _vice versa_.

As all seed vessels are destined to produce seeds, so are the stamina

to bear the pollen, or fecundating powder. All seeds contain within

their membranes a certain medullary substance, which swells when dipped

into warm water. All pollen, likewise, contains in its membrane an

elastic substance, which, although very subtle, and almost invisible,

by means of warm water often explodes with great vehemence. While

plants are in flower, the pollen falls from their antheræ, and is

dispersed abroad, as seeds are dislodged from their situation when

the fruit is ripe. At the same time that the pollen is scattered, the

pistillum presents its stigma, which is then in its highest vigour,

and, for a portion of the day at least, is moistened with a fine dew.

The stamina either surround this stigma, or if the flowers are of the

drooping kind, they are bent towards one side, so that the pollen can

easily find access to the stigma, where it not only adheres by means of

the dew of that part, but the moisture occasions its bursting, by which

means its contents are discharged. That issued from it being mixed with

the fluid of the stigma, is conveyed to rudiments of the seed. Many

evident instances of this present themselves to our notice; but I have

nowhere seen it more manifest than in the Jacobean Lily (_Amarylis

formosissima_), the pistillum of which, when sufficient heat is

given the plant to make it flower in perfection, is bent downwards and

from its stigma issues a drop of limpid fluid, so large that one would

think it in danger of falling to the ground. It is, however, gradually

reabsorbed into the style about three or four o’clock and becomes

invisible until about ten the next morning, when it appears again; by

noon it attains its largest dimensions; and in the afternoon, by a

gentle and scarcely perceptible decrease it returns to its source. If

we shake the antheræ over the stigma, so that the pollen may fall on

this limpid drop, we see the fluid soon after become turbid and assume

a yellow color; and we perceive little rivulets, or opaque streaks

running from the stigma towards the rudiments of the seed. Some time

afterwards, when the drop has totally disappeared, the pollen may be

observed adhering to the stigma, but of an irregular figure, having

lost its original form. No one, therefore, can assent to what Morland

and others have asserted, that the pollen passes into the stigma,

pervades the style and enters the tender rudiments of the seed, as

Leeuwenhoeck supposed his worms to enter the ova. A most evident proof

of the falsehood of this opinion may be obtained from any species of

_Mirabilis_ (Marvel of Peru), whose pollen is so very large that

it almost exceeds the style itself in thickness, and, falling on the

stigma, adheres firmly to it; that organ sucking and exhausting the

pollen, as a cuttle fish devours everything that comes within its

grasp. One evening in the month of August, I removed all the stamina

from three flowers of the _Mirabilis Longiflora_, at the same time

destroying all the rest of the flowers which were expanded; I sprinkled

these three flowers with the pollen of _Mirabilis Jalappa_; the

seed-buds swelled, but did not ripen. Another evening I performed a

similar experiment, only sprinkling the flowers with the pollen of the

same species; all these flowers produced ripe seeds.

Some writers have believed that the stamina are parts of the

fructification, which serve only to discharge an impure or

excrementitious matter, and by no means formed for so important a work

as generation. But it is very evident that these authors have not

sufficiently examined the subject; for, as in many vegetables, some

flowers are furnished with stamina only, and others only with pistilla;

it is altogether impossible that stamina situated at so very great a

distance from the fruit, as on a different branch, or perhaps on a

separate plant, should serve to convey any impurities from the embryo.

No physiologist could demonstrate, _a priori_, the necessity of

the masculine fluid to the rendering the eggs of animals prolific, but

experience has established it beyond a doubt. We therefore judge _a

posteriori_ principally, of the same effect in plants.

In the month of January, 1760, the _Antholyza Cunonia_ flowered

in a pot in my parlour, but produced no fruit, the air of the room not

being sufficiently agitated to waft the pollen to the stigma. One day,

about noon, feeling the stigma very moist, I plucked off one of the

antheræ, by means of a fine pair of forceps, and gently rubbed it on

one part of the expanded stigmata. The spike of flowers remained eight

or ten days longer; when I observed, in gathering the branch for my

herbarium, that the fruit of that flower only on which the experiment

had been made, had swelled to the size of a bean. I then dissected this

fruit and discovered that one of the three cells contained seeds in

considerable number, the other two being entirely withered.

In the month of April I sowed the seeds of hemp (_Cannabis_) in

two different pots. The young plants came up so plentifully, that each

pot contained thirty or forty. I placed each by the light of a window,

but in different and remote apartments. The hemp grew extremely well

in both pots. In one of them I permitted the male and female plants

to remain together, to flower and bear fruit, which ripened in July,

being macerated in water, and committed to the earth, sprung up in

twelve days. From the other, however, I removed all the male plants,

as soon as they were old enough for me to distinguish them from the

females. The remaining females grew very well, and presented their long

pistilla in great abundance, these flowers continuing a very long time,

as if in expectation of their mates; while the plants in the other pot

had already ripened their fruit, their pistilla having, quite in a

different manner, faded as soon as the males had discharged all their

pollen. It was truly a beautiful and truly admirable spectacle to see

the unimpregnated females preserve their pistilla so long green and

flourishing, not permitting them to begin to fade till they had been

for a very considerable time exposed in vain, to the access of the

male pollen.

Afterwards, when these virgin plants began to decay through age, I

examined all their calyces in the presence of several botanists and

found them large and flourishing, although every one of the seed-buds

was brown, compressed, membranaceous, and dry, not exhibiting any

appearance of cotyledons or pulp. Hence I am perfectly convinced that

the circumstance which authors have recorded, of the female hemp having

produced seeds, although deprived of the male, could only have happened

by means of pollen brought by the wind from some distant place. No

experiment can be more easily performed than the above; none more

satisfactory in demonstrating the generation of plants.

The _Clutia tenella_ was in like manner kept growing in my window

during the months of June and July. The male plant was in one pot,

the female in another. The latter abounded with fruit, not one of its

flowers proving abortive. I removed the two pots into different windows

of the same apartment; still all the female flowers continued to become

fruitful. At length I took away the male entirely, leaving the female

alone, and cutting off all the flowers which it had already borne.

Every day new ones appeared from the axila of every leaf; each remained

eight or ten days, after which their foot stalks turning yellow, they

fell barren to the ground. A botanical friend, who had amused himself

with observing this phenomenon with me, persuaded me to bring, from the

stove in the garden, a single male flower, which he placed over one of

the female ones, then in perfection, tying a piece of red silk around

its pistillum. The next day the male flower was taken away, and this

single seed-bud remained, and bore fruit. Afterwards I took another

male flower out of the same stove, and with a pair of slender forceps

pinched off one of its antheræ, which I afterwards gently scratched

with a feather, so that a very small portion of its pollen was

discharged upon one of the three stigmata of a female flower, the other

two stigmata being covered with paper. This fruit likewise attained its

due size, and on being cut transversely, exhibited one cell filled with

a large seed, and the other two empty. The rest of the flowers, being

unimpregnated, faded and fell off. This experiment may be performed

with as little trouble as the former.

The _Datisca cannabina_ came up in my garden from seed ten years

ago, and has every year been plentifully increased by means of its

perennial root. Flowers in great number have been produced by it; but,

being all female, they proved abortive. Being desirous of producing

male plants, I obtained more seeds from Paris. Some more plants were

raised; but these likewise to my great mortification, all proved

females, and bore flowers, but no fruit. In the year 1757 I received

another parcel of seeds. From these I obtained a few male plants, which

flowered in 1758. These were planted at a great distance from the

females; and when their flowers were just ready to emit their pollen,

holding a paper under them, I gently shook the spike of panicle with

my finger, till the paper was almost covered with the yellow powder. I

carried this to the females, which were flowering in another part of

the garden, and placed it over them. The cold nights of the year in

which this experiment was made, destroyed these Datiscas, with many

other plants, much earlier than usual. Nevertheless, when I examined

the flowers of those plants, which I had sprinkled with the fertilizing

powder, I found the seeds of their due magnitude; while in the more

remote Datiscas, which had not been impregnated with pollen, no traces

of seeds were visible.

Several species of Momordica, cultivated by us, like other Indian

vegetables, in close stoves, have frequently borne female flowers;

which, although at first very vigorous, after a short time have

constantly faded and turned yellow, without perfecting any seed, till

I instructed the gardener, as soon as he observed a female flower, to

gather a male one, and place it above the female. By this contrivance

we are so certain of obtaining fruit that we dare pledge ourselves to

make any female flowers fertile that shall be fixed on.

The _Jatropha urens_ has flowered every year in my hot-house; but

the female flowers coming before the males, in a week’s time dropped

their petals and faded before the latter were opened; from which cause

no fruit has been produced, but the _germina_ themselves have

fallen off. We have therefore never had any fruit of the Jatropha till

the year 1752, when the male flowers were in vigour on a tall tree,

at the same time that the females began to appear on a small Jatropha

which was growing in a garden-pot. I placed this pot under the other

tree, by which means the female flowers bore seeds, which grew on being

sown. I have frequently amused myself with taking the male flowers from

one plant, and scattering them over the female flowers of another, and

have always found the seeds of the latter impregnated by it.

Two years ago I placed a piece of paper under some of these male

flowers and afterwards folded up the pollen which had fallen upon it,

preserving it so folded up, if I remember right, four or six weeks,

at the end of which time another branch of the same Jatropha was in

flower. I then took the pollen, which I had so long preserved in paper,

and strewed it over three female flowers, the only ones at that time

expanded. The three females proved fruitful, while all the rest, which

grew in the same bunch, fell off abortive.

The interior petals of the _Ornithogalum_, commonly but improperly

called _Canadense_, cohere so closely together that they only just

admit the air to the germen and will scarcely permit the pollen of

another flower to pass; this plant produced every day new flowers and

fruit, the fructification never failing in any instance; I therefore,

with the utmost care, extracted the antheræ from one of the flowers

with a hooked needle, and as I hoped, this single flower proved barren.

This experiment was repeated about a week after with the same success.

I removed all of the antheræ out of a flower of _Chelidonium

corniculatum_ (scarlet-horned poppy), which was growing in a remote

part of the garden, upon the first opening of its petals, and stripped

off all the rest of the flowers; another day I treated another flower

of the same plant in a similar manner, but sprinkled the pistillum of

this with the pollen borrowed from another plant of the same species;

the result was, that the first flower produced no fruit, but the second

afforded very perfect seed. My design in this experiment was to prove

that the mere removal of the antheræ from a flower is not in itself

sufficient to render the germen abortive.

Having the _Nicotiana fruticosa_ growing in a garden-pot, and

producing plenty of flowers and seed, I extracted the antheræ from the

newly expanded flowers before they had burst, at the same time cutting

away all the other flowers; this germen produced no fruit, nor did it

even swell.

I removed an urn, in which the _Asphodelus fistulosus_ was

growing, to one corner of the garden, and from one of the flowers

which had lately opened, I extracted its antheræ; this caused the

impregnation to fail. Another day I treated another flower in the same

manner; but, bringing a flower from a plant in a different part of the

garden, with which I sprinkled the pistillum of the mutilated one, its

germen became by that means fruitful.

_Ixia chinensis_, flowering in my stove, the windows of which

were shut, all its flowers proved abortive. I therefore took one of

its antheræ in a pair of pincers, and with them sprinkled the stigmata

of two flowers, and the next day one stigma only of a third flower;

the seed-buds of these flowers remained, grew to a large size and bore

seed, the fruit of the third, however, contained ripe seed only in one

of its cells.

To relate more experiments would only be to fatigue the reader

unnecessarily. All nature proclaims the truth I have endeavored to

inculcate, and every flower bears witness to it. Any person may make

the experiment for himself with any plant he pleases, only taking

care to place the pot in which it is growing, in the window of a room

sufficiently out of reach of other flowers; and I will venture to

promise him that he will obtain no perfect fruit unless pollen has

access to the pistillum.

Logan’s experiments on the Mays are perfectly satisfactory, and

manifestly show that the pollen does not enter the style, or arrive

at the germen, but that it is exhausted by the genital fluid of the

pistillum. And as in animals no conception can take place, unless the

genital fluid of the female be discharged at the same moment as the

impregnating liquor of the male; so in plants, generation fails, unless

the stigma be moist with prolific dew.

Husbandmen know, by long experience, that if rain falls while rye is

in flower, by coagulating the pollen of its antheræ, it occasions the

emptiness of many husks in the ear.

Gardeners remark the same thing every year in fruit trees. Their

blossoms produce no fruit if they have unfortunately been exposed to

long-continued rains.

Aquatic plants rise above the water at the time of flowering, and

afterwards again subside, for no other reason, than that the pollen may

safely reach the stigma.

The white water-lily (_Nymphaea alba_) raises itself every morning

out of the water and opens its flowers, so that by noon at least three

inches of its flower-stalk may be seen above the surface. In the

evening it is closely shut up, and withdrawn again; for about four

o’clock in the afternoon the flower closes, and remains all night under

water; which was observed full two thousand years since, even as long

ago as the time of Theophrastus, who has described this circumstance

in the _Nymphaea Lotus_, a plant so much resembling our white

water-lily that they are only distinguished from each other by the

leaves of the Lotus being indented. Theophrastus gives the following

account of this vegetable, in his _History of Plants_, book IV.,

chap. 10: “It is said to withdraw its flowers into the Euphrates,

which continue to descend till midnight, to so great a depth that at

daybreak they are out of reach of the hand; after which it rises again,

and in the course of the morning appears above the water, and expands

its flowers, rising higher and higher, till it is a considerable

height above the surface.” The very same thing may be observed in the

_Nymphaea alba_.

Many flowers close themselves in the evening and before rain, lest the

pollen should be coagulated; but after the discharge of the pollen

they always remain open. Such of them as do not shut up, incline their

flowers downward in those circumstances, and several flowers, which

come forth in the moisture of spring, droop perpetually. The manner in

which the Parnassia and Saxifrage move their antheræ to the stigma is

well known. The common Rue, a plant everywhere to be met with, moves

one of its antheræ every day to the stigma, till all of them in their

turns have deposited their pollen there.

The Neapolitan star flower (_Ornithogalum nutans_) has six broad

stamina, which stand close together in the form of a bell, the three

external ones being but half the length of the others; so that it seems

impossible for their antheræ ever to convey their pollen to the stigma;

but nature, by an admirable contrivance, bends the summits of these

external stamina inwards between the other filaments, so that they are

enabled to accomplish their purpose.

The Plaintain tree (_Musa_) bears two kinds of hermaphrodite

flowers; some have imperfect antheræ, others only the rudiments of

stigmata; as the last mentioned kind appear after the others, they

cannot impregnate them, consequently no seeds are produced in our

gardens, and scarcely ever on the plants cultivated in India. An event

happened this year, which I have long wished for; two plaintain-trees

flowering with me so fortunately that one of them brought forth its

first female blossoms at the time that male ones began to appear on the

other. I eagerly ran to collect antheræ from the first plant, in order

to scatter them over the newly-expanded females, in hopes of obtaining

seed from them, which no botanist has yet been able to do. But when I

came to examine the antheræ I found even the largest of them absolutely

empty and void of pollen, consequently unfit for impregnating the

females; the seeds of this plant, therefore, can never be perfected in

our gardens. I do not doubt, however, that real male plants of this

species may be found in its native country, bearing flowers without

fruit, which the gardeners have neglected; while the females in this

country produce imperfect fruit, without seeds, like the female fig;

and, like that tree, are increased easily by suckers. The fruit,

therefore, of the plaintain-tree scarcely attains anything like its due

size, the larger seed-buds only ripening, without containing anything

in them.

The day would sooner fail me than examples. A female date-bearing palm

flowered many years at Berlin, without producing any seeds. But the

Berlin people taking care to have some of the blossoms of the male

tree, which was then flowering at Leipsic, sent them by the post, they

obtained fruit by that means; and some dates, the offspring of this

impregnation, being planted in my garden, sprung up, and to this day

continue to grow vigorously. Kœmpfer formerly told us how necessary

it was found by the oriental people, who live upon the produce of

palm-trees, and are the true Lotophagi, to plant some male trees among

the females, if they hoped for any fruit; hence, it is the practice of

those who make war in that part of the world to cut down all the male

palms, that a famine may afflict their proprietors; sometimes even

the inhabitants themselves destroy the male trees, when they dread an

invasion, that their enemies may find no sustenance in the country.

Leaving these instances, and innumerable others, which are so well

known to botanists that they would by no means bear the appearance of

novelty, and can only be doubted by those persons who neither have

observed nature, nor will they take the trouble to study her, I pass

to a fresh subject, concerning which much new light is wanted; I mean

hybrid, or mule vegetables, the existence and origin of which we shall

now consider.

I shall enumerate three or four real mule plants, to whose origin I

have been an eye-witness.

1. _Veronica spuria_, described in Amœnitates Acad. vol. III. p.

35, came from the impregnation of _Veronic maratima_ by _Verbena

officinalis_; it is easily propagated by cuttings, and agrees

perfectly with its mother in fructification, and with its father in

leaves.

2. _Delphinium hybridum_, sprung up in a part of the garden where

_Delphinium clatum_ and _Aconitum Napellus_ grew together;

it resembles its mother as much in its internal parts, that is, in

fructification as it does its father (the _Aconitum_) in outward

structure, or leaves; and, owing its origin to plants so nearly allied

to each other, it propagates itself by seed; some of which I now send

with this Dissertation.

3. _Hieracium Taraxici_, gathered in 1753 upon our mountains by

Dr. Solander, in its thick, brown, woolly calyx; in its stem being

hairy towards the top, and in its bracteæ, as well as in every part of

its fructification, resembles so perfectly its mother, _Hieracium

alpinum_, that an inexperienced person might mistake one for the

other; but in the smoothness of its leaves, in their indentations and

whole structure, it so manifestly agrees with its father, _Leontodon

Taraxacum_ (Dandelion), that there can be no doubt of its origin.

4. _Tragopogon hybridum_ attracted my notice the autumn before

last, in a part of the garden where I had planted _Tragopogon

pratense_, and _Tragopogon porrifolium_; but winter coming on,

destroyed its seeds. Last year, while the _Tragopogon pratense_

was in flower I rubbed off its pollen early in the morning, and

about eight o’clock sprinkled its stigmata with some pollen of the

_Tragopogon porrifolium_, marking the calyces by tying a thread

round them. I afterwards gathered the seeds when ripe, and sowed them

that autumn in another place; they grew, and produced this year, 1759,

purple flowers yellow at the base, seeds of which I now send. I doubt

whether any experiment demonstrates the generation of plants more

certainly than this.

There can be no doubt that these are all new species produced by

hybrid generation. And hence we learn, that a mule offspring is

the exact image of its mother in its medullary substance, internal

nature, or fructification, but resembles its father in leaves. This

is a foundation upon which naturalists may build much. For it seems

probable that many plants, which now appear different species of

the same _genus_, may in the beginning have been but one plant,

having arisen merely from hybrid generation. Many of those Geraniums

which grow at the Cape of Good Hope, and have never been found wild

anywhere but in the south parts of Africa, and which, as they are

distinguished from all other Geraniums by their single-leaved calyx,

many-flowered foot-stalk, irregular corolla, seven fertile stamina,

and three mutilated ones, and by their naked seeds furnished with

downy awns; so they agree together in all these characters, although

very various in their roots, stems and leaves; these Geraniums, I say,

would almost induce a botanist to believe that the species of one

_genus_ in vegetables are only so many different plants as there

have been different associations with the flowers of one species, and

consequently a _genus_ is nothing else than a number of plants

sprung from the same mother by different fathers. But whether all

these species be the offspring of time; whether, in the beginning

of all things, the Creator limited the number of future species, I

dare not presume to determine. I am, however, convinced this mode of

multiplying plants does not interfere with the system or general scheme

of nature; as I daily observe that insects, which live upon one species

of a particular _genus_, are contented with another of the same

_genus_.

A person who has once seen the _Achyranthes aspera_, and remarked

its spike, the parts of its flower, its small and peculiarly formed

nectaria, as well as its calyces bent backwards as the fruit ripens,

would think it very easy at any time to distinguish these flowers

from all others in the universe; but when he finds the flowers of

_Achyranthes indica_ agreeing with them even in their minutest

parts, and at the same time observes the large, thick, obtuse,

undulated leaves of the last-mentioned plant, he will think he sees

_Achyranthes aspera_ masked in the foliage of _Xanthium

strumarium_. But I forbear to mention any more instances.

Here is a new employment for botanists, to attempt the production of

new species of vegetables by scattering the pollen of various plants

over various widowed females. And if these remarks should meet with

a favourable reception, I shall be the more induced to dedicate what

remains of my life to such experiments, which recommend themselves by

being at the same time agreeable and useful. I am persuaded by many

considerations that those numerous and most valuable varieties of

plants which are used for culinary purposes, have been produced in

this manner, as the several kinds of cabbages, lettuces, etc.; and I

apprehend this is the reason of their not being changed by a difference

of soil. Hence I cannot give my assent to the opinion of those who

imagine all varieties to have been occasioned by change of soil; for,

if this were the case, the plants would return to their original form,

if removed again to their original situation.

FOOTNOTES:

[Footnote 13: From the _Publications of the Linnaean Society_.]

XII

JOSEPH BLACK

1728-1799

_Joseph Black, born in 1728 at Bordeaux, France, was educated in

Belfast and at the University of Glasgow. Before he took his M.D.

degree he showed that alkalies were formed, not by their absorbing

“phlogiston,” but by their having carbonic acid gas, or “fixed air,”

as a component. In 1753 he was appointed lecturer on chemistry at

Glasgow, and in 1776 became professor of chemistry at Edinburgh. In

1763 he announced his discovery of latent heat, a principle which

has been of great practical value. He died in Edinburgh, December 6,

1799._

THE DISCOVERY OF CARBONIC ACID GAS[14]

Hoffman, in one of his observations, gives the history of a powder

called _Magnesia Alba_, which has been long used, and esteemed as

a mild and tasteless purgative; but the method of preparing it was not

generally known before he made it public.

It was originally obtained from a liquor called the _Mother of

nitre_, which is produced in the following manner:

Salt-petre is separated from the brine which first affords it, or from

the water with which it is washed out of nitrous earths, by the process

commonly used in crystallizing salts. In this process, the brine is

gradually diminished, and at length reduced to a small quantity of

an unctuous bitter saline liquor, affording no more salt-petre by

evaporation, but, if urged with a brisk fire, drying up into a confused

mass, which attracts water strongly, and becomes fluid again when

exposed to the open air.

To this liquor the workmen have given the name of the _Mother of

nitre_; and Hoffman, finding it composed of the magnesia united

to an acid, obtained a separation of these, either by exposing the

compound to a strong fire, in which the acid was dissipated, and the

magnesia remained behind, or by the addition of an alkali, which

attracted the acid to itself: and this last method he recommends as

the best. He likewise makes an inquiry into the nature and virtues

of the powder thus prepared; and observes, that it is an absorbent

earth, which joins readily with all acids, and must necessarily destroy

any acidity it meets in the stomach; but that its purgative power is

uncertain, for sometimes it has not the least effect of that kind.

As it is a mere insipid earth, he rationally concludes it to be a

purgative only when converted into a sort of neutral salt by an acid

in the stomach, and that its effect is therefore proportional to the

quantity of this acid.

Although magnesia appears from this history of it, to be a very

innocent medicine; yet, having observed that some hypochondriacs,

who used it frequently, were subject to flatulencies and spasms, he

seems to have suspected it of some noxious quality. The circumstances,

however, which gave rise to his suspicion, may very possibly have

proceeded from the imprudence of his patients; who, trusting too much

to magnesia (which is properly a palliative in that disease) and

neglecting the assistance of other remedies, allowed their disorder

to increase upon them. It may, indeed, be alleged that magnesia, as a

purgative, is not the most eligible medicine for such constitutions, as

they agree best with those that strengthen, stimulate, and warm; which

the saline purges, commonly used, are not observed to do. But there

seems at last to be no objection to its use, when children are troubled

with an acid in their stomach: for, gentle purging, in this case, is

very proper; and it is often more conveniently procured by means of

magnesia, than of any other medicine, on account of its being entirely

insipid.

The above-mentioned Author, observing, some time after, that a bitter

saline liquor, similar to that obtained from the brine of salt-petre,

was likewise produced by the evaporation of those waters which contain

common salt, had the curiosity to try if this would also yield a

magnesia. The experiment succeeded: And he thus found out another

process for obtaining this powder; and at the same time assured

himself, by experiments, that the product from both was exactly the

same.

My curiosity led me, some time ago, to inquire more particularly into

the nature of magnesia, and especially to compare its properties with

those of the other absorbent earths, of which there plainly appeared to

me to be very different kinds, although commonly confounded together

under one name. I was indeed led to this examination of the absorbent

earths, partly by the hope of discovering a new sort of lime and

lime-water, which might possibly be a more powerful solvent of the

stone, than that commonly used; but was disappointed in my expectations.

I have had no opportunity of seeing Hoffman’s first magnesia, or the

liquor from which it is prepared, and have therefore been obliged to

make my experiments upon the second.

In order to prepare it, I at first employed the bitter saline liquor

called _bittern_, which remains in the pans after the evaporation

of sea-water. But as that liquor is not always easily procured, I

afterwards made use of a salt called Epsom salt, which is separated

from the bittern by crystallization, and is evidently composed of

magnesia and the vitriolic acid.

There is likewise a spurious kind of Glauber salt, which yields plenty

of magnesia, and seems to be no other than Epsom salt, of sea-water

reduced to crystals of a larger size. And common salt also affords

a small quantity of this powder; because, being separated from the

bittern by one hasty crystallization only, it necessarily contains a

portion of that liquor.

Those who would prepare a magnesia from Epsom salt, may use the

following process:

Dissolve equal quantities of Epsom salt, and of pearl ashes,

separately, in a sufficient quantity of water; purify each solution

from its dregs, and mix them accurately together by violent agitation.

Then make them just to boil over a brisk fire.

Add now to the mixture, three or four times its quantity of hot water;

after a little agitation, allow the magnesia to settle to the bottom,

and decant off as much of the water as possible. Pour on the same

quantity of cold water; and, after settling, decant it off in the

same manner. Repeat this washing with the cold water ten or twelve

times, or even oftener, if the magnesia be required perfectly pure for

chemical experiments.

When it is sufficiently washed, the water may be strained and squeezed

from it in a linen cloth; for very little of the magnesia passes

through.

The alkali in the mixture, uniting with the acid, separates it from

the magnesia; which, not being of itself soluble in water, must

consequently appear immediately under a solid form. But the powder

which thus appears is not entirely magnesia; part of it is the neutral

salt formed from the union of the acid and alkali. This neutral salt

is found, upon examination, to agree in all respects with vitriolated

tartar, and requires a large quantity of hot water to dissolve it. As

much of it is therefore dissolved as the water can take up; the rest

is dispersed through the mixture, in the form of a powder. Hence the

necessity of washing the magnesia with so much trouble; for the first

effusion of hot water is intended to dissolve the whole of the salt,

and the subsequent additions of cold water to wash away this solution.

The caution given, of boiling the mixture, is not unnecessary: if it

be neglected, the whole of the magnesia is not accurately separated at

once; and, by allowing it to rest for some time, that powder concretes

into minute grains, which, when viewed with the microscope, appear to

be assemblages of needles diverging from a point. This happens more

especially when the solutions of the Epsom salt, and of the alkali,

are diluted with too much water before they are mixed together. Thus,

if a dram of Epsom salt, and of salt of tartar, be dissolved each in

four ounces of water, and be mixed, and then allowed to rest three or

four days, the whole of the magnesia will be formed into these grains.

Or, if we filtrate the mixture soon after it is made, and heat the

clear liquor which passes through, it will become turbid, and deposit a

magnesia.

An ounce of magnesia was exposed in a crucible, for about an hour, to

such a heat as is sufficient to melt copper. When taken out, it weighed

three drams and one scruple, or had lost 7-12 of its former weight.

I repeated, with the magnesia prepared in this manner, most of those

experiments I had already made upon it before calcination, and the

result was as follows:--

It dissolves in all the acids, and with these composes salts exactly

similar to those described in the first set of experiments: But, what

is particularly to be remarked, it is dissolved without any the least

degree of effervescence.

It slowly precipitates the corrosive sublimate of mercury, in the form

of a black powder.

It separates the volatile alkali in salt-ammoniac from the acid, when

it is mixed with a warm solution of that salt. But it does not separate

an acid from a calcareous earth, nor does it introduce the least change

upon lime-water.

Lastly, when a dram of it is digested with an ounce of water in a

bottle for some hours, it does not make any the least change in the

water. The magnesia, when dried, is found to have gained ten grains;

but it neither effervesces with acids, nor does it sensibly affect

lime-water.

Observing magnesia to lose such a remarkable proportion of its weight

in the fire, my next attempts were directed to the investigation of

this volatile part; and, among other experiments, the following seemed

to throw some light upon it:--

Three ounces of magnesia were distilled in a glass retort and receiver,

the fire being gradually increased until the magnesia was obscurely red

hot. When all was cool, I found only five drams of a whitish water in

the receiver, which had a faint smell of the spirit of hartshorn, gave

a green colour to the juice of violets, and rendered the solutions of

corrosive sublimate, and of silver, very slightly turbid. But it did

not sensibly effervesce with acids.

The magnesia, when taken out of the retort, weighed an ounce, three

drams, and thirty grains, or had lost more than half of its weight. It

still effervesced pretty briskly with acids, though not so strongly as

before this operation.

The fire should have been raised here to the degree requisite for

the perfect calcination of magnesia. But, even from this imperfect

experiment, it is evident, that, of the volatile parts contained in

that powder, a small proportion only is water; the rest cannot, it

seems, be retained in vessels, under a visible form. Chemists have

often observed in their distillations that part of a body has vanished

from their senses notwithstanding the utmost care to retain it; and

they have always found, upon further inquiry, that subtle part to be

air, which having been imprisoned in the body, under a solid form, was

set free, and rendered fluid and elastic by the fire. We may therefore

safely conclude, that the volatile matter lost in the calcination of

magnesia, is mostly air; and hence the calcined magnesia does not emit

air, or make an effervescence when mixed with acids.

The water, from its properties, seems to contain a small portion of

volatile alkali, which was probably formed from the earth, air and

water, from some of these combined together; and perhaps also from a

small quantity of inflammable matter, which adhered accidently to the

magnesia. Whenever chemists meet with this salt, they are inclined to

ascribe its origin to some animal or putrid vegetable substance; and

this they have always done, when they obtained it from the calcareous

earths, all of which afford a small quantity of it. There is, however,

no doubt, that it can sometimes be produced independently of any such

mixture, since many fresh vegetables, and tartar, afford a considerable

quantity of it. And how can it, in the present instance, be supposed,

that any animal or vegetable matter adhered to the magnesia, while it

was dissolved by an acid, separated from this by an alkali, and washed

with so much water?

Two drams of magnesia were calcined in a crucible, in the manner

described above, and thus reduced to two scruples and twelve grains.

This calcined magnesia was dissolved in a sufficient quantity of spirit

of vitriol, and then again separated from the acid by the addition of

an alkali, of which a large quantity is necessary for this purpose. The

magnesia being very well washed and dried, weighed one dram and fifty

grains. It effervesced violently, or emitted a large quantity of air,

when thrown into acids; formed a red powder, when mixed with a solution

of sublimate; separated the calcareous earths from an acid, and

sweetened lime-water; and had thus recovered all those properties which

it had but just now lost by calcination. Nor had it only recovered

its original properties, but acquired besides an addition of weight,

nearly equal to what had been lost in the fire; and as it is found to

effervesce with acids, part of the addition must certainly be air.

This air seems to have been furnished by the alkali, from which it

was separated by the acid; for Dr. Hales has clearly proved, that

alkaline salts contain a large quantity of fixed air, which they emit

in great abundance when joined to a pure acid. In the present case, the

alkali is really joined to an acid, but without any visible emission

of air; and yet the air is not retained in it; for the neutral salt,

into which it is converted, is the same in quantity, and in every other

respect, as if the acid employed had not been previously saturated with

magnesia, but offered to the alkali in its pure state, and had driven

the air out of it in their conflict. It seems therefore evident, that

the air was forced from the alkali by the acid, and lodged itself in

the magnesia.

These considerations led me to try a few experiments, whereby I might

know what quantity of air is expelled from an alkali, or from magnesia,

by acids.

Two drams of a pure fixed alkaline salt, and an ounce of water, were

put into a Florentine flask, which, together with its contents, weighed

two ounces and two drams. Some oil of vitriol diluted with water was

dropped in, until the salt was exactly saturated; which it was found to

be, when two drams, two scruples and three grains of this acid had been

added. The phial with its contents now weighed two ounces, four drams

and fifteen grains. One scruple, therefore, and eight grains, were lost

during the ebullition; of which a trifling portion may be water, or

something of the same kind; the rest is air.

FOOTNOTES:

[Footnote 14: From _Experiments upon Magnesia, Quicklime, and some

other Alkaline Substances_ (1775).]

XIII

JOSEPH PRIESTLEY

1733-1804

_Joseph Priestley, born in Yorkshire, England, March 13, 1733, was

a Unitarian minister. In 1774 he discovered oxygen, which he called

“dephlogisticated air.” Because of his liberal political ideas he was

persecuted by his countrymen, and in 1794 emigrated to Northumberland,

Pennsylvania, where he lived until his death, February 6, 1804._

THE DISCOVERY OF OXYGEN[15]

Presently, after my return from abroad, I went to work upon the

_mercurius calcinatus_, which I had procured from Mr. Cadet; and,

with a very moderate degree of heat, I got from about one-fourth of

an ounce of it, an ounce-measure of air, which I observed to be not

readily imbibed, either by the substance itself from which it had

been expelled (for I suffered them to continue a long time together

before I transferred the air to any other place) or by water, in which

I suffered this air to stand a considerable time before I made any

experiment upon it.

In this air, as I had expected, a candle burned with a vivid flame; but

what I observed new at this time (November 19), and which surprised me

no less than the fact I had discovered before, was, that, whereas a

few moments agitation in water will deprive the modified nitrous air

of its property of admitting a candle to burn in it; yet, after more

than ten times as much agitation as would be sufficient to produce this

alteration in the nitrous air, no sensible change was produced in this.

A candle still burned in it with a strong flame; and it did not, in

the least, diminish common air, which I have observed that nitrous air,

in this state, in some measure does.

But I was much more surprised, when, after two days, in which this air

had continued in contact with water (by which it was diminished about

one-twentieth of its bulk) I agitated it violently in water about five

minutes, and found that a candle still burned in it as well as in

common air. The same degree of agitation would have made phlogisticated

nitrous air fit for respiration indeed, but it would certainly have

extinguished a candle.

These facts fully convinced me, that there must be a very material

difference between the constitution of air from _mercurius

calcinatus_, and that of phlogisticated nitrous air, notwithstanding

their resemblance in some particulars. But though I did not doubt that

the air from _mercurius calcinatus_ was fit for respiration, after

being agitated in water, as every kind of air without exception, on

which I have tried the experiment, had been, I still did not suspect

that it was respirable in the first instance; so far was I from having

any idea of this air being, what it really was, much superior, in this

respect, to the air of the atmosphere.

In this ignorance of the real nature of this kind of air, I continued

from this time (November) to the 1st of March following; having, in the

meantime, been intent upon my experiments on the vitriolic acid air

above recited, and the various modifications of air produced by spirit

of nitre, an account of which will follow. But in the course of this

month, I not only ascertained the nature of this kind of air, though

very gradually, but was led to it by the complete discovery of the

constitution of the air we breathe.

Till this 1st of March, 1775, I had so little suspicion of the air from

_mercurius calcinatus_, &c., being wholesome, that I had not even

thought of applying it to the test of nitrous air; but thinking (as my

reader must imagine I frequently must have done) on the candle burning

in it after long agitation in water, it occurred to me at last to make

the experiment; and putting one measure of nitrous air to two measures

of this air, I found, not only that it was diminished, but that it was

diminished quite as much as common air, and that the redness of the

mixture was likewise equal to that of a similar mixture of nitrous and

common air.

After this I had no doubt but that the air from _mercurius

calcinatus_ was fit for respiration, and that it had all the other

properties of genuine common air. But I did not take notice of what I

might have observed, if I had not been so fully possessed by the notion

of there being no air better than common air, that the redness was

really deeper, and the diminution something greater than common air

would have admitted.

Moreover, this advance in the way of truth, in reality, threw me back

into error, making me give up the hypothesis I had first formed, viz.

that the _mercurius calcinatus_ had extracted spirit of nitre

from the air; for I now concluded, that all the constituent parts of

the air were equally, and in their proper proportion, imbibed in the

preparation of this substance, and also in the process of making red

lead. For at the same time that I made the above mentioned experiment

on the air from _mercurius calcinatus_, I likewise observed that

the air which I had extracted from red lead, after the fixed air was

washed out of it, was of the same nature, being diminished by nitrous

air like common air: but, at the same time, I was puzzled to find that

air from the red precipitate was diminished in the same manner, though

the process for making this substance is quite different from that of

making the two others. But to this circumstance I happened not to give

much attention.

I wish my reader be not quite tired with the frequent repetition of the

word surprise, and others of similar import; but I must go on in that

style a little longer. For the next day I was more surprised than ever

I had been before, with finding that, after the above-mentioned mixture

of nitrous air and the air from _mercurius calcinatus_, had stood

all night, (in which time the whole diminution must have taken place;

and, consequently, had it been common air, it must have been made

perfectly noxious, and entirely unfit for respiration or inflammation)

a candle burned in it, and even better than in common air.

I cannot, at this distance of time, recollect what it was that I had in

view in making this experiment; but I know I had no expectation of the

real issue of it. Having acquired a considerable degree of readiness in

making experiments of this kind, a very slight and evanescent motive

would be sufficient to induce me to do it. If, however, I had not

happened, for some other purpose, to have had a lighted candle before

me I should probably never have made the trial; and the whole train

of my future experiments relating to this kind of air might have been

prevented.

Still, however, having no conception of the real cause of this

phenomenon, I considered it as something very extraordinary; but as

a property that was peculiar to air that was extracted from these

substances, and adventitious; and I always spoke of the air to my

acquaintance as being substantially the same thing with common air.

I particularly remember my telling Dr. Price, that I was myself

perfectly satisfied of its being common air, as it appeared to be so

by the test of nitrous air; though, for the satisfaction of others, I

wanted a mouse to make the proof quite complete.

On the 8th of this month I procured a mouse, and put it into a glass

vessel, containing two ounce-measures of the air from _mercuris

calcinatus_. Had it been common air, a full-grown mouse, as this

was, would have lived in it about a quarter of an hour. In this air,

however, my mouse lived a full half hour; and though it was taken out

seemingly dead, it appeared to have been only exceedingly chilled; for,

upon being held to fire, it presently revived, and appeared not to have

received any harm from the experiment.

By this I was confirmed in my conclusion, that the air extracted

from _mercurius calcinates_, &c., was, at least, as good as

common air; but I did not certainly conclude that it was any better;

because, though one mouse would live only a quarter of an hour in a

given quantity of air, I knew it was not impossible but that another

mouse might have lived in it half an hour; so little accuracy is

there in this method of ascertaining the goodness of air; and indeed

I have never had recourse to it for my own satisfaction, since the

discovery of that most ready, accurate, and elegant test that nitrous

air furnishes. But in this case I had a view to publishing the most

generally satisfactory account of my experiments that the nature of the

thing would admit of.

This experiment with the mouse, when I had reflected upon it some time,

gave me so much suspicion that the air into which I had put it was

better than common air, that I was induced, the day after, to apply

the test of nitrous air to a small part of that very quantity of air

which the mouse had breathed so long; so that, had it been common air,

I was satisfied it must have been very nearly, if not altogether, as

noxious as possible, so as not to be affected by nitrous air; when,

to my surprise again, I found that though it had been breathed so

long, it was still better than common air. For after mixing it with

nitrous air, in the usual proportion of two to one, it was diminished

in the proportion of four and one-half to three and one-half; that

is, the nitrous air had made it two-ninths less than before, and this

in a very short space of time; whereas I had never found that, in the

longest time, any common air was reduced more than one-fifth of its

bulk by any proportion of nitrous air, nor more than one-fourth by any

phlogistic process whatever. Thinking of this extraordinary fact upon

my pillow, the next morning I put another measure of nitrous air to the

same mixture, and, to my utter astonishment, found that it was farther

diminished to almost one-half of its original quantity. I then put a

third measure to it; but this did not diminish it any farther; but,

however, left it one measure less than it was even after the mouse had

been taken out of it.

Being now fully satisfied that this air, even after the mouse had

breathed it half an hour, was much better than common air; and having

a quantity of it still left, sufficient for the experiment, viz. an

ounce-measure and a half, I put the mouse into it; when I observed that

it seemed to feel no shock upon being put into it, evident signs of

which would have been visible, if the air had not been very wholesome;

but that it remained perfectly at its ease another full half hour, when

I took it out quite lively and vigorous. Measuring the air the next

day, I found it to be reduced from one and one-half to two-thirds of an

ounce-measure. And after this, if I remember well (for in my register

of the day I only find it noted, that it was considerably diminished

by nitrous air), it was nearly as good as common air. It was evident,

indeed, from the mouse having been taken out quite vigorous, that the

air could not have been rendered very noxious.

For my farther satisfaction I procured another mouse, and putting it

into less than two ounce-measures of air extracted from _mercurius

calcinatus_ and air from red precipitate (which, having found

them to be of the same quality, I had mixed together) it lived

three-quarters of an hour. But not having had the precaution to set the

vessel in a warm place, I suspect that the mouse died of cold. However,

as it had lived three times as long as it could probably have lived in

the same quantity of common air, and I did not expect much accuracy

from this kind of a test, I did not think it necessary to make any more

experiments with mice.

Being now fully satisfied of the superior goodness of this kind of air,

I proceeded to measure that degree of purity, with as much accuracy

as I could, by the test of nitrous air; and I began with putting one

measure of nitrous air to two measures of this air, as if I had been

examining common air; and now I observed that the diminution was

evidently greater than common air would have suffered by the same

treatment. A second measure of nitrous air reduced it to two-thirds

of its original quantity, and a third measure to one-half. Suspecting

that the diminution could not proceed much farther, I then added only

half a measure of nitrous air, by which it was diminished still more;

but not much, and another half-measure made it more than half of its

original quantity; so that, in this case, two measures of this air took

more than two measures of nitrous air, and yet remained less than half

of what it was. Five measures brought it pretty exactly to its original

dimensions.

At the same time, air from the red precipitate was diminished in

the same proportion as that from _mercurius calcinatus_, five

measures of nitrous air being received by two measures of this without

any increase of dimensions. Now as common air takes about one-half

of its bulk of nitrous air, before it begins to receive any addition

to its dimensions from more nitrous air, and this air took more than

four half-measures before it ceased to be diminished by more nitrous

air, and even five half-measures made no addition to its original

dimensions, I conclude that it was between four and five times as good

as common air. It will be seen that I have since procured air better

than this, even between five and six times as good as the best common

air that I have ever met with.

FOOTNOTES:

[Footnote 15: From _Experiments and Observations on Different Kinds

of Air_, Vol. II, (1775).]

XIV

HENRY CAVENDISH

1731-1810

_Henry Cavendish, the discoverer of hydrogen, was born of English

parents in Nice, October 10, 1731. He studied at Cambridge University,

England, and in 1760 joined the Royal Society, devoting his great

fortune to the advancement of science. He discovered hydrogen in 1766,

and later, using Priestley’s discovery of oxygen, found that the two

gases combined under certain physical conditions to produce water.

Besides his studies in chemistry, he made some interesting discoveries

in physics. In 1783 he proposed the theory that heat was a motion

rather than a substance; and in 1798 he computed the density of the

earth to be about five and a half times that of water. He died at

Clapham, February 24, 1810._

THE COMBINATION OF HYDROGEN AND OXYGEN INTO WATER[16]

In Dr. Priestley’s last volume of experiments is related an experiment

of Mr. Warltire’s, in which it is said that, on firing a mixture of

common and inflammable air by electricity in a close copper vessel

holding about three pints, a loss of weight was always perceived, on

an average about two grains, though the vessel was stopped in such a

manner that no air could escape by the explosion. It is also related,

that on repeating the experiment in glass vessels, the inside of the

glass, though clean and dry before, immediately became dewy; which

confirmed an opinion he had long entertained, that common air deposits

its moisture by phlogistication. As the latter experiment seemed likely

to throw great light on the subject I had in view, I thought it well

worth examining more closely. The first experiment also, if there was

no mistake in it, would be very extraordinary and curious; but it did

not succeed with me; for though the vessel I used held more than Mr.

Warltire’s, namely, 24,000 grains of water, and though the experiment

was repeated several times with different proportions of common and

inflammable air, I could never perceive a loss of weight of more than

one-fifth of a grain, and commonly none at all. It must be observed,

however, that though there were some of the experiments in which it

seemed to diminish a little in weight, there were none in which it

increased.

In all the experiments, the inside of the glass globe became dewy,

as observed by Mr. Warltire; but not the least sooty matter could be

perceived. Care was taken in all of them to find how much the air was

diminished by the explosion, and to observe its test. The result is as

follows, the bulk of the inflammable air being expressed in decimals of

the common air:

------+-----------+----------+-------------+------------+--------

| | |Air Remaining|Test of this|

Common|Inflammable|Diminution| after the | Air in the |Standard

Air | Air | | Explosion |First Method|

------+-----------+----------+-------------+------------+--------

1 | 1.241 | .686 | 1.555 | .055 | .0

| 1.955 | .642 | 1.423 | .063 | .0

| .706 | .647 | 1.059 | .066 | .0

| .423 | .612 | .811 | .097 | .03

| .331 | .476 | .855 | .339 | .27

| .206 | .294 | .912 | .648 | .58

------+-----------+----------+-------------+------------+---------

In these experiments the inflammable air was procured from zinc, as it

was in all my experiments, except where otherwise expressed: but I made

two more experiments, to try whether there was any difference between

the air from zinc and that from iron, the quantity of inflammable air

being the same in both, namely, 0.331 of the common; but I could not

find any difference to be depended on between the two kinds of air,

either in the diminution which they suffered by the explosion, or the

test of the burnt air.

From the fourth experiment it appears, that 423 measures of inflammable

air are nearly sufficient to phlogisticate completely 1000 of common

air; and that the bulk of the remaining air after the explosion is then

very little more than four-fifths of the common air employed; so that

as common air cannot be reduced to a much less bulk than that by any

method of phlogistication, we may safely conclude, that when they are

mixed in this proportion, and exploded, almost all the inflammable air,

and about one-fifth part of the common air, lose their elasticity, and

are condensed into the dew which lines the glass.

The better to examine the nature of this dew, 500,000 grain measures

of inflammable air were burnt with about two and one-half times the

quantity of common air, and the burnt air made to pass through a glass

cylinder eight feet long and three-quarters of an inch in diameter,

in order to deposit the dew. The two airs were conveyed slowly into

this cylinder by separate copper pipes, passing through a brass plate

which stopped up the end of the cylinder; and as neither inflammable

nor common air can burn by themselves, there was no danger of the flame

spreading into the magazines from which they were conveyed. Each of

these magazines consisted of a large tin vessel, inverted into another

vessel just big enough to receive it. The inner vessel communicated

with the copper pipe, and the air was forced out of it by pouring water

into the outer vessel; and in order that the quantity of common air

expelled should be two and one-half times that of the inflammable, the

water was let into the outer vessels by two holes in the bottom of the

same tin pan, the hole which conveyed the water into that vessel in

which the common air was confined being two and one-half times as big

as the other.

In trying the experiment, the magazines being first filled with their

respective airs, the glass cylinder was taken off, and water let, by

the two holes, into the outer vessel, till the airs began to issue from

the ends of the copper pipes; they were then set on fire by a candle,

and the cylinder put on again in its place. By this means upwards of

135 grains of water were condensed in the cylinder, which had no taste

nor smell, and which left no sensible sediment when evaporated to

dryness; neither did it yield any pungent smell during evaporation; in

short, it seemed pure water.

In my first experiment, the cylinder near that part where the air

was fired was a little tinged with sooty matter, but very slightly

so; and that little seemed to proceed from the putty with which the

apparatus was luted, and which was heated by the flame; for in another

experiment, in which it is contrived so that the luting should not be

much heated, scarce any sooty tinge could be perceived.

By the experiments with the globe it appeared, that when inflammable

and common air are exploded in a proper proportion, almost all the

inflammable air, and nearly one-fifth of the common air, lose their

elasticity, and are condensed into dew. And by this experiment it

appears, that this dew is plain water, and consequently that almost all

the inflammable air and about one-fifth of the common air, are turned

into pure water.

In order to examine the nature of the matter condensed on firing a

mixture of dephlogisticated and inflammable air, I took a glass globe

holding 8,800 grain measures, furnished with a brass cock and an

apparatus for firing air by electricity. This globe was well exhausted

by an air-pump, and then filled with a mixture of inflammable and

dephlogisticated air, by shutting the cock, fastening a bent glass tube

to its mouth, and letting up the end of it into a glass jar inverted

into water, and containing a mixture of 19,500 grain measures of

dephlogisticated air, and 37,000 of inflammable; so that, upon opening

the cock, some of this mixed air rushed through the bent tube, and

filled the globe. The cock was then shut, and the included air fired by

electricity, by which means almost all of it lost its elasticity. The

cock was then again opened, so as to let in more of the same air, to

supply the place of that destroyed by the explosion, which was again

fired, and the operation continued till almost the whole of the mixture

was let into the globe and exploded. By this means, though the globe

held not more than the sixth part of the mixture, almost the whole of

it was exploded therein, without any fresh exhaustion of the globe.

As I was desirous to try the quantity and test of this burnt air,

without letting any water into the globe, which would have prevented my

examining the nature of the condensed matter, I took a larger globe,

furnished also with a stop cock, exhausted it by an air-pump, and

screwed it on upon the cock of the former globe; upon which, by opening

both cocks, the air rushed out of the smaller globe into the larger,

till it became of equal density in both; then, by shutting the cock of

the larger globe, unscrewing it again from the former, and opening it

under water, I was enabled to find the quantity of the burnt air in

it; and consequently, as the proportion which the contents of the two

globes bore to each other was known, could tell the quantity of burnt

air in the small globe before the communication was made between them.

By this means the whole quantity of the burnt air was found to be 2,950

grain measures; its standard was 1.85.

The liquor condensed in the globe, in weight about thirty grains, was

sensibly acid to the taste, and by saturation with fixed alkali, and

evaporation, yielded near two grains of nitre; so that it consisted

of water united to a small quantity of nitrous acid. No sooty matter

was deposited in the globe. The dephlogisticated air used in this

experiment was procured from red precipitate, that is, from a solution

of quicksilver in spirit of nitre distilled till it acquires a red

colour.

As it was suspected, that the acid contained in the condensed liquor

was no essential part of the dephlogisticated air, but was owing to

some acid vapour which came over in making it and had not been absorbed

by the water, the experiment was repeated in the same manner, with some

more of the same air, which had been previously washed with water, by

keeping it a day or two in a bottle with some water, and shaking it

frequently; whereas that used in the preceding experiment had never

passed through water, except in preparing it. The condensed liquor was

still acid.

The experiment was also repeated with dephlogisticated air, procured

from red lead by means of oil of vitriol; the liquor condensed was

acid, but by an accident I was prevented from determining the nature of

the acid.

I also procured some dephlogisticated air from the leaves of plants, in

the manner of Doctors Ingenhousz and Priestley, and exploded it with

inflammable air as before; the condensed liquor still continued acid,

and of the nitrous kind.

In all these experiments the proportion of inflammable air was such,

that the burnt air was not much phlogisticated; and it was observed,

that the less phlogisticated it was, the more acid was the condensed

liquor. I therefore made another experiment, with some more of the

same air from plants, in which the proportion of inflammable air was

greater, so that the burnt air was almost completely phlogisticated,

its standard being 1-10. The condensed liquor was then not at all acid,

but seemed pure water; so that it appears, that with this kind of

dephlogisticated air, the condensed liquor is not at all acid, when the

two airs are mixed in such a proportion that the burnt air is almost

completely phlogisticated, but is considerably so when it is not much

phlogisticated.

In order to see whether the same thing would obtain with air procured

from red precipitate, I made two more experiments with that kind

of air, the air in both being taken from the same bottle, and the

experiment tried in the same manner, except that the proportions of

inflammable air were different. In the first, in which the burnt air

was almost completely phlogisticated, the condensed liquor was not at

all acid. In the second, in which its standard was 1.86, that is, not

much phlogisticated, it was considerably acid; so that with this air,

as well as with that from plants, the condensed liquor contains, or is

entirely free from, acid, according as the burnt air is less or more

phlogisticated; and there can be little doubt but that the same rule

obtains with any other kind of dephlogisticated air.

In order to see whether the acid, formed by the explosion of

dephlogisticated air obtained by means of the vitriolic acid, would

also be of the nitrous kind, I procured some air from turbith mineral,

and exploded it with inflammable air, the proportion being such that

the burnt air was not much phlogisticated. The condensed liquor

manifested an acidity, which appeared, by saturation with a solution

of salt of tartar, to be of the nitrous kind; and it was found, by the

addition of some _terra ponderosa salita_, to contain little or no

vitriolic acid.

When inflammable air was exploded with common air, in such a proportion

that the standard of the burnt air was about 4-10, the condensed

liquor was not in the least acid. There is no difference, however, in

this respect between common air, and dephlogisticated air mixed with

phlogisticated in such a proportion as to reduce it to the standard of

common air; for some dephlogisticated air from red precipitate, being

reduced to this standard by the addition of perfectly phlogisticated

air, and then exploded with the same proportion of inflammable air as

the common air was in the foregoing experiment, the condensed liquor

was not in the least acid.

From the foregoing experiments it appears, that when a mixture of

inflammable and dephlogisticated air is exploded in such proportion

that the burnt air is not much phlogisticated, the condensed liquor

contains a little acid, which is always of the nitrous kind,

whatever substance the dephlogisticated air is procured from; but

if the proportion be such that the burnt air is almost entirely

phlogisticated, the condensed liquor is not at all acid, but seems

pure water, without any addition whatever; and as, when they are mixed

in that proportion, very little air remains after the explosion,

almost the whole being condensed, it follows that almost the whole

of the inflammable and dephlogisticated air is converted into pure

water. It is not easy, indeed, to determine from these experiments

what proportion the burnt air, remaining after the explosions, bore to

the dephlogisticated air employed, as neither the small nor the large

globe could be perfectly exhausted of air, and there was no saying

with exactness what quantity was left in them; but in most of them,

after allowing for this uncertainty, the true quantity of burnt air

seemed not more than 1-17 of the dephlogisticated air employed, or

1-50 of the mixture. It seems, however, unnecessary to determine this

point exactly, as the quantity is so small, that there can be little

doubt but that it proceeds only from the impurities mixed with the

dephlogisticated and inflammable air, and consequently that, if those

airs could be obtained perfectly pure, the whole would be condensed.

With respect to common air, and dephlogisticated air reduced by the

addition of phlogisticated air to the standard of common air, the

case is different; as the liquor condensed in exploding them with

inflammable air, I believe I may say in any proportion, is not at all

acid; perhaps because if they are mixed in such a proportion as that

the burnt air is not much phlogisticated, the explosion is too weak,

and not accompanied with sufficient heat.

All the foregoing experiments, on the explosion of inflammable air

with common and dephlogisticated airs, except those which relate to

the cause of the acid found in the water, were made in the summer

of the year 1781, and were mentioned by me to Dr. Priestley, who

in consequence of it made some experiments of the same kind, as he

relates in a paper printed in the preceding volume of the Transactions.

During the last summer also, a friend of mine gave some account of

them to M. Lavoisier, as well as of the conclusion drawn from them

that dephlogisticated air is only water deprived of phlogiston; but

at that time so far was M. Lavoisier from thinking any such opinion

warranted, that, till he was prevailed upon to repeat the experiment

himself, he found some difficulty in believing that nearly the whole

of the two airs could be converted into water. It is remarkable, that

neither of these gentlemen found any acid in the water produced by the

combustion; which might proceed from the latter having burnt two airs

in a different manner from what I did; and from the former having used

a different kind of inflammable air, namely, that from charcoal, and

perhaps having used a greater proportion of it.

FOOTNOTES:

[Footnote 16: From _Experiments with Airs--Transactions of Royal

Society of London_ (1784).]

XV

SIR WILLIAM HERSCHEL

1738-1822

_Sir William Herschel was born in Hanover, Germany, November 15,

1738, the son of a bandmaster. At an early age he was compelled to

earn his own living by playing in the band of the Hanoverian Guards.

In 1766, after some years of financial straits, he found work as

an organist at Bath. Studying languages and mathematics without

assistance from tutors, he became interested in “the music of the

spheres” which developed into a scientific attitude in astronomy. He

managed, in spite of his poverty, to construct specula for a telescope

and in 1781, with one of his own instruments, he discovered the

planet Uranus, one of the most romantic discoveries in the history of

science. Among his other discoveries were two of the satellites of

Uranus, two more of Saturn, and the fact that the moon was without

atmosphere; he also described many of the binary stars, discovered

many nebulous stars (which prepared the way for the nebular theory of

the universe), and made the inference from the movements of the stars

that the whole solar system was rushing towards the constellation

of Hercules. After his death, August 25, 1822, his son, Sir John

Herschel, continued his work in astronomy._

I

THE DISCOVERY OF URANUS[17]

ACCOUNT OF A COMET

On Tuesday, the 13th of March, 1781, between 10 and 11 in the evening,

while examining the small stars in the neighborhood of H Geminorum, I

perceived one that appeared visibly larger than the rest: being struck

with its uncommon magnitude, I compared it to H Geminorum and the

small star in the quartile between Auriga and Gemini, and finding it

so much larger than either of them, suspected it to be a comet. I was

then engaged in a series of observations on the parallax of the fixed

stars, which I hope soon to have the honour of laying before the R.S.,

and those observations requiring very high powers, I had ready at hand

several magnifiers of 227, 460, 932, 1536, 2010, &c., all of which I

have successfully used on that occasion. The power I had on when I

first saw the comet was 227. From experience I knew that the diameters

of the fixed stars are not proportionally magnified with higher powers,

as the planets are; I therefore now put on the powers of 460 and 932,

and found the diameter of the comet increased in proportion to the

power, as it ought to be, on the supposition of its not being a fixed

star, while the diameters of the stars to which I compared it, were not

increased in the same ratio. Also, that the comet being magnified much

beyond what its light would admit of, appeared hazy and ill-defined

with these great powers, while the stars preserved that lustre and

distinctness which from many thousand observations I knew they would

retain. The sequel has shown that my surmises were well founded, this

proving to be the comet we have lately observed.

II

ON THE NAME OF THE NEW PLANET

By the observations of the most eminent astronomers in Europe it

appears that the new star, which I had the honour of pointing out

to them in March, 1781, is a primary planet of our solar system. A

body so nearly related to us by its similar condition and situation,

in the unbounded expanse of the starry heavens, must often be the

subject of conversation, not only of astronomers, but of every lover

of science in general. This consideration, then, makes it necessary

to give it a name, by which it may be distinguished from the rest of

the planets and fixed stars. In the fabulous ages of ancient times

the appellations of Mercury, Venus, Mars, Jupiter, and Saturn, were

given to the planets, as being the names of their principal heroes

and divinities. In the present more philosophical era, it would

hardly be allowable to have recourse to the same method, and call

on Juno, Apollo, Pallas or Minerva, for a name to our new heavenly

body. The first consideration in any particular event, or remarkable

incident, seems to be its chronology; if in any future age it should be

asked, when this last-found planet was discovered it would be a very

satisfactory answer to say, “In the reign of King George the Third.” As

a philosopher, then, the name of Georgium Sidus presents itself to me,

as an appellation which will conveniently convey the information of the

time and country where and when it was brought to view.

III

ON NEBULOUS STARS, PROPERLY SO CALLED

In one of his late examinations of a space in the heavens, which

he had not reviewed before, Dr. H. discovered a star of about the

eighth magnitude, surrounded with a faintly luminous atmosphere, of a

considerable extent. The phenomenon was so striking that he could not

help reflecting on the circumstance that attended it, which appeared to

be of a very instructive nature, and such as might lead to inferences

which will throw a considerable light on some points relating to the

construction of the heavens.

Cloudy or nebulous stars have been mentioned by several astronomers;

but this name ought not to be applied to the objects which they have

pointed out as such; for, on examination, they proved to be either

mere clusters of stars, plainly to be distinguished with his large

instruments, or such nebulous appearances as might be reasonably

supposed to be occasioned by a multitude of stars at a vast distance.

The milky way itself consists entirely of stars, and by imperceptible

degrees he was led on from most evident congeries of stars to other

groups in which the lucid points were smaller, but still very plainly

to be seen; and from them to such wherein they could but barely be

suspected, till he arrived at last to spots in which no trace of a star

was to be discerned. But then the gradations to these later were by

such well-connected steps as left no room for doubt but that all these

phenomena were equally occasioned by stars, variously dispersed in the

immense expanse of the universe.

When Dr. H. pursued these researches, he was in the situation of a

natural philosopher who follows the various species of animals and

insects from the height of their perfection down to the lowest ebb of

life; when, arriving at the vegetable kingdom, he can scarcely point

out to us the precise boundary where the animal ceases and the plant

begins; and may even go so far as to suspect them not to be essentially

different. But recollecting himself, he compares, for instance, one

of the human species to a tree, and all doubt of the subject vanishes

before him. In the same manner we pass through gentle steps from a

coarse cluster of stars, such as the Pleiades, the Præserpe, the milky

way, the cluster in the Crab, the nebula in Hercules, that near the

preceding hip of Bootis, the 17th, 38th, 41st of the 7th class of his

catalogues, the 10th, 20th, 35th of the 6th class, the 33d, 48th, 213th

of the 1st, the 12th, 150th, 756th of the 2d, and the 18th, 140th,

725th of the 3d, without any hesitation, till we find ourselves brought

to an object such as the nebula in Orion, where we are still inclined

to remain in the once adopted idea, of stars exceedingly remote,

and inconceivably crowded, as being the occasion of that remarkable

appearance. It seems, therefore, to require a more dissimilar object

to set us right again. A glance like that of the naturalist, who casts

his eye from the perfect animal to the perfect vegetable, is wanting to

remove the veil from the mind of the astronomer. The object mentioned

above is the phenomenon that was wanting for this purpose. View, for

instance, the 19th cluster of the 6th class, and afterwards cast your

eye on this cloudy star, and the result will be no less decisive than

that of the naturalist alluded to. Our judgment will be, that the

nebulosity about the star is not of a starry nature.

But that we may not be too precipitate in these new decisions, let us

enter more at large into the various grounds which induced us formerly

to surmise, that every visible object, in the extended and distant

heavens, was of the starry kind, and collate them with those which now

offer themselves for the contrary opinion. It has been observed, on a

former occasion, that all the smaller parts of other great systems,

such as the planets, their rings and satellites, the comets, and such

other bodies of the like nature as may belong to them, can never be

perceived by us, on account of the faintness of light reflected from

small opaque objects: in the present remarks, therefore, all these are

to be entirely set aside.

A well connected series of objects, such as mentioned above, has led

us to infer that all nebulæ consist of stars. This being admitted, we

were authorized to extend our analogical way of reasoning a little

further. Many of the nebulæ had no other appearance than that whitish

cloudiness, on the blue ground on which they seemed to be projected;

and why the same cause should not be assigned to explain the most

extensive nebulosities, as well as those that amounted only to a

few minutes of a degree in size, did not appear. It could not be

inconsistent to call up a telescopic milky way, at an immense distance,

to account for such a phenomenon; and if any part of the nebulosity

seemed detached from the rest, or contained a visible star or two,

the probability of seeing a few near stars, apparently scattered over

the far distant regions of myriads of sidereal collections, rendered

nebulous by their distance, would also clear up these singularities.

In order to be more easily understood in his remarks on the comparative

disposition of the heavenly bodies, Dr. H. mentions some of the

particulars which introduced the ideas of connection and disjunction:

for these, being properly founded on an examination of objects that

may be reviewed at any time, will be of considerable importance to the

validity of what we may advance with regard to the lately discovered

nebulous stars. On June 27, 1786, he saw a beautiful cluster of very

small stars of various sizes, about 15' in diameter, and very rich

of stars. On viewing this object, it is impossible to withhold our

assent to the idea which occurs, that these stars are connected so far

with one another as to be gathered together, within a certain space,

of little extent when compared to the vast expanse of the heavens.

As this phenomenon has been repeatedly seen in a thousand cases, Dr.

H. thinks he may justly lay great stress on the idea of such stars

being connected. On September 9, 1779, he discovered a very small star

near _ε_ Bootis. The question here occurring, whether it had any

connection with _ε_ or not, was determined in the negative; for,

considering the number of stars scattered in a variety of places, it is

very far from being uncommon, that a star at a great distance should

happen to be nearly in a line drawn from the sun through _ε_, and

thus constitute the observed double star. September 7, 1782, when Dr.

H. first saw the planetary nebula near υ Aquarii, he pronounced it to

be a system whose parts were connected together. Without entering

into any kind of calculation, it is evident that a certain degree of

light within a very small space, joined to the particular shape this

object presents to us, which is nearly round, and even in its deviation

consistent with regularity, being a little elliptical, ought naturally

to give us the idea of a conjunction in the things that produce it.

And a considerable addition to this argument may be derived from a

repetition of the same phenomenon, in nine or ten more of a similar

construction.

When Dr. H. examined the cluster of stars, following the head of the

Great Dog, he found on March 19, 1786, that there was within this

cluster a round, resolvable nebula, of about 2' in diameter, and nearly

an equal degree of light throughout. Here, considering that the cluster

was free from nebulosity in other parts, and that many such clusters,

as well as such nebulæ, exist in divers parts of the heavens, it seemed

very probable that the nebula was unconnected with the cluster; and

that a similar reason would as easily account for this appearance as

it had resolved the phenomenon of the double star near e Bootis; that

is, a casual situation of our sun and the two other objects nearly in

a line. And though it may be rather more remarkable, that this should

happen with two compound systems, which are not by far so numerous

as single stars, we have, to make up for this singularity, a much

larger space in which it may take place, the cluster being of a very

considerable extent.

On February 15, 1786, Dr. H. discovered that one of his planetary

nebulæ had a spot in the centre, which was more luminous than the rest,

and with long attention, a very bright, round, well-defined centre

became visible. He remained not a single moment in doubt, but that

the bright centre was connected with the rest of the apparent disc.

October 6, 1785, he found a very bright, round nebula, of about 1-1/2'

in diameter. It has a large, bright nucleus in the middle, which is

undoubtedly connected with the luminous parts about it. And though

we must confess, that if this phenomenon, and many more of the same

nature, recorded in the catalogues of nebulæ, consist of clustering

stars, we find ourselves involved in some difficulty to account for the

extraordinary condensation of them about the centre; yet the idea of

a connection between the outward parts and these very condensed ones

within, is by no means lessened on that account.

There is a telescopic milky way, which Dr. H. has traced out in the

heavens in many sweeps made from the year 1783 to 1789. It takes up

a space of more than 60 square degrees of the heavens, and there are

thousands of stars scattered over it: among others, four that form a

trapezium, and are situated in the well known nebula of Orion, which

is included in the above extent. All these stars, as well as the four

mentioned, he takes to be entirely unconnected with the nebulosity

which involves them in appearance. Among them is also _δ_ Orionis,

a cloudy star, improperly so called by former astronomers; but it does

not seem to be connected with the milkiness any more than the rest.

Dr. H. now comes to some other phenomena, that, from their singularity,

merit undoubtedly a very full discussion. Among the reasons which

induced us to embrace the opinion that all very faint milky nebulosity

ought to be ascribed to an assemblage of stars is, that we could

not easily assign any other cause of sufficient importance for such

luminous appearances, to reach us at the immense distance we must

suppose ourselves to be from them. But if an argument of considerable

force should now be brought forward, to show the existence of luminous

matter, in a state of modification very different from the construction

of a sun or star, all objections, drawn from our incapacity of

accounting for new phenomena on old principles, he thinks, will lose

their validity.

Hitherto Dr. H. has been showing, by various instances in objects whose

places are given, in what manner we may form ideas of connection, and

its contrary, by an attentive inspection of them only; he now relates

a series of observations, with remarks on them as they are delivered,

from which he afterwards draws a few simple conclusions, that seem to

be of considerable importance.

October 16, 1784. A star of about the ninth magnitude, surrounded by a

milky nebulosity, or chevelure, of about 3' in diameter. The nebulosity

is very faint, and a little extended or elliptical, the extent being

not far from the meridian, or a little from north preceding to south

following. The chevelure involves a small star, which is about 1-1/2'

north of the cloudy star; other stars of equal magnitude are perfectly

free from this appearance. (R.A. 5h 57m 4s. P.D. 96° 22'). His present

judgment concerning this remarkable object is, that the nebulosity

belongs to the star which is situated in its centre. The small one, on

the contrary, which is mentioned as involved, being one of many that

are profusely scattered over this rich neighbourhood, he supposes to

be quite unconnected with this phenomenon. A circle of 3' in diameter

is sufficiently large to admit another small star, without any bias to

the judgment he formed concerning the one in question. It might appear

singular, that such an object should not have immediately suggested

all the remarks contained in this paper; but about things that appear

new we ought not to form opinions too hastily, and his observations

on the construction of the heavens were then but entered on. In this

case, therefore, it was the safest way to lay down a rule not to reason

on the phenomena that might offer themselves, till he should be in

possession of a sufficient stock of materials to guide his researches.

October 16, 1784. A small star of about the 11th or 12th magnitude,

very faintly affected with milky nebulosity; other stars of the same

magnitude were perfectly free from this appearance. Another observation

mentions five or six small stars within the space of 3 or 4', all very

faintly affected in the same manner, and the nebulosity suspected to

be a little stronger about each star. But a third observation rather

opposes this increase of the faintly luminous appearance. (R. A. 6h

Om 33s. P. D. 96° 13'). Here the connection between the stars and the

nebulosity is not so evident as to amount to conviction; for which

reason we shall pass on to the next.

* * * * *

November 25, 1788. A star of about the 9th magnitude, surrounded with

very faint milky nebulosity; other stars of the same size are perfectly

free from that appearance. Less than 1' in diameter. The star is either

not round or double (a).

March 23, 1789. A bright, considerably well-defined nucleus, with a

very faint, small, round chevelure (b). The connection admits of no

doubt; but the object is not perhaps of the same nature with those

called cloudy stars.

April 14, 1789. A considerable, bright, round nebula; having a large

place in the middle of nearly an equal brightness; but less bright

towards the margin (c). This seems rather to approach the planetary

sort.

March 5, 1790. A pretty considerable star of the 9th or 10th

magnitude, visibly affected with a very faint nebulosity of little

extent, all around. A power of 300 showed the nebulosity of greater

extent (d). The connection is not to be doubted.

March 19, 1790. A very bright nucleus, with a small, very faint

chevelure, exactly round. In a low situation, where the chevelure

could hardly be seen, this object would put on the appearance of an

ill-defined, planetary nebula, of 6, 8 or 10" diameter (e).

November 13, 1790. A most singular phenomenon! A star of about the 8th

magnitude, with a faint luminous atmosphere, of a circular form, and

of about 3' in diameter. The star is perfectly in the centre, and the

atmosphere is so diluted, faint, and equal throughout, that there can

be no surmise of its consisting of stars; nor can there be a doubt of

the evident connection between the atmosphere and the star. Another

star not much less in brightness, and in the same field with the above,

was perfectly free from any such appearance. This last object is so

decisive in every particular, Dr. H. says, that we need not hesitate

to admit it as a pattern, from which we are authorised to draw the

following important consequences:

Supposing the connection between the star and its surrounding

nebulosity to be allowed, we argue, that one of the two following cases

must necessarily be admitted: In the first place, if the nebulosity

consist of stars that are very remote, which appear nebulous on account

of the small angles their mutual distances subtend at the eye, by which

they will not only, as it were, run into each other, but also appear

extremely faint and diluted; then, what must be the enormous size of

the central point, which outshines all the rest in so superlative a

degree as to admit of no comparison! In the next place, if the star be

larger than common, how very small and compressed must be those other

luminous points that are the occasion of the nebulosity which surrounds

the central one! As, by the former supposition, the luminous central

point must far exceed the standard of what we call a star, so, in the

latter, the shining matter about the centre will be much too small to

come under the same denomination; we therefore either have a central

body which is not a star, or have a star which is involved in a shining

fluid, of a nature totally unknown to us. Dr. H. can adopt no other

sentiment than the latter, since the probability is certainly not for

the existence of so enormous a body as would be required to shine like

a star of the eighth magnitude, at a distance sufficiently great to

cause a vast system of stars to put on the appearance of a very diluted

milky nebulosity.

But what a field of novelty is here opened to our conceptions! A

shining fluid, of a brightness sufficient to reach us from the remote

regions of a star of the 8th, 9th, 10th, or 12th magnitude, and of an

extent so considerable as to take up 3, 4, 5, or 6 minutes in diameter!

Can we compare it to the coruscation of the electric fluid in the

aurora borealis? Or to the more magnificent cone of the zodiacal light

as we see it in the spring or autumn? The latter, notwithstanding Dr.

H. has observed it to reach at least 90° from the sun, is yet of so

little extent and brightness, as probably not to be perceived even by

the inhabitants of Saturn or the Georgian planet, and must be utterly

invisible at the remoteness of the nearest fixed star.

More extensive views may be derived from this proof of the existence

of a shining matter. Perhaps it has been too hastily surmised that

all milky nebulosity, of which there is so much in the heavens, is

owing to starlight only. These nebulous stars may serve as a clue to

unravel other mysterious phenomena. If the shining fluid that surrounds

them is not so essentially connected with these nebulous stars, but

that it can also exist without them, which seems to be sufficiently

probable, and will be examined hereafter, we may with great facility

explain that very extensive, telescopic nebulosity, which, as before

mentioned, is expanded over more than 60° of the heavens, about the

constellation of Orion; a luminous matter accounting much better for it

than clustering stars at a distance. In this case we may also pretty

nearly guess at its situation, which must commence somewhere about the

range of the stars of the 7th magnitude, or a little farther from us,

and extend unequally in some places perhaps to the regions of those

of the 9th, 10th, 11th, and 12th. The foundation for this surmise is,

that not unlikely some of the stars that happen to be situated in a

more condensed part of it, or that perhaps by their own attraction

draw together some quantity of this fluid greater than what they are

entitled to by their situation in it, will, of course, assume the

appearance of cloudy stars; and many of those named are either in this

stratum of luminous matter, or very near it.

It has been said above, that in nebulous stars the existence of the

shining fluid does not seem to be so essentially connected with the

central points that it might not also exist without them. For this

opinion we may assign several reasons. One of them is the greater

resemblance of the chevelure of these stars and the diffused extensive

nebulosity mentioned before, which renders it highly probable that

they are of the same nature. Now, if this be admitted, the separate

existence of the luminous matter, or its independence of a central

star, is fully proved. We may also judge, very confidently, that the

light of this shining fluid is no kind of reflection from the star in

the centre; for, as we have already observed, reflected light could

never reach us at the great distance we are from such objects. Besides,

how impenetrable would be an atmosphere of a sufficient density to

reflect so great a quantity of light! And yet we observe, that the

outward parts of the chevelure are nearly as bright as those that are

close to the star; so that this supposed atmosphere ought to give no

obstruction to the passage of the central rays. If therefore this

matter is self-luminous, it seems more fit to produce a star by its

condensation than to depend on the star for its existence.

Many other diffused nebulosities, besides that about the constellation

of Orion, have been observed or suspected; but some of them are

probably very distant, and run far out into space. For instance, about

5m in time preceding _x_ Cygni, Dr. H. suspects as much of it

as covers near 4 square degrees; and much about the same quantity

44m preceding the 125 Tauri. A space of almost 8 square degrees, 6m

preceding _α_ Trianguli, seems to be tinged with milky nebulosity.

Three minutes preceding the 46 Eridani, strong, milky nebulosity is

expanded over more than 2 square degrees. Fifty-four minutes preceding

the 13th _Canum venaticorum_, and again 48m preceding the same

star, the field of view affected with whitish nebulosity throughout

the whole breadth of the sweep, which was 2° 39'. Four minutes

following the 57 Cygni a considerable space is filled with faint,

milky nebulosity, which is pretty bright in some places, and contains

the 37th nebula of the 5th class, in the brightest part of it. In the

neighbourhood of the 44th Piscium, very faint nebulosity appears to

be diffused over more than 9 square degrees of the heavens. Now all

these phenomena, as we have already seen, will admit of a much easier

explanation by a luminous fluid than by stars at an immense distance.

The nature of planetary nebulæ, which has hitherto been involved in

much darkness, may now be explained with some degree of satisfaction,

since the uniform and very considerable brightness of their apparent

disc accords remarkably well with a much condensed, luminous fluid;

whereas, to suppose them to consist of clustering stars, will not so

completely account for the milkiness or soft tint of their light, to

produce which it would be required that the condensation of the stars

should be carried to an almost inconceivable degree of accumulation.

The surmise of the regeneration of stars, by means of planetary nebulæ,

expressed in a former paper, will become more probable, as all the

luminous matter contained in one of them, when gathered together into a

body of the size of a star, would have nearly such a quantity of light

as we find the planetary nebulæ to give. To prove this experimentally,

we may view them with a telescope that does not magnify sufficiently

to show their extent, by which means we shall gather all their light

together into a point, when they will be found to assume the appearance

of small stars; that is, of stars at the distance of those which we

call of the 8th, 9th, or 10th magnitude. Indeed this idea is greatly

supported by the discovery of a well-defined, lucid point, resembling

a star, in the centre of one of them; for the argument which has been

used, in the case of nebulous stars, to show the probability of the

existence of luminous matter, which rested on the disparity between a

bright point and its surrounding shining fluid, may here be alleged

with equal justice. If the point be a generating star, the further

accumulation of the already much condensed, luminous matter may

complete it in time.

How far the light that is perpetually emitted from millions of suns may

be concerned in this shining fluid, it might be presumptuous to attempt

to determine; but, notwithstanding the inconceivable subtilty of the

particles of light, when the number of the emitting bodies is almost

infinitely great, and the time of the continual emission indefinitely

long, the quantity of emitted particles may well become adequate to the

constitution of a shining fluid, or luminous matter, provided a cause

can be found that may retain them from flying off, or reunite them. But

such a cause cannot be difficult to guess at, when we know that light

is so easily reflected, refracted, inflected and deflected; and that,

in the immense range of its course, it must pass through innumerable

systems, where it cannot but frequently meet with many obstacles to

its rectilinear progression not to mention the great counteraction

of the united attractive force of whole sidereal systems, which must

be continually exerting their power on the particles while they are

endeavouring to fly off. However, we shall lay no stress on a surmise

of this kind, as the means of verifying it are wanting; nor is it of

any immediate consequence to us to know the origin of the luminous

matter. Let it suffice, that its existence is rendered evident, by

means of nebulous stars.

FOOTNOTES:

[Footnote 17: This excerpt and the one following are from the report

by Herschel in the _Transactions of the Royal Society of London_;

the third is an abstract from the same report, the conclusion, however,

being by Herschel.]

XVI

KARL WILHELM SCHEELE

1742-1786

_Karl Wilhelm Scheele, who discovered independently of the English

chemists the double constitution of air, was born in Stralsund,

Pomerania, December 19, 1742. At an early age he manifested interest

in pharmacy, and during his career as an apothecary engaged in various

experiments in chemistry. He published his “Treatise on Air and Fire”

in 1777. He died at Köping, May 21, 1786._

THE CONSTITUENTS OF AIR[18]

1. It is the object and chief business of chemistry to separate

skilfully substances into their constituents, to discover their

properties, and to compound them in different ways. How difficult it

is, however, to carry out such operations with the greatest accuracy,

can only be unknown to one who either has never undertaken this

occupation, or at least has not done so with sufficient attention.

2. Hitherto chemical investigators are not agreed as to how many

elements or fundamental materials compose all substances. In fact this

is one of the most difficult problems; some indeed hold that there

remains no further hope of searching out the elements of substances.

Poor comfort for those who feel their greatest pleasure in the

investigation of natural things! Far is he mistaken, who endeavours

to confine chemistry, this noble science, within such narrow bounds!

Others believe that earth and phlogiston are the things from which all

material nature has derived its origin. The majority seem completely

attached to the peripatetic elements.