Nitrat of potash, usually called nitre or saltpetre, contains very

little water of cristallization, perhaps even none at all; yet this salt

liquifies in a degree of heat very little superior to that of boiling

water. This liquifaction cannot therefore be produced by means of the

water of cristallization, but in consequence of the salt being very

fusible in its nature, and from its passing from the solid to the liquid

state of aggregation, when but a little raised above the temperature of

boiling water. All salts are in this manner susceptible of being

liquified by caloric, but in higher or lower degrees of temperature.

Some of these, as the acetites of potash and soda, liquify with a very

moderate heat, whilst others, as sulphat of potash, lime, &c. require

the strongest fires we are capable of producing. This liquifaction of

salts by caloric produces exactly the same phenomena with the melting of

ice; it is accomplished in each salt by a determinate degree of heat,

which remains invariably the same during the whole time of the

liquifaction. Caloric is employed, and becomes fixed during the melting

of the salt, and is, on the contrary, disengaged when the salt

coagulates. These are general phenomena which universally occur during

the passage of every species of substance from the solid to the fluid

state of aggregation, and from fluid to solid.

These phenomena arising from solution by caloric are always less or more

conjoined with those which take place during solutions in water. We

cannot pour water upon a salt, on purpose to dissolve it, without

employing a compound solvent, both water and caloric; hence we may

distinguish several different cases of solution, according to the nature

and mode of existence of each salt. If, for instance, a salt be

difficultly soluble in water, and readily so by caloric, it evidently

follows, that this salt will be difficultly soluble in cold water, and

considerably in hot water; such is nitrat of potash, and more especially

oxygenated muriat of potash. If another salt be little soluble both in

water and caloric, the difference of its solubility in cold and warm

water will be very inconsiderable; sulphat of lime is of this kind. From

these considerations, it follows, that there is a necessary relation

between the following circumstances; the solubility of a salt in cold

water, its solubility in boiling water, and the degree of temperature at

which the same salt liquifies by caloric, unassisted by water; and that

the difference of solubility in hot and cold water is so much greater in

proportion to its ready solution in caloric, or in proportion to its

susceptibility of liquifying in a low degree of temperature.

The above is a general view of solution; but, for want of particular

facts, and sufficiently exact experiments, it is still nothing more than

an approximation towards a particular theory. The means of compleating

this part of chemical science is extremely simple; we have only to

ascertain how much of each salt is dissolved by a certain quantity of

water at different degrees of temperature; and as, by the experiments

published by Mr de la Place and me, the quantity of caloric contained in

a pound of water at each degree of the thermometer is accurately known,

it will be very easy to determine, by simple experiments, the proportion

of water and caloric required for solution by each salt, what quantity

of caloric is absorbed by each at the moment of liquifaction, and how

much is disengaged at the moment of cristallization. Hence the reason

why salts are more rapidly soluble in hot than in cold water is

perfectly evident. In all solutions of salts caloric is employed; when

that is furnished intermediately from the surrounding bodies, it can

only arrive slowly to the salt; whereas this is greatly accelerated when

the requisite caloric exists ready combined with the water of solution.

In general, the specific gravity of water is augmented by holding salts

in solution; but there are some exceptions to the rule. Some time hence,

the quantities of radical, of oxygen, and of base, which constitute each

neutral salt, the quantity of water and caloric necessary for solution,

the increased specific gravity communicated to water, and the figure of

the elementary particles of the cristals, will all be accurately known.

From these all the circumstances and phenomena of cristallization will

be explained, and by these means this part of chemistry will be

compleated. Mr Seguin has formed the plan of a thorough investigation of

this kind, which he is extremely capable of executing.

The solution of salts in water requires no particular apparatus; small

glass phials of different sizes, Pl. II. Fig. 16. and 17. pans of

earthern ware, A, Fig. 1. and 2. long-necked matrasses, Fig. 14. and

pans or basons of copper or of silver, Fig. 13. and 15. answer very well

for these operations.

SECT. II.

_Of Lixiviation._

This is an operation used in chemistry and manufactures for separating

substances which are soluble in water from such as are insoluble. The

large vat or tub, Pl. II. Fig. 12. having a hole D near its bottom,

containing a wooden spiget and fosset or metallic stop-cock DE, is

generally used for this purpose. A thin stratum of straw is placed at

the bottom of the tub; over this, the substance to be lixiviated is laid

and covered by a cloth, then hot or cold water, according to the degree

of solubility of the saline matter, is poured on. When the water is

supposed to have dissolved all the saline parts, it is let off by the

stop-cock; and, as some of the water charged with salt necessarily

adheres to the straw and insoluble matters, several fresh quantities of

water are poured on. The straw serves to secure a proper passage for the

water, and may be compared to the straws or glass rods used in

filtrating, to keep the paper from touching the sides of the funnel. The

cloth which is laid over the matters under lixiviation prevents the

water from making a hollow in these substances where it is poured on,

through which it might escape without acting upon the whole mass.

This operation is less or more imitated in chemical experiments; but as

in these, especially with analytical views, greater exactness is

required, particular precautions must be employed, so as not to leave

any saline or soluble part in the residuum. More water must be employed

than in ordinary lixiviations, and the substances ought to be previously

stirred up in the water before the clear liquor is drawn off, otherwise

the whole mass might not be equally lixiviated, and some parts might

even escape altogether from the action of the water. We must likewise

employ fresh portions of water in considerable quantity, until it comes

off entirely free from salt, which we may ascertain by means of the

hydrometer formerly described.

In experiments with small quantities, this operation is conveniently

performed in jugs or matrasses of glass, and by filtrating the liquor

through paper in a glass funnel. When the substance is in larger

quantity, it may be lixiviated in a kettle of boiling water, and

filtrated through paper supported by cloth in the wooden frame, Pl. II.

Fig. 3. and 4.; and in operations in the large way, the tub already

mentioned must be used.

SECT. III.

_Of Evaporation._

This operation is used for separating two substances from each other, of

which one at least must be fluid, and whose degrees of volatility are

considerably different. By this means we obtain a salt, which has been

dissolved in water, in its concrete form; the water, by heating, becomes

combined with caloric, which renders it volatile, while the particles of

the salt being brought nearer to each other, and within the sphere of

their mutual attraction, unite into the solid state.

As it was long thought that the air had great influence upon the

quantity of fluid evaporated, it will be proper to point out the errors

which this opinion has produced. There certainly is a constant slow

evaporation from fluids exposed to the free air; and, though this

species of evaporation may be considered in some degree as a solution in

air, yet caloric has considerable influence in producing it, as is

evident from the refrigeration which always accompanies this process;

hence we may consider this gradual evaporation as a compound solution

made partly in air, and partly in caloric. But the evaporation which

takes place from a fluid kept continually boiling, is quite different in

its nature, and in it the evaporation produced by the action of the air

is exceedingly inconsiderable in comparison with that which is

occasioned by caloric. This latter species may be termed _vaporization_

rather than _evaporation_. This process is not accelerated in proportion

to the extent of evaporating surface, but in proportion to the

quantities of caloric which combine with the fluid. Too free a current

of cold air is often hurtful to this process, as it tends to carry off

caloric from the water, and consequently retards its conversion into

vapour. Hence there is no inconvenience produced by covering, in a

certain degree, the vessels in which liquids are evaporated by continual

boiling, provided the covering body be of such a nature as does not

strongly draw off the caloric, or, to use an expression of Dr

Franklin's, provided it be a bad conductor of heat. In this case, the

vapours escape through such opening as is left, and at least as much is

evaporated, frequently more than when free access is allowed to the

external air.

As during evaporation the fluid carried off by caloric is entirely lost,

being sacrificed for the sake of the fixed substances with which it was

combined, this process is only employed where the fluid is of small

value, as water, for instance. But, when the fluid is of more

consequence, we have recourse to distillation, in which process we

preserve both the fixed substance and the volatile fluid. The vessels

employed for evaporation are basons or pans of copper, silver, or lead,

Pl. II. Fig. 13. and 15. or capsules of glass, porcellain, or stone

ware, Pl. II. A, Fig. 1. and 2. Pl. III. Fig. 3 and 4. The best utensils

for this purpose are made of the bottoms of glass retorts and matrasses,

as their equal thinness renders them more fit than any other kind of

glass vessel for bearing a brisk fire and sudden alterations of heat and

cold without breaking.

As the method of cutting these glass vessels is no where described in

books, I shall here give a description of it, that they may be made by

chemists for themselves out of spoiled retorts, matrasses, and

recipients, at a much cheaper rate than any which can be procured from

glass manufacturers. The instrument, Pl. III. Fig. 5. consisting of an

iron ring AC, fixed to the rod AB, having a wooden handle D, is employed

as follows: Make the ring red hot in the fire, and put it upon the

matrass G, Fig. 6. which is to be cut; when the glass is sufficiently

heated, throw on a little cold water, and it will generally break

exactly at the circular line heated by the ring.

Small flasks or phials of thin glass are exceeding good vessels for

evaporating small quantities of fluid; they are very cheap, and stand

the fire remarkably. One or more of these may be placed upon a second

grate above the furnace, Pl. III. Fig. 2. where they will only

experience a gentle heat. By this means a great number of experiments

may be carried on at one time. A glass retort, placed in a sand bath,

and covered with a dome of baked earth, Pl. III. Fig. 1. answers pretty

well for evaporations; but in this way it is always considerably slower,

and is even liable to accidents; as the sand heats unequally, and the

glass cannot dilate in the same unequal manner, the retort is very

liable to break. Sometimes the sand serves exactly the office of the

iron ring formerly mentioned; for, if a single drop of vapour, condensed

into liquid, happens to fall upon the heated part of the vessel, it

breaks circularly at that place. When a very intense fire is necessary,

earthen crucibles may be used; but we generally use the word

_evaporation_ to express what is produced by the temperature of boiling

water, or not much higher.

SECT. IV.

_Of Cristallization._

In this process the integrant parts of a solid body, separated from each

other by the intervention of a fluid, are made to exert the mutual

attraction of aggregation, so as to coalesce and reproduce a solid mass.

When the particles of a body are only separated by caloric, and the

substance is thereby retained in the liquid state, all that is necessary

for making it cristallize, is to remove a part of the caloric which is

lodged between its particles, or, in other words, to cool it. If this

refrigeration be slow, and the body be at the same time left at rest,

its particles assume a regular arrangement, and cristallization,

properly so called, takes place; but, if the refrigeration is made

rapidly, or if the liquor be agitated at the moment of its passage to

the concrete state, the cristallization is irregular and confused.

The same phenomena occur with watery solutions, or rather in those made

partly in water, and partly by caloric. So long as there remains a

sufficiency of water and caloric to keep the particles of the body

asunder beyond the sphere of their mutual attraction, the salt remains

in the fluid state; but, whenever either caloric or water is not present

in sufficient quantity, and the attraction of the particles for each

other becomes superior to the power which keeps them asunder, the salt

recovers its concrete form, and the cristals produced are the more

regular in proportion as the evaporation has been slower and more

tranquilly performed.

All the phenomena we formerly mentioned as taking place during the

solution of salts, occur in a contrary sense during their

cristallization. Caloric is disengaged at the instant of their assuming

the solid state, which furnishes an additional proof of salt being held

in solution by the compound action of water and caloric. Hence, to cause

salts to cristallize which readily liquify by means of caloric, it is

not sufficient to carry off the water which held them in solution, but

the caloric united to them must likewise be removed. Nitrat of potash,

oxygenated muriat of potash, alum, sulphat of soda, &c. are examples of

this circumstance, as, to make these salts cristallize, refrigeration

must be added to evaporation. Such salts, on the contrary, as require

little caloric for being kept in solution, and which, from that

circumstance, are nearly equally soluble in cold and warm water, are

cristallizable by simply carrying off the water which holds them in

solution, and even recover their solid state in boiling water; such are

sulphat of lime, muriat of potash and of soda, and several others.

The art of refining saltpetre depends upon these properties of salts,

and upon their different degrees of solubility in hot and cold water.

This salt, as produced in the manufactories by the first operation, is

composed of many different salts; some are deliquescent, and not

susceptible of being cristallized, such as the nitrat and muriat of

lime; others are almost equally soluble in hot and cold water, as the

muriats of potash and of soda; and, lastly, the saltpetre, or nitrat of

potash, is greatly more soluble in hot than it is in cold water. The

operation is begun, by pouring upon this mixture of salts as much water

as will hold even the least soluble, the muriats of soda and of potash,

in solution; so long as it is hot, this quantity readily dissolves all

the saltpetre, but, upon cooling, the greater part of this salt

cristallizes, leaving about a sixth part remaining dissolved, and mixed

with the nitrat of lime and the two muriats. The nitre obtained by this

process is still somewhat impregnated with other salts, because it has

been cristallized from water in which these abound: It is completely

purified from these by a second solution in a small quantity of boiling

water, and second cristallization. The water remaining after these

cristallizations of nitre is still loaded with a mixture of saltpetre,

and other salts; by farther evaporation, crude saltpetre, or

rough-petre, as the workmen call it, is procured from it, and this is

purified by two fresh solutions and cristallizations.

The deliquescent earthy salts which do not contain the nitric acid are

rejected in this manufacture; but those which consist of that acid

neutralized by an earthy base are dissolved in water, the earth is

precipitated by means of potash, and allowed to subside; the clear

liquor is then decanted, evaporated, and allowed to cristallize. The

above management for refining saltpetre may serve as a general rule for

separating salts from each other which happen to be mixed together. The

nature of each must be considered, the proportion in which each

dissolves in given quantities of water, and the different solubility of

each in hot and cold water. If to these we add the property which some

salts possess, of being soluble in alkohol, or in a mixture of alkohol

and water, we have many resources for separating salts from each other

by means of cristallization, though it must be allowed that it is

extremely difficult to render this separation perfectly complete.

The vessels used for cristallization are pans of earthen ware, A, Pl.

II. Fig. 1. and 2. and large flat dishes, Pl. III. Fig. 7. When a saline

solution is to be exposed to a slow evaporation in the heat of the

atmosphere, with free access of air, vessels of some depth, Pl. III.

Fig. 3. must be employed, that there may be a considerable body of

liquid; by this means the cristals produced are of considerable size,

and remarkably regular in their figure.

Every species of salt cristallizes in a peculiar form, and even each

salt varies in the form of its cristals according to circumstances,

which take place during cristallization. We must not from thence

conclude that the saline particles of each species are indeterminate in

their figures: The primative particles of all bodies, especially of

salts, are perfectly constant in their specific forms; but the cristals

which form in our experiments are composed of congeries of minute

particles, which, though perfectly equal in size and shape, may assume

very dissimilar arrangements, and consequently produce a vast variety of

regular forms, which have not the smallest apparent resemblance to each

other, nor to the original cristal. This subject has been very ably

treated by the Abbe Hauey, in several memoirs presented to the Academy,

and in his work upon the structure of cristals: It is only necessary to

extend generally to the class of salts the principles he has

particularly applied to some cristalized stones.

SECT. V.

_Of Simple Distillation._

As distillation has two distinct objects to accomplish, it is divisible

into simple and compound; and, in this section, I mean to confine myself

entirely to the former. When two bodies, of which one is more volatile

than the other, or has more affinity to caloric, are submitted to

distillation, our intention is to separate them from each other: The

more volatile substance assumes the form of gas, and is afterwards

condensed by refrigeration in proper vessels. In this case distillation,

like evaporation, becomes a species of mechanical operation, which

separates two substances from each other without decomposing or altering

the nature of either. In evaporation, our only object is to preserve the

fixed body, without paying any regard to the volatile matter; whereas,

in distillation, our principal attention is generally paid to the

volatile substance, unless when we intend to preserve both the one and

the other. Hence, simple distillation is nothing more than evaporation

produced in close vessels.

The most simple distilling vessel is a species of bottle or matrass, A,

Pl. III. Fig. 8. which has been bent from its original form BC to BD,

and which is then called a retort; when used, it is placed either in a

reverberatory furnace, Pl. XIII. Fig. 2. or in a sand bath under a dome

of baked earth, Pl. III. Fig. 1. To receive and condense the products,

we adapt a recipient, E, Pl. III. Fig. 9. which is luted to the retort.

Sometimes, more especially in pharmaceutical operations, the glass or

stone ware cucurbit, A, with its capital B, Pl. III. Fig. 12, or the

glass alembic and capital, Fig. 13. of one piece, is employed. This

latter is managed by means of a tubulated opening T, fitted with a

ground stopper of cristal; the capital, both of the cucurbit and

alembic, has a furrow or trench, r r, intended for conveying the

condensed liquor into the beak RS, by which it runs out. As, in almost

all distillations, expansive vapours are produced, which might burst the

vessels employed, we are under the necessity of having a small hole, T,

Fig. 9. in the balloon or recipient, through which these may find vent;

hence, in this way of distilling, all the products which are permanently

aeriform are entirely lost, and even such as difficultly lose that state

have not sufficient space to condense in the balloon: This apparatus is

not, therefore, proper for experiments of investigation, and can only be

admitted in the ordinary operations of the laboratory or in pharmacy. In

the article appropriated for compound distillation, I shall explain the

various methods which have been contrived for preserving the whole

products from bodies in this process.

As glass or earthen vessels are very brittle, and do not readily bear

sudden alterations of heat and cold, every well regulated laboratory

ought to have one or more alembics of metal for distilling water,

spiritous liquors, essential oils, &c. This apparatus consists of a

cucurbit and capital of tinned copper or brass, Pl. III. Fig. 15. and

16. which, when judged proper, may be placed in the water bath, D, Fig.

17. In distillations, especially of spiritous liquors, the capital must

be furnished with a refrigetory, SS, Fig. 16. kept continually filled

with cold water; when the water becomes heated, it is let off by the

stop-cock, R, and renewed with a fresh supply of cold water. As the

fluid distilled is converted into gas by means of caloric furnished by

the fire of the furnace, it is evident that it could not condense, and,

consequently, that no distillation, properly speaking, could take place,

unless it is made to deposit in the capital all the caloric it received

in the cucurbit; with this view, the sides of the capital must always be

preserved at a lower temperature than is necessary for keeping the

distilling substance in the state of gas, and the water in the

refrigetory is intended for this purpose. Water is converted into gas

by the temperature of 80 deg. (212 deg.), alkohol by 67 deg. (182.75

deg.), ether by 32 deg. (104 deg.); hence these substances cannot be

distilled, or, rather, they will fly off in the state of gas, unless

the temperature of the refrigetory be kept under these respective

degrees.

In the distillation of spiritous, and other expansive liquors, the above

described refrigetory is not sufficient for condensing all the vapours

which arise; in this case, therefore, instead of receiving the distilled

liquor immediately from the beak, TU, of the capital into a recipient, a

worm is interposed between them. This instrument is represented Pl. III.

Fig. 18. contained in a worm tub of tinned copper, it consists of a

metallic tube bent into a considerable number of spiral revolutions. The

vessel which contains the worm is kept full of cold water, which is

renewed as it grows warm. This contrivance is employed in all

distilleries of spirits, without the intervention of a capital and

refrigetory, properly so called. The one represented in the plate is

furnished with two worms, one of them being particularly appropriated to

distillations of odoriferous substances.

In some simple distillations it is necessary to interpose an adopter

between the retort and receiver, as shown Pl. III. Fig, 11. This may

serve two different purposes, either to separate two products of

different degrees of volatility, or to remove the receiver to a greater

distance from the furnace, that it may be less heated. But these, and

several other more complicated instruments of ancient contrivance, are

far from producing the accuracy requisite in modern chemistry, as will

be readily perceived when I come to treat of compound distillation.

SECT. VI.

_Of Sublimation._

This term is applied to the distillation of substances which condense in

a concrete or solid form, such as the sublimation of sulphur, and of

muriat of ammoniac, or sal ammoniac. These operations may be

conveniently performed in the ordinary distilling vessels already

described, though, in the sublimation of sulphur, a species of vessels,

named Alludels, have been usually employed. These are vessels of stone

or porcelain ware, which adjust to each other over a cucurbit containing

the sulphur to be sublimed. One of the best subliming vessels, for

substances which are not very volatile, is a flask, or phial of glass,

sunk about two thirds into a sand bath; but in this way we are apt to

lose a part of the products. When these are wished to be entirely

preserved, we must have recourse to the pneumato-chemical distilling

apparatus, to be described in the following chapter.

CHAP. VI.

_Of Pneumato-chemical Distillations, Metallic Dissolutions, and some

other operations which require very complicated instruments._

SECT. I.

_Of Compound and Pneumato-chemical Distillations._

In the preceding chapter, I have only treated of distillation as a

simple operation, by which two substances, differing in degrees of

volatility, may be separated from each other; but distillation often

actually decomposes the substances submitted to its action, and becomes

one of the most complicated operations in chemistry. In every

distillation, the substance distilled must be brought to the state of

gas, in the cucurbit or retort, by combination with caloric: In simple

distillation, this caloric is given out in the refrigeratory or in the

worm, and the substance again recovers its liquid or solid form, but the

substances submitted to compound distillation are absolutely

decompounded; one part, as for instance the charcoal they contain,

remains fixed in the retort, and all the rest of the elements are

reduced to gasses of different kinds. Some of these are susceptible of

being condensed, and of recovering their solid or liquid forms, whilst

others are permanently aeriform; one part of these are absorbable by

water, some by the alkalies, and others are not susceptible of being

absorbed at all. An ordinary distilling apparatus, such as has been

described in the preceding chapter, is quite insufficient for retaining

or for separating these diversified products, and we are obliged to have

recourse, for this purpose, to methods of a more complicated nature.

The apparatus I am about to describe is calculated for the most

complicated distillations, and may be simplified according to

circumstances. It consists of a tubulated glass retort A, Pl. IV. Fig.

1. having its beak fitted to a tubulated balloon or recipient BC; to the

upper orifice D of the balloon a bent tube DEfg is adjusted, which, at

its other extremity g, is plunged into the liquor contained in the

bottle L, with three necks xxx. Three other similar bottles are

connected with this first one, by means of three similar bent tubes

disposed in the same manner; and the farthest neck of the last bottle is

connected with a jar in a pneumato-chemical apparatus, by means of a

bent tube[60]. A determinate weight of distilled water is usually put

into the first bottle, and the other three have each a solution of

caustic potash in water. The weight of all these bottles, and of the

water and alkaline solution they contain, must be accurately

ascertained. Every thing being thus disposed, the junctures between the

retort and recipient, and of the tube D of the latter, must be luted

with fat lute, covered over with slips of linen, spread with lime and

white of egg; all the other junctures are to be secured by a lute made

of wax and rosin melted together.

When all these dispositions are completed, and when, by means of heat

applied to the retort A, the substance it contains becomes decomposed,

it is evident that the least volatile products must condense or sublime

in the beak or neck of the retort itself, where most of the concrete

substances will fix themselves. The more volatile substances, as the

lighter oils, ammoniac, and several others, will condense in the

recipient GC, whilst the gasses, which are not susceptible of

condensation by cold, will pass on by the tubes, and boil up through the

liquors in the several bottles. Such as are absorbable by water will

remain in the first bottle, and those which caustic alkali can absorb

will remain in the others; whilst such gasses as are not susceptible of

absorption, either by water or alkalies, will escape by the tube RM, at

the end of which they may be received into jars in a pneumato-chemical

apparatus. The charcoal and fixed earth, &c. which form the substance or

residuum, anciently called _caput mortuum_, remain behind in the retort.

In this manner of operating, we have always a very material proof of the

accuracy of the analysis, as the whole weights of the products taken

together, after the process is finished, must be exactly equal to the

weight of the original substance submitted to distillation. Hence, for

instance, if we have operated upon eight ounces of starch or gum arabic,

the weight of the charry residuum in the retort, together with that of

all the products gathered in its neck and the balloon, and of all the

gas received into the jars by the tube RM added to the additional weight

acquired by the bottles, must, when taken together, be exactly eight

ounces. If the product be less or more, it proceeds from error, and the

experiment must be repeated until a satisfactory result be procured,

which ought not to differ more than six or eight grains in the pound

from the weight of the substance submitted to experiment.

In experiments of this kind, I for a long time met with an almost

insurmountable difficulty, which must at last have obliged me to desist

altogether, but for a very simple method of avoiding it, pointed out to

me by Mr Hassenfratz. The smallest diminution in the heat of the

furnace, and many other circumstances inseparable from this kind of

experiments, cause frequent reabsorptions of gas; the water in the

cistern of the pneumato-chemical apparatus rushes into the last bottle

through the tube RM, the same circumstance happens from one bottle into

another, and the fluid is often forced even into the recipient C. This

accident is prevented by using bottles having three necks, as

represented in the plate, into one of which, in each bottle, a capillary

glass-tube St, st, st, st, is adapted, so as to have its lower

extremity t immersed in the liquor. If any absorption takes place,

either in the retort, or in any of the bottles, a sufficient quantity of

external air enters, by means of these tubes, to fill up the void; and

we get rid of the inconvenience at the price of having a small mixture

of common air with the products of the experiment, which is thereby

prevented from failing altogether. Though these tubes admit the external

air, they cannot permit any of the gasseous substances to escape, as

they are always shut below by the water of the bottles.

It is evident that, in the course of experiments with this apparatus,

the liquor of the bottles must rise in these tubes in proportion to the

pressure sustained by the gas or air contained in the bottles; and this

pressure is determined by the height and gravity of the column of fluid

contained in all the subsequent bottles. If we suppose that each bottle

contains three inches of fluid, and that there are three inches of water

in the cistern of the connected apparatus above the orifice of the tube

RM, and allowing the gravity of the fluids to be only equal to that of

water, it follows that the air in the first bottle must sustain a

pressure equal to twelve inches of water; the water must therefore rise

twelve inches in the tube S, connected with the first bottle, nine

inches in that belonging to the second, six inches in the third, and

three in the last; wherefore these tubes must be made somewhat more than

twelve, nine, six, and three inches long respectively, allowance being

made for oscillatory motions, which often take place in the liquids. It

is sometimes necessary to introduce a similar tube between the retort

and recipient; and, as the tube is not immersed in fluid at its lower

extremity, until some has collected in the progress of the distillation,

its upper end must be shut at first with a little lute, so as to be

opened according to necessity, or after there is sufficient liquid in

the recipient to secure its lower extremity.

This apparatus cannot be used in very accurate experiments, when the

substances intended to be operated upon have a very rapid action upon

each other, or when one of them can only be introduced in small

successive portions, as in such as produce violent effervescence when

mixed together. In such cases, we employ a tubulated retort A, Pl. VII.

Fig. 1. into which one of the substances is introduced, preferring

always the solid body, if any such is to be treated, we then lute to the

opening of the retort a bent tube BCDA, terminating at its upper

extremity B in a funnel, and at its other end A in a capillary opening.

The fluid material of the experiment is poured into the retort by means

of this funnel, which must be made of such a length, from B to C, that

the column of liquid introduced may counterbalance the resistance

produced by the liquors contained in all the bottles, Pl. IV. Fig. 1.

Those who have not been accustomed to use the above described distilling

apparatus may perhaps be startled at the great number of openings which

require luting, and the time necessary for making all the previous

preparations in experiments of this kind. It is very true that, if we

take into account all the necessary weighings of materials and products,

both before and after the experiments, these preparatory and succeeding

steps require much more time and attention than the experiment itself.

But, when the experiment succeeds properly, we are well rewarded for all

the time and trouble bestowed, as by one process carried on in this

accurate manner much more just and extensive knowledge is acquired of

the nature of the vegetable or animal substance thus submitted to

investigation, than by many weeks assiduous labour in the ordinary

method of proceeding.

When in want of bottles with three orifices, those with two may be used;

it is even possible to introduce all the three tubes at one opening, so

as to employ ordinary wide-mouthed bottles, provided the opening be

sufficiently large. In this case we must carefully fit the bottles with

corks very accurately cut, and boiled in a mixture of oil, wax, and

turpentine. These corks are pierced with the necessary holes for

receiving the tubes by means of a round file, as in Pl. IV. Fig. 8.

SECT. II.

_Of Metallic Dissolutions._

I have already pointed out the difference between solution of salts in

water and metallic dissolutions. The former requires no particular

vessels, whereas the latter requires very complicated vessels of late

invention, that we may not lose any of the products of the experiment,

and may thereby procure truly conclusive results of the phenomena which

occur. The metals, in general, dissolve in acids with effervescence,

which is only a motion excited in the solvent by the disengagement of a

great number of bubbles of air or aeriform fluid, which proceed from the

surface of the metal, and break at the surface of the liquid.

Mr Cavendish and Dr Priestley were the first inventors of a proper

apparatus for collecting these elastic fluids. That of Dr Priestley is

extremely simple, and consists of a bottle A, Pl. VII. Fig. 2. with its

cork B, through which passes the bent glass tube BC, which is engaged

under a jar filled with water in the pneumato-chemical apparatus, or

simply in a bason full of water. The metal is first introduced into the

bottle, the acid is then poured over it, and the bottle is instantly

closed with its cork and tube, as represented in the plate. But this

apparatus has its inconveniencies. When the acid is much concentrated,

or the metal much divided, the effervescence begins before we have time

to cork the bottle properly, and some gas escapes, by which we are

prevented from ascertaining the quantity disengaged with rigorous

exactness. In the next place, when we are obliged to employ heat, or

when heat is produced by the process, a part of the acid distills, and

mixes with the water of the pneumato-chemical apparatus, by which means

we are deceived in our calculation of the quantity of acid decomposed.

Besides these, the water in the cistern of the apparatus absorbs all the

gas produced which is susceptible of absorption, and renders it

impossible to collect these without loss.

To remedy these inconveniencies, I at first used a bottle with two

necks, Pl. VII. Fig. 3. into one of which the glass funnel BC is luted

so as to prevent any air escaping; a glass rod DE is fitted with emery

to the funnel, so as to serve the purpose of a stopper. When it is used,

the matter to be dissolved is first introduced into the bottle, and the

acid is then permitted to pass in as slowly as we please, by raising the

glass rod gently as often as is necessary until saturation is produced.

Another method has been since employed, which serves the same purpose,

and is preferable to the last described in some instances. This consists

in adapting to one of the mouths of the bottle A, Pl. VII. Fig. 4. a

bent tube DEFG, having a capillary opening at D, and ending in a funnel

at G. This tube is securely luted to the mouth C of the bottle. When any

liquid is poured into the funnel, it falls down to F; and, if a

sufficient quantity be added, it passes by the curvature E, and falls

slowly into the bottle, so long as fresh liquor is supplied at the

funnel. The liquor can never be forced out of the tube, and no gas can

escape through it, because the weight of the liquid serves the purpose

of an accurate cork.

To prevent any distillation of acid, especially in dissolutions

accompanied with heat, this tube is adapted to the retort A, Pl. VII.

Fig. 1. and a small tubulated recipient, M, is applied, in which any

liquor which may distill is condensed. On purpose to separate any gas

that is absorbable by water, we add the double necked bottle L, half

filled with a solution of caustic potash; the alkali absorbs any

carbonic acid gas, and usually only one or two other gasses pass into

the jar of the connected pneumato-chemical apparatus through the tube

NO. In the first chapter of this third part we have directed how these

are to be separated and examined. If one bottle of alkaline solution be

not thought sufficient, two, three, or more, may be added.

SECT. III.

_Apparatus necessary in Experiments upon Vinous and Putrefactive

Fermentations._

For these operations a peculiar apparatus, especially intended for this

kind of experiment, is requisite. The one I am about to describe is

finally adopted, as the best calculated for the purpose, after numerous

corrections and improvements. It consists of a large matrass, A, Pl. X.

fig. 1. holding about twelve pints, with a cap of brass a b, strongly

cemented to its mouth, and into which is screwed a bent tube c d,

furnished with a stop-cock e. To this tube is joined the glass

recipient B, having three openings, one of which communicates with the

bottle C, placed below it. To the posterior opening of this recipient is

fitted a glass tube g h i, cemented at g and i to collets of

brass, and intended to contain a very deliquescent concrete neutral

salt, such as nitrat or muriat of lime, acetite of potash, &c. This tube

communicates with two bottles D and E, filled to x and y with a

solution of caustic potash.

All the parts of this machine are joined together by accurate screws,

and the touching parts have greased leather interposed, to prevent any

passage of air. Each piece is likewise furnished with two stop-cocks, by

which its two extremities may be closed, so that we can weigh each

separately at any period of the operation.

The fermentable matter, such as sugar, with a proper quantity of yeast,

and diluted with water, is put into the matrass. Sometimes, when the

fermentation is too rapid, a considerable quantity of froth is produced,

which not only fills the neck of the matrass, but passes into the

recipient, and from thence runs down into the bottle C. On purpose to

collect this scum and must, and to prevent it from reaching the tube

filled with deliquescent salts, the recipient and connected bottle are

made of considerable capacity.

In the vinous fermentation, only carbonic acid gas is disengaged,

carrying with it a small proportion of water in solution. A great part

of this water is deposited in passing through the tube g h i, which is

filled with a deliquescent salt in gross powder, and the quantity is

ascertained by the augmentation of the weight of the salt. The carbonic

acid gas bubbles up through the alkaline solution in the bottle D, to

which it is conveyed by the tube k l m. Any small portion which may

not be absorbed by this first bottle is secured by the solution in the

second bottle E, so that nothing, in general, passes into the jar F,

except the common air contained in the vessels at the commencement of

the experiment.

The same apparatus answers extremely well for experiments upon the

putrefactive fermentation; but, in this case, a considerable quantity of

hydrogen gas is disengaged through the tube q r s t u, by which it is

conveyed into the jar F; and, as this disengagement is very rapid,

especially in summer, the jar must be frequently changed. These

putrefactive fermentations require constant attendance from the above

circumstance, whereas the vinous fermentation hardly needs any. By means

of this apparatus we can ascertain, with great precision, the weights of

the substances submitted to fermentation, and of the liquid and aeriform

products which are disengaged. What has been already said in Part I.

Chap. XIII. upon the products of the vinous fermentation, may be

consulted.

SECT. IV.

_Apparatus for the Decomposition of Water._

Having already given an account, in the first part of this work, of the

experiments relative to the decomposition of water, I shall avoid any

unnecessary repetitions, and only give a few summary observations upon

the subject in this section. The principal substances which have the

power of decomposing water are iron and charcoal; for which purpose,

they require to be made red hot, otherwise the water is only reduced

into vapours, and condenses afterwards by refrigeration, without

sustaining the smallest alteration. In a red heat, on the contrary, iron

or charcoal carry off the oxygen from its union with hydrogen; in the

first case, black oxyd of iron is produced, and the hydrogen is

disengaged pure in form of gas; in the other case, carbonic acid gas is

formed, which disengages, mixed with the hydrogen gas; and this latter

is commonly carbonated, or holds charcoal in solution.

A musket barrel, without its breach pin, answers exceedingly well for

the decomposition of water, by means of iron, and one should be chosen

of considerable length, and pretty strong. When too short, so as to run

the risk of heating the lute too much, a tube of copper is to be

strongly soldered to one end. The barrel is placed in a long furnace,

CDEF, Pl. VII. Fig. 11. so as to have a few degrees of inclination from

E to F; a glass retort A, is luted to the upper extremity E, which

contains water, and is placed upon the furnace VVXX. The lower extremity

F is luted to a worm SS, which is connected with the tubulated bottle H,

in which any water distilled without decomposition, during the

operation, collects, and the disengaged gas is carried by the tube KK to

jars in a pneumato-chemical apparatus. Instead of the retort a funnel

may be employed, having its lower part shut by a stop-cock, through

which the water is allowed to drop gradually into the gun-barrel.

Immediately upon getting into contact with the heated part of the iron,

the water is converted into steam, and the experiment proceeds in the

same manner as if it were furnished in vapours from the retort.

In the experiment made by Mr Meusnier and me before a committee of the

Academy, we used every precaution to obtain the greatest possible

precision in the result of our experiment, having even exhausted all the

vessels employed before we began, so that the hydrogen gas obtained

might be free from any mixture of azotic gas. The results of that

experiment will hereafter be given at large in a particular memoir.

In numerous experiments, we are obliged to use tubes of glass,

porcelain, or copper, instead of gun-barrels; but glass has the

disadvantage of being easily melted and flattened, if the heat be in the

smallest degree raised too high; and porcelain is mostly full of small

minute pores, through which the gas escapes, especially when compressed

by a column of water. For these reasons I procured a tube of brass,

which Mr de la Briche got cast and bored out of the solid for me at

Strasburg, under his own inspection. This tube is extremely convenient

for decomposing alkohol, which resolves into charcoal, carbonic acid

gas, and hydrogen gas; it may likewise be used with the same advantage

for decomposing water by means of charcoal, and in a great number of

experiments of this nature.

FOOTNOTES:

[60] The representation of this apparatus, Pl. IV. Fig. 1. will convey a

much better idea of its disposition than can possibly be given by the

most laboured description.--E.

CHAP. VII.

_Of the Composition and Application of Lutes._

The necessity of properly securing the junctures of chemical vessels to

prevent the escape of any of the products of experiments, must be

sufficiently apparent; for this purpose lutes are employed, which ought

to be of such a nature as to be equally impenetrable to the most subtile

substances, as glass itself, through which only caloric can escape.

This first object of lutes is very well accomplished by bees wax, melted

with about an eighth part of turpentine. This lute is very easily

managed, sticks very closely to glass, and is very difficultly

penetrable; it may be rendered more consistent, and less or more hard or

pliable, by adding different kinds of resinous matters. Though this

species of lute answers extremely well for retaining gasses and vapours,

there are many chemical experiments which produce considerable heat, by

which this lute becomes liquified, and consequently the expansive

vapours must very readily force through and escape.

For such cases, the following fat lute is the best hitherto discovered,

though not without its disadvantages, which shall be pointed out. Take

very pure and dry unbaked clay, reduced to a very fine powder, put this

into a brass mortar, and beat it for several hours with a heavy iron

pestle, dropping in slowly some boiled lintseed oil; this is oil which

has been oxygenated, and has acquired a drying quality, by being boiled

with litharge. This lute is more tenacious, and applies better, if amber

varnish be used instead of the above oil. To make this varnish, melt

some yellow amber in an iron laddle, by which operation it loses a part

of its succinic acid, and essential oil, and mix it with lintseed oil.

Though the lute prepared with this varnish is better than that made with

boiled oil, yet, as its additional expence is hardly compensated by its

superior quality, it is seldom used.

The above fat lute is capable of sustaining a very violent degree of

heat, is impenetrable by acids and spiritous liquors, and adheres

exceedingly well to metals, stone ware, or glass, providing they have

been previously rendered perfectly dry. But if, unfortunately, any of

the liquor in the course of an experiment gets through, either between

the glass and the lute, or between the layers of the lute itself, so as

to moisten the part, it is extremely difficult to close the opening.

This is the chief inconvenience which attends the use of fat lute, and

perhaps the only one it is subject to. As it is apt to soften by heat,

we must surround all the junctures with slips of wet bladder applied

over the luting, and fixed on by pack-thread tied round both above and

below the joint; the bladder, and consequently the lute below, must be

farther secured by a number of turns of pack-thread all over it. By

these precautions, we are free from every danger of accident; and the

junctures secured in this manner may be considered, in experiments, as

hermetically sealed.

It frequently happens that the figure of the junctures prevents the

application of ligatures, which is the case with the three-necked

bottles formerly described; and it even requires great address to apply

the twine without shaking the apparatus; so that, where a number of

junctures require luting, we are apt to displace several while securing

one. In these cases, we may substitute slips of linen, spread with white

of egg and lime mixed together, instead of the wet bladder. These are

applied while still moist, and very speedily dry and acquire

considerable hardness. Strong glue dissolved in water may answer instead

of white of egg. These fillets are usefully applied likewise over

junctures luted together with wax and rosin.

Before applying a lute, all the junctures of the vessels must be

accurately and firmly fitted to each other, so as not to admit of being

moved. If the beak of a retort is to be luted to the neck of a

recipient, they ought to fit pretty accurately; otherwise we must fix

them, by introducing short pieces of soft wood or of cork. If the

disproportion between the two be very considerable, we must employ a

cork which fits the neck of the recipient, having a circular hole of

proper dimensions to admit the beak of the retort. The same precaution

is necessary in adapting bent tubes to the necks of bottles in the

apparatus represented Pl. IV. Fig. 1. and others of a similar nature.

Each mouth of each bottle must be fitted with a cork, having a hole made

with a round file of a proper size for containing the tube. And, when

one mouth is intended to admit two or more tubes, which frequently

happens when we have not a sufficient number of bottles with two or

three necks, we must use a cork with two or three holes, Pl. IV. Fig. 8.

When the whole apparatus is thus solidly joined, so that no part can

play upon another, we begin to lute. The lute is softened by kneading

and rolling it between the fingers, with the assistance of heat, if

necessary. It is rolled into little cylindrical pieces, and applied to

the junctures, taking great care to make it apply close, and adhere

firmly, in every part; a second roll is applied over the first, so as to

pass it on each side, and so on till each juncture be sufficiently

covered; after this, the slips of bladder, or of linen, as above

directed, must be carefully applied over all. Though this operation may

appear extremely simple, yet it requires peculiar delicacy and

management; great care must be taken not to disturb one juncture whilst

luting another, and more especially when applying the fillets and

ligatures.

Before beginning any experiment, the closeness of the luting ought

always to be previously tried, either by slightly heating the retort A,

Pl. IV. Fig. 1, or by blowing in a little air by some of the

perpendicular tubes S s s s; the alteration of pressure causes a

change in the level of the liquid in these tubes. If the apparatus be

accurately luted, this alteration of level will be permanent; whereas,

if there be the smallest, opening in any of the junctures, the liquid

will very soon recover its former level. It must always be remembered,

that the whole success of experiments in modern chemistry depends upon

the exactness of this operation, which therefore requires the utmost

patience, and most attentive accuracy.

It would be of infinite service to enable chemists, especially those who

are engaged in pneumatic processes, to dispense with the use of lutes,

or at least to diminish the number necessary in complicated instruments.

I once thought of having my apparatus constructed so as to unite in all

its parts by fitting with emery, in the way of bottles with cristal

stoppers; but the execution of this plan was extremely difficult. I have

since thought it preferable to substitute columns of a few lines of

mercury in place of lutes, and have got an apparatus constructed upon

this principle, which appears capable of very convenient application in

a great number of circumstances.

It consists of a double necked bottle A, Pl. XII. Fig. 12.; the interior

neck bc communicates with the inside of the bottle, and the exterior

neck or rim de leaves an interval between the two necks, forming a

deep gutter intended to contain the mercury. The cap or lid of glass B

enters this gutter, and is properly fitted to it, having notches in its

lower edge for the passage of the tubes which convey the gas. These

tubes, instead of entering directly into the bottles as in the ordinary

apparatus, have a double bend for making them enter the gutter, as

represented in Fig. 13. and for making them fit the notches of the cap

B; they rise again from the gutter to enter the inside of the bottle

over the border of the inner mouth. When the tubes are disposed in their

proper places, and the cap firmly fitted on, the gutter is filled with

mercury, by which means the bottle is completely excluded from any

communication, excepting through the tubes. This apparatus may be very

convenient in many operations in which the substances employed have no

action upon Mercury. Pl. XII. Fig. 14. represents an apparatus upon this

principle properly fitted together.

Mr Seguin, to whose active and intelligent assistance I have been very

frequently much indebted, has bespoken for me, at the glass-houses, some

retorts hermetically united to their recipients, by which luting will be

altogether unnecessary.

CHAP. VIII.

_Of Operations upon Combustion and Deflagration._

SECT. I.

_Of Combustion in general._

Combustion, according to what has been already said in the First Part of

this Work, is the decomposition of oxygen gas produced by a combustible

body. The oxygen which forms the base of this gas is absorbed by, and

enters into, combination with the burning body, while the caloric and

light are set free. Every combustion, therefore, necessarily supposes

oxygenation; whereas, on the contrary, every oxygenation does not

necessarily imply concomitant combustion; because combustion, properly

so called, cannot take place without disengagement of caloric and light.

Before combustion can take place, it is necessary that the base of

oxygen gas should have greater affinity to the combustible body than it

has to caloric; and this elective attraction, to use Bergman's

expression, can only take place at a certain degree of temperature,

which is different for each combustible substance; hence the necessity

of giving a first motion or beginning to every combustion by the

approach of a heated body. This necessity of heating any body we mean to

burn depends upon certain considerations, which have not hitherto been

attended to by any natural philosopher, for which reason I shall enlarge

a little upon the subject in this place.

Nature is at present in a state of equilibrium, which cannot have been

attained until all the spontaneous combustions or oxygenations possible

in the ordinary degrees of temperature had taken place. Hence, no new

combustions or oxygenations can happen without destroying this

equilibrium, and raising the combustible substances to a superior degree

of temperature. To illustrate this abstract view of the matter by

example: Let us suppose the usual temperature of the earth a little

changed, and that it is raised only to the degree of boiling water; it

is evident, that, in this case, phosphorus, which is combustible in a

considerably lower degree of temperature, would no longer exist in

nature in its pure and simple state, but would always be procured in its

acid or oxygenated state, and its radical would become one of the

substances unknown to chemistry. By gradually increasing the

temperature of the earth the same circumstance would successively happen

to all the bodies capable of combustion; and, at last, every possible

combustion having taken place, there would no longer exist any

combustible body whatever, as every substance susceptible of that

operation would be oxygenated, and consequently incombustible.

There cannot therefore exist, so far as relates to us, any combustible

body, except such as are incombustible in the ordinary temperatures of

the earth; or, what is the same thing, in other words, that it is

essential to the nature of every combustible body not to possess the

property of combustion, unless heated, or raised to the degree of

temperature at which its combustion naturally takes place. When this

degree is once produced, combustion commences, and the caloric which is

disengaged by the decomposition of the oxygen gas keeps up the

temperature necessary for continuing combustion. When this is not the

case, that is, when the disengaged caloric is insufficient for keeping

up the necessary temperature, the combustion ceases: This circumstance

is expressed in common language by saying, that a body burns ill, or

with difficulty.

Although combustion possesses some circumstances in common with

distillation, especially with the compound kind of that operation, they

differ in a very material point. In distillation there is a separation

of one part of the elements of the substance from each other, and a

combination of these, in a new order, occasioned by the affinities which

take place in the increased temperature produced during distillation:

This likewise happens in combustion, but with this farther circumstance,

that a new element, not originally in the body, is brought into action;

oxygen is added to the substance submitted to the operation, and caloric

is disengaged.

The necessity of employing oxygen in the state of gas in all experiments

with combustion, and the rigorous determination of the quantities

employed, render this kind of operations peculiarly troublesome. As

almost all the products of combustion are disengaged in the state of

gas, it is still more difficult to retain them than even those furnished

during compound distillation; hence this precaution was entirely

neglected by the ancient chemists; and this set of experiments

exclusively belong to modern chemistry.

Having thus pointed out, in a general way, the objects to be had in view

in experiments upon combustion, I proceed, in the following sections of

this chapter, to describe the different instruments I have used with

this view. The following arrangement is formed, not upon the nature of

the combustible bodies, but upon that of the instruments necessary for

combustion.

SECT. II.

_Of the Combustion of Phosphorus._

In these combustions we begin by filling a jar, capable at least of

holding six pints, with oxygen gas in the water apparatus, Pl. V. Fig.

1.; when it is perfectly full, so that the gas begins to flow out below,

the jar, A, is carried to the mercury apparatus, Pl. IV. Fig. 3. We then

dry the surface of the mercury, both within and without the jar, by

means of blotting-paper, taking care to keep the paper for some time

entirely immersed in the mercury before it is introduced under the jar,

lest we let in any common air, which sticks very obstinately to the

surface of the paper. The body to be submitted to combustion, being

first very accurately weighed in nice scales, is placed in a small flat

shallow dish, D, of iron or porcelain; this is covered by the larger cup

P, which serves the office of a diving bell, and the whole is passed

through the mercury into the jar, after which the larger cup is retired.

The difficulty of passing the materials of combustion in this manner

through the mercury may be avoided by raising one of the sides of the

jar, A, for a moment, and slipping in the little cup, D, with the

combustible body as quickly as possible. In this manner of operating, a

small quantity of common air gets into the jar, but it is so very

inconsiderable as not to injure either the progress or accuracy of the

experiment in any sensible degree.

When the cup, D, is introduced under the jar, we suck out a part of the

oxygen gas, so as to raise the mercury to EF, as formerly directed, Part

I. Chap. V. otherwise, when the combustible body is set on fire, the gas

becoming dilated would be in part forced out, and we should no longer be

able to make any accurate calculation of the quantities before and after

the experiment. A very convenient mode of drawing out the air is by

means of an air-pump syringe adapted to the syphon, GHI, by which the

mercury may be raised to any degree under twenty-eight inches. Very

inflammable bodies, as phosphorus, are set on fire by means of the

crooked iron wire, MN, Pl. IV. Fig. 16. made red hot, and passed quickly

through the mercury. Such as are less easily set on fire have a small

portion of tinder, upon which a minute particle of phosphorus is fixed,

laid upon them before using the red hot iron.

In the first moment of combustion the air, being heated, rarifies, and

the mercury descends; but when, as in combustions of phosphorus and

iron, no elastic fluid is formed, absorption becomes presently very

sensible, and the mercury rises high into the jar. Great attention must

be used not to burn too large a quantity of any substance in a given

quantity of gas, otherwise, towards the end of the experiment, the cup

would approach so near the top of the jar as to endanger breaking it by

the great heat produced, and the sudden refrigeration from the cold

mercury. For the methods of measuring the volume of the gasses, and for

correcting the measures according to the heighth of the barometer and

thermometer, &c. see Chap. II. Sect. V. and VI. of this part.

The above process answers very well for burning all the concrete

substances, and even for the fixed oils: These last are burnt in lamps

under the jar, and are readily set on fire by means of tinder,

phosphorus, and hot iron. But it is dangerous for substances susceptible

of evaporating in a moderate heat, such as ether, alkohol, and the

essential oils; these substances dissolve in considerable quantity in

oxygen gas; and, when set on fire, a dangerous and sudden explosion

takes place, which carries up the jar to a great height, and dashes it

in a thousand pieces. From two such explosions some of the members of

the Academy and myself escaped very narrowly. Besides, though this

manner of operating is sufficient for determining pretty accurately the

quantity of oxygen gas absorbed, and of carbonic acid produced, as water

is likewise formed in all experiments upon vegetable and animal matters

which contain an excess of hydrogen, this apparatus can neither collect

it nor determine its quantity. The experiment with phosphorus is even

incomplete in this way, as it is impossible to demonstrate that the

weight of the phosphoric acid produced is equal to the sum of the

weights of the phosphorus burnt and oxygen gas absorbed during the

process. I have been therefore obliged to vary the instruments according

to circumstances, and to employ several of different kinds, which I

shall describe in their order, beginning with that used for burning

phosphorus.

Take a large balloon, A, Pl. IV. Fig. 4. of cristal or white glass, with

an opening, EF, about two inches and a half, or three inches, diameter,

to which a cap of brass is accurately fitted with emery, and which has

two holes for the passage of the tubes xxx, yyy. Before shutting the

balloon with its cover, place within it the stand, BC, supporting the

cup of porcelain, D, which contains the phosphorus. Then lute on the cap

with fat lute, and allow it to dry for some days, and weigh the whole

accurately; after this exhaust the balloon by means of an air-pump

connected with the tube xxx, and fill it with oxygen gas by the tube

yyy, from the gazometer, Pl. VIII. Fig. 1. described Chap. II. Sect

II. of this part. The phosphorus is then set on fire by means of a

burning-glass, and is allowed to burn till the cloud of concrete

phosphoric acid stops the combustion, oxygen gas being continually

supplied from the gazometer. When the apparatus has cooled, it is

weighed and unluted; the tare of the instrument being allowed, the

weight is that of the phosphoric acid contained. It is proper, for

greater accuracy, to examine the air or gas contained in the balloon

after combustion, as it may happen to be somewhat heavier or lighter

than common air; and this difference of weight must be taken into

account in the calculations upon the results of the experiment.

SECT. III.

_Of the Combustion of Charcoal._

The apparatus I have employed for this process consists of a small

conical furnace of hammered copper, represented in perspective, Pl. XII.

Fig. 9. and internally displayed Fig. 11. It is divided into the

furnace, ABC, where the charcoal is burnt, the grate, d e, and the

ash-hole, F; the tube, GH, in the middle of the dome of the furnace

serves to introduce the charcoal, and as a chimney for carrying off the

air which has served for combustion. Through the tube, l m n, which

communicates with the gazometer, the hydrogen gas, or air, intended for

supporting the combustion, is conveyed into the ash-hole, F, whence it

is forced, by the application of pressure to the gazometer, to pass

through the grate, d e, and to blow upon the burning charcoal placed

immediately above.

Oxygen gas, which forms 28/100 of atmospheric air, is changed into

carbonic acid gas during combustion with charcoal, whilst the azotic gas

of the air is not altered at all. Hence, after the combustion of

charcoal in atmospheric air, a mixture of carbonic acid gas and azotic

gas must remain; to allow this mixture to pass off, the tube, o p, is

adapted to the chimney, GH, by means of a screw at G, and conveys the

gas into bottles half filled with solution of caustic potash. The

carbonic acid gas is absorbed by the alkali, and the azotic gas is

conveyed into a second gazometer, where its quantity is ascertained.

The weight of the furnace, ABC, is first accurately determined, then

introduce the tube RS, of known weight, by the chimney, GH, till its

lower end S, rests upon the grate, d e, which it occupies entirely; in

the next place, fill the furnace with charcoal, and weigh the whole

again, to know the exact quantity of charcoal submitted to experiment.

The furnace is now put in its place, the tube, l m n, is screwed to

that which communicates with the gazometer, and the tube, o p, to that

which communicates with the bottles of alkaline solution. Every thing

being in readiness, the stop-cock of the gazometer is opened, a small

piece of burning charcoal is thrown into the tube, RS, which is

instantly withdrawn, and the tube, o p, is screwed to the chimney, GH.

The little piece of charcoal falls upon the grate, and in this manner

gets below the whole charcoal, and is kept on fire by the stream of air

from the gazometer. To be certain that the combustion is begun, and goes

on properly, the tube, q r s, is fixed to the furnace, having a piece

of glass cemented to its upper extremity, s, through which we can see

if the charcoal be on fire.

I neglected to observe above, that the furnace, and its appendages, are

plunged in water in the cistern, TVXY, Fig. 11. Pl. XII. to which ice

may be added to moderate the heat, if necessary; though the heat is by

no means very considerable, as there is no air but what comes from the

gazometer, and no more of the charcoal burns at one time than what is

immediately over the grate.

As one piece of charcoal is consumed another falls down into its place,

in consequence of the declivity of the sides of the furnace; this gets

into the stream of air from the grate, d e, and is burnt; and so on,

successively, till the whole charcoal is consumed. The air which has

served the purpose of the combustion passes through the mass of

charcoal, and is forced by the pressure of the gazometer to escape

through the tube, o p, and to pass through the bottles of alkaline

solution.

This experiment furnishes all the necessary data for a complete analysis

of atmospheric air and of charcoal. We know the weight of charcoal

consumed; the gazometer gives us the measure of the air employed; the

quantity and quality of gas remaining after combustion may be

determined, as it is received, either in another gazometer, or in jars,

in a pneumato-chemical apparatus; the weight of ashes remaining in the

ash-hole is readily ascertained; and, finally, the additional weight

acquired by the bottles of alkaline solution gives the exact quantity of

carbonic acid formed during the process. By this experiment we may

likewise determine, with sufficient accuracy, the proportions in which

charcoal and oxygen enter into the composition of carbonic acid.

In a future memoir I shall give an account to the Academy of a series of

experiments I have undertaken, with this instrument, upon all the

vegetable and animal charcoals. By some very slight alterations, this

machine may be made to answer for observing the principal phenomena of

respiration.

SECT. IV.

_Of the Combustion of Oils._

Oils are more compound in their nature than charcoal, being formed by

the combination of at least two elements, charcoal and hydrogen; of

course, after their combustion in common air, water, carbonic acid gas,

and azotic gas, remain. Hence the apparatus employed for their

combustion requires to be adapted for collecting these three products,

and is consequently more complicated than the charcoal furnace.

The apparatus I employ for this purpose is composed of a large jar or

pitcher A, Pl. XII. Fig. 4. surrounded at its upper edge by a rim of

iron properly cemented at DE, and receding from the jar at BC, so as to

leave a furrow or gutter xx, between it and the outside of the jar,

somewhat more than two inches deep. The cover or lid of the jar, Fig. 5.

is likewise surrounded by an iron rim f g, which adjusts into the

gutter xx, Fig. 4. which being filled with mercury, has the effect of

closing the jar hermetically in an instant, without using any lute; and,

as the gutter will hold about two inches of mercury, the air in the jar

may be made to sustain the pressure of more than two feet of water,

without danger of its escaping.

The lid has four holes, T h i k, for the passage of an equal

number of tubes. The opening T is furnished with a leather box, through

which passes the rod, Fig. 3. intended for raising and lowering the wick

of the lamp, as will be afterwards directed. The three other holes are

intended for the passage of three several tubes, one of which conveys

the oil to the lamp, a second conveys air for keeping up the combustion,

and the third carries off the air, after it has served for combustion.

The lamp in which the oil is burnt is represented Fig. 2; a is the

reservoir of oil, having a funnel by which it is filled; b c d e f g h

is a syphon which conveys the oil to the lamp 11; 7, 8, 9, 10, is the

tube which conveys the air for combustion from the gazometer to the same

lamp. The tube b c is formed externally, at its lower end b, into a

male screw, which turns in a female screw in the lid of the reservoir of

oil a; so that, by turning the reservoir one way or the other, it is

made to rise or fall, by which the oil is kept at the necessary level.

When the syphon is to be filled, and the communication formed between

the reservoir of oil and the lamp, the stop-cock c is shut, and that

at e opened, oil is poured in by the opening f at the top of the

syphon, till it rises within three or four lines of the upper edge of

the lamp, the stop-cock k is then shut, and that at c opened; the

oil is then poured in at f, till the branch b c d of the syphon is

filled, and then the stop-cock e is closed. The two branches of the

syphon being now completely filled, a communication is fully established

between the reservoir and the lamp.

In Pl. XII. Fig. 1. all the parts of the lamp 11, Fig. 2. are

represented magnified, to show them distinctly. The tube i k carries

the oil from the reservoir to the cavity a a a a, which contains the

wick; the tube 9, 10, brings the air from the gazometer for keeping up

the combustion; this air spreads through the cavity d d d d, and, by

means of the passages c c c c and b b b b, is distributed on each

side of the wick, after the principles of the lamps constructed by

Argand, Quinquet, and Lange.

To render the whole of this complicated apparatus more easily

understood, and that its description may make all others of the same

kind more readily followed, it is represented, completely connected

together for use, in Pl. XI. The gazometer P furnishes air for the

combustion by the tube and stop-cock 1, 2; the tube 2, 3, communicates

with a second gazometer, which is filled whilst the first one is

emptying during the process, that there may be no interruption to the

combustion; 4, 5, is a tube of glass filled with deliquescent salts, for

drying the air as much as possible in its passage; and the weight of

this tube and its contained salts, at the beginning of the experiment,

being known, it is easy to determine the quantity of water absorbed by

them from the air. From this deliquescent tube the air is conducted

through the pipe 5, 6, 7, 8, 9, 10, to the lamp 11, where it spreads on

both sides of the wick, as before described, and feeds the flame. One

part of this air, which serves to keep up the combustion of the oil,

forms carbonic acid gas and water, by oxygenating its elements. Part of

this water condenses upon the sides of the pitcher A, and another part

is held in solution in the air by means of caloric furnished by the

combustion. This air is forced by the compression of the gazometer to

pass through the tube 12, 13, 14, 15, into the bottle 16, and the worm

17, 18, where the water is fully condensed from the refrigeration of the

air; and, if any water still remains in solution, it is absorbed by

deliquescent salts contained in the tube 19, 20.

All these precautions are solely intended for collecting and determining

the quantity of water formed during the experiment; the carbonic acid

and azotic gas remains to be ascertained. The former is absorbed by

caustic alkaline solution in the bottles 22 and 25. I have only

represented two of these in the figure, but nine at least are requisite;

and the last of the series may be half filled with lime-water, which is

the most certain reagent for indicating the presence of carbonic acid;

if the lime-water is not rendered turbid, we may be certain that no

sensible quantity of that acid remains in the air.

The rest of the air which has served for combustion, and which chiefly

consists of azotic gas, though still mixed with a considerable portion

of oxygen gas, which has escaped unchanged from the combustion, is

carried through a third tube 28, 29, of deliquescent salts, to deprive

it of any moisture it may have acquired in the bottles of alkaline

solution and lime-water, and from thence by the tube 29, 30, into a

gazometer, where its quantity is ascertained. Small essays are then

taken from it, which are exposed to a solution of sulphuret of potash,

to ascertain the proportions of oxygen and azotic gas it contains.

In the combustion of oils the wick becomes charred at last, and

obstructs the rise of the oil; besides, if we raise the wick above a

certain height, more oil rises through its capillary tubes than the

stream of air is capable of consuming, and smoke is produced. Hence it

is necessary to be able to lengthen or shorten the wick without opening

the apparatus; this is accomplished by means of the rod 31, 32, 33, 34,

which passes through a leather-box, and is connected with the support of

the wick; and that the motion of this rod, and consequently of the wick,

may be regulated with the utmost smoothness and facility; it is moved at

pleasure by a pinnion which plays in a toothed rack. The rod, with its

appendages, are represented Pl. XII. Fig. 3. It appeared to me, that the

combustion would be assisted by surrounding the flame of the lamp with a

small glass jar open at both ends, as represented in its place in Pl.

XI.

I shall not enter into a more detailed description of the construction

of this apparatus, which is still capable of being altered and modified

in many respects, but shall only add, that when it is to be used in

experiment, the lamp and reservoir with the contained oil must be

accurately weighed, after which it is placed as before directed, and

lighted; having then formed the connection between the air in the

gazometer and the lamp, the external jar A, Pl. XI. is fixed over all,

and secured by means of the board BC and two rods of iron which connect

this board with the lid, and are screwed to it. A small quantity of oil

is burnt while the jar is adjusting to the lid, and the product of that

combustion is lost; there is likewise a small portion of air from the

gazometer lost at the same time. Both of these are of very

inconsiderable consequence in extensive experiments, and they are even

capable of being valued in our calculation of the results.

In a particular memoir, I shall give an account to the Academy of the

difficulties inseparable from this kind of experiments: These are so

insurmountable and troublesome, that I have not hitherto been able to

obtain any rigorous determination of the quantities of the products. I

have sufficient proof, however, that the fixed oils are entirely

resolved during combustion into water and carbonic acid gas, and

consequently that they are composed of hydrogen and charcoal; but I have

no certain knowledge respecting the proportions of these ingredients.

SECT. V.

_Of the Combustion of Alkohol._

The combustion of alkohol may be very readily performed in the apparatus

already described for the combustion of charcoal and phosphorus. A lamp

filled with alkohol is placed under the jar A, Pl. IV. Fig. 3. a small

morsel of phosphorus is placed upon the wick of the lamp, which is set

on fire by means of the hot iron, as before directed. This process is,

however, liable to considerable inconveniency; it is dangerous to make

use of oxygen gas at the beginning of the experiment for fear of

deflagration, which is even liable to happen when common air is

employed. An instance of this had very near proved fatal to myself, in

presence of some members of the Academy. Instead of preparing the

experiment, as usual, at the time it was to be performed, I had disposed

every thing in order the evening before; the atmospheric air of the jar

had thereby sufficient time to dissolve a good deal of the alkohol; and

this evaporation had even been considerably promoted by the height of

the column of mercury, which I had raised to EF, Pl. IV. Fig. 3. The

moment I attempted to set the little morsel of phosphorus on fire by

means of the red hot iron, a violent explosion took place, which threw

the jar with great violence against the floor of the laboratory, and

dashed it in a thousand pieces.

Hence we can only operate upon very small quantities, such as ten or

twelve grains of alkohol, in this manner; and the errors which may be

committed in experiments upon such small quantities prevents our placing

any confidence in their results. I endeavoured to prolong the

combustion, in the experiments contained in the Memoirs of the Academy

for 1784, p. 593. by lighting the alkohol first in common air, and

furnishing oxygen gas afterwards to the jar, in proportion as it

consumed; but the carbonic acid gas produced by the process became a

great hinderance to the combustion, the more so that alkohol is but

difficultly combustible, especially in worse than common air; so that

even in this way very small quantities only could be burnt.

Perhaps this combustion might succeed better in the oil apparatus, Pl.

XI.; but I have not hitherto ventured to try it. The jar A in which the

combustion is performed is near 1400 cubical inches in dimension; and,

were an explosion to take place in such a vessel, its consequences would

be very terrible, and very difficult to guard against. I have not,

however, despaired of making the attempt.

From all these difficulties, I have been hitherto obliged to confine

myself to experiments upon very small quantities of alkohol, or at least

to combustions made in open vessels, such as that represented in Pl. IX.

Fig. 5. which will be described in Section VII. of this chapter. If I am

ever able to remove these difficulties, I shall resume this

investigation.

SECT. VI.

_Of the Combustion of Ether._

Tho' the combustion of ether in close vessels does not present the same

difficulties as that of alkohol, yet it involves some of a different

kind, not more easily overcome, and which still prevent the progress of

my experiments. I endeavoured to profit by the property which ether

possesses of dissolving in atmospheric air, and rendering it inflammable

without explosion. For this purpose, I constructed the reservoir of

ether a b c d, Plate XII. Fig. 8. to which air is brought from the

gazometer by the tube 1, 2, 3, 4. This air spreads, in the first place,

in the double lid ac of the reservoir, from which it passes through

seven tubes ef, gh, ik, &c. which descend to the bottom of the

ether, and it is forced by the pressure of the gazometer to boil up

through the ether in the reservoir. We may replace the ether in this

first reservoir, in proportion as it is dissolved and carried off by the

air, by means of the supplementary reservoir E, connected by a brass

tube fifteen or eighteen inches long, and shut by a stop-cock. This

length of the connecting tube is to enable the descending ether to

overcome the resistance occasioned by the pressure of the air from the

gazometer.

The air, thus loaded with vapours of ether, is conducted by the tube 5,

6, 7, 8, 9, to the jar A, into which it is allowed to escape through a

capillary opening, at the extremity of which it is set on fire. The air,

when it has served the purpose of combustion, passes through the bottle

16, Pl. XI. the worm 17, 18, and the deliquescent tube 19, 20, after

which it passes through the alkaline bottles; in these its carbonic acid

gas is absorbed, the water formed during the experiment having been

previously deposited in the former parts of the apparatus.

When I caused construct this apparatus, I supposed that the combination

of atmospheric air and ether formed in the reservoir a b c d, Pl. XII.

Fig. 8. was in proper proportion for supporting combustion; but in this

I was mistaken; for there is a very considerable quantity of excess of

ether; so that an additional quantity of atmospheric air is necessary

to enable it to burn fully. Hence a lamp constructed upon these

principles will burn in common air, which furnishes the quantity of

oxygen necessary for combustion, but will not burn in close vessels in

which the air is not renewed. From this circumstance, my ether lamp went

out soon after being lighted and shut up in the jar A, Pl. XII. Fig. 8.

To remedy this defect, I endeavoured to bring atmospheric air to the

lamp by the lateral tube 10, 11, 12, 13, 14, 15, which I distributed

circularly round the flame; but the flame is so exceedingly rare, that

it is blown out by the gentlest possible stream of air, so that I have

not hitherto succeeded in burning ether. I do not, however, despair of

being able to accomplish it by means of some changes I am about to have

made upon this apparatus.

SECT. VII.

_Of the Combustion of Hydrogen Gas, and the Formation of Water._

In the formation of water, two substances, hydrogen and oxygen, which

are both in the aeriform state before combustion, are transformed into

liquid or water by the operation. This experiment would be very easy,

and would require very simple instruments, if it were possible to

procure the two gasses perfectly pure, so that they might burn without

any residuum. We might, in that case, operate in very small vessels,

and, by continually furnishing the two gasses in proper proportions,

might continue the combustion indefinitely. But, hitherto, chemists have

only employed oxygen gas, mixed with azotic gas; from which

circumstance, they have only been able to keep up the combustion of

hydrogen gas for a very limited time in close vessels, because, as the

residuum of azotic gas is continually increasing, the air becomes at

last so much contaminated, that the flame weakens and goes out. This

inconvenience is so much the greater in proportion as the oxygen gas

employed is less pure. From this circumstance, we must either be

satisfied with operating upon small quantities, or must exhaust the

vessels at intervals, to get rid of the residuum of azotic gas; but, in

this case, a portion of the water formed during the experiment is

evaporated by the exhaustion; and the resulting error is the more

dangerous to the accuracy of the process, that we have no certain means

of valuing it.

These considerations make me desirous to repeat the principal

experiments of pneumatic chemistry with oxygen gas entirely free from

any admixture of azotic gas; and this may be procured from oxygenated

muriat of potash. The oxygen gas extracted from this salt does not

appear to contain azote, unless accidentally, so that, by proper

precautions, it may be obtained perfectly pure. In the mean time, the

apparatus employed by Mr Meusnier and me for the combustion of hydrogen

gas, which is described in the experiment for recomposition of water,

Part I. Chap. VIII. and need not be here repeated, will answer the

purpose; when pure gasses are procured, this apparatus will require no

alterations, except that the capacity of the vessels may then be

diminished. See Pl. IV. Fig. 5.

The combustion, when once begun, continues for a considerable time, but

weakens gradually, in proportion as the quantity of azotic gas remaining

from the combustion increases, till at last the azotic gas is in such

over proportion that the combustion can no longer be supported, and the

flame goes out. This spontaneous extinction must be prevented, because,

as the hydrogen gas is pressed upon in its reservoir, by an inch and a

half of water, whilst the oxygen gas suffers a pressure only of three

lines, a mixture of the two would take place in the balloon, which would

at last be forced by the superior pressure into the reservoir of oxygen

gas. Wherefore the combustion must be stopped, by shutting the

stop-cock of the tube dDd whenever the flame grows very feeble; for

which purpose it must be attentively watched.

There is another apparatus for combustion, which, though we cannot with

it perform experiments with the same scrupulous exactness as with the

preceding instruments, gives very striking results that are extremely

proper to be shewn in courses of philosophical chemistry. It consists of

a worm EF, Pl. IX. Fig. 5. contained in a metallic cooller ABCD. To the

upper part of this worm E, the chimney GH is fixed, which is composed of

two tubes, the inner of which is a continuation of the worm, and the

outer one is a case of tin-plate, which surrounds it at about an inch

distance, and the interval is filled up with sand. At the inferior

extremity K of the inner tube, a glass tube is fixed, to which we adopt

the Argand lamp LM for burning alkohol, &c.

Things being thus disposed, and the lamp being filled with a determinate

quantity of alkohol, it is set on fire; the water which is formed during

the combustion rises in the chimney KE, and being condensed in the worm,

runs out at its extremity F into the bottle P. The double tube of the

chimney, filled with sand in the interstice, is to prevent the tube from

cooling in its upper part, and condensing the water; otherwise, it

would fall back in the tube, and we should not be able to ascertain its

quantity, and besides it might fall in drops upon the wick, and

extinguish the flame. The intention of this construction, is to keep the

chimney always hot, and the worm always cool, that the water may be

preserved in the state of vapour whilst rising, and may be condensed

immediately upon getting into the descending part of the apparatus. By

this instrument, which was contrived by Mr Meusnier, and which is

described by me in the Memoirs of the Academy for 1784, p. 593. we may,

with attention to keep the worm always cold, collect nearly seventeen

ounces of water from the combustion of sixteen ounces of alkohol.

SECT. VIII.

_Of the Oxydation of Metals._

The term _oxydation_ or _calcination_ is chiefly used to signify the

process by which metals exposed to a certain degree of heat are

converted into oxyds, by absorbing oxygen from the air. This combination

takes place in consequence of oxygen possessing a greater affinity to

metals, at a certain temperature, than to caloric, which becomes

disengaged in its free state; but, as this disengagement, when made in

common air, is slow and progressive, it is scarcely evident to the

senses. It is quite otherwise, however, when oxydation takes place in

oxygen gas; for, being produced with much greater rapidity, it is

generally accompanied with heat and light, so as evidently to show that

metallic substances are real combustible bodies.

All the metals have not the same degree of affinity to oxygen. Gold,

silver, and platina, for instance, are incapable of taking it away from

its combination with caloric, even in the greatest known heat; whereas

the other metals absorb it in a larger or smaller quantity, until the

affinities of the metal to oxygen, and of the latter to caloric, are in

exact equilibrium. Indeed, this state of equilibrium of affinities may

be assumed as a general law of nature in all combinations.

In all operations of this nature, the oxydation of metals is accelerated

by giving free access to the air; it is sometimes much assisted by

joining the action of a bellows, which directs a stream of air over the

surface of the metal. This process becomes greatly more rapid if a

stream of oxygen gas be used, which is readily done by means of the

gazometer formerly described. The metal, in this case, throws out a

brilliant flame, and the oxydation is very quickly accomplished; but

this method can only be used in very confined experiments, on account of

the expence of procuring oxygen gas. In the essay of ores, and in all

the common operations of the laboratory, the calcination or oxydation of

metals is usually performed in a dish of baked clay, Pl. IV. Fig. 6.

commonly called a _roasting test_, placed in a strong furnace. The

substances to be oxydated are frequently stirred, on purpose to present

fresh surfaces to the air.

Whenever this operation is performed upon a metal which is not volatile,

and from which nothing flies off into the surrounding air during the

process, the metal acquires additional weight; but the cause of this

increased weight during oxydation could never have been discovered by

means of experiments performed in free air; and it is only since these

operations have been performed in close vessels, and in determinate

quantities of air, that any just conjectures have been formed concerning

the cause of this phenomenon. The first method for this purpose is due

to Dr Priestley, who exposes the metal to be calcined in a porcelain cup

N, Pl. IV. Fig. 11. placed upon the stand IK, under a jar A, in the

bason BCDE, full of water; the water is made to rise up to GH, by

sucking out the air with a syphon, and the focus of a burning glass is

made to fall upon the metal. In a few minutes the oxydation takes

place, a part of the oxygen contained in the air combines with the

metal, and a proportional diminution of the volume of air is produced;

what remains is nothing more than azotic gas, still however mixed with a

small quantity of oxygen gas. I have given an account of a series of

experiments made with this apparatus in my Physical and Chemical Essays,

first published in 1773. Mercury may be used instead of water in this

experiment, whereby the results are rendered still more conclusive.

Another process for this purpose was invented by Mr Boyle, and of which

I gave an account in the Memoirs of the Academy for 1774, p. 351. The

metal is introduced into a retort, Pl. III. Fig. 20. the beak of which

is hermetically sealed; the metal is then oxydated by means of heat

applied with great precaution. The weight of the vessel, and its

contained substances, is not at all changed by this process, until the

extremity of the neck of the retort is broken; but, when that is done,

the external air rushes in with a hissing noise. This operation is

attended with danger, unless a part of the air is driven out of the

retort, by means of heat, before it is hermetically sealed, as otherwise

the retort would be apt to burst by the dilation of the air when placed

in the furnace. The quantity of air driven out may be received under a

jar in the pneumato-chemical apparatus, by which its quantity, and that

of the air remaining in the retort, is ascertained. I have not

multiplied my experiments upon oxydation of metals so much as I could

have wished; neither have I obtained satisfactory results with any metal

except tin. It is much to be wished that some person would undertake a

series of experiments upon oxydation of metals in the several gasses;

the subject is important, and would fully repay any trouble which this

kind of experiment might occasion.

As all the oxyds of mercury are capable of revivifying without addition,

and restore the oxygen gas they had before absorbed, this seemed to be

the most proper metal for becoming the subject of conclusive experiments

upon oxydation. I formerly endeavoured to accomplish the oxydation of

mercury in close vessels, by filling a retort, containing a small

quantity of mercury, with oxygen gas, and adapting a bladder half full

of the same gas to its beak; See Pl. IV. Fig. 12. Afterwards, by heating

the mercury in the retort for a very long time, I succeeded in oxydating

a very small portion, so as to form a little red oxyd floating upon the

surface of the running mercury; but the quantity was so small, that the

smallest error committed in the determination of the quantities of

oxygen gas before and after the operation must have thrown very great

uncertainty upon the results of the experiment. I was, besides,

dissatisfied with this process, and not without cause, lest any air

might have escaped through the pores of the bladder, more especially as

it becomes shrivelled by the heat of the furnace, unless covered over

with cloths kept constantly wet.

This experiment is performed with more certainty in the apparatus

described in the Memoirs of the Academy for 1775, p. 580. This consists

of a retort, A, Pl. IV. Fig. 2. having a crooked glass tube BCDE of ten

or twelve lines internal diameter, melted on to its beak, and which is

engaged under the bell glass FG, standing with its mouth downwards, in a

bason filled with water or mercury. The retort is placed upon the bars

of the furnace MMNN, Pl. IV. Fig. 2. or in a sand bath, and by means of

this apparatus we may, in the course of several days, oxydate a small

quantity of mercury in common air; the red oxyd floats upon the surface,

from which it may be collected and revivified, so as to compare the

quantity of oxygen gas obtained in revivification with the absorption

which took place during oxydation. This kind of experiment can only be

performed upon a small scale, so that no very certain conclusions can be

drawn from them[61].

The combustion of iron in oxygen gas being a true oxydation of that

metal, ought to be mentioned in this place. The apparatus employed by Mr

Ingenhousz for this operation is represented in Pl. IV. Fig. 17.; but,

having already described it sufficiently in Chap. III. I shall refer the

reader to what is said of it in that place. Iron may likewise be

oxydated by combustion in vessels filled with oxygen gas, in the way

already directed for phosphorus and charcoal. This apparatus is

represented Pl. IV. Fig. 3. and described in the fifth chapter of the

first part of this work. We learn from Mr Ingenhousz, that all the

metals, except gold, silver, and mercury, may be burnt or oxydated in

the same manner, by reducing them into very fine wire, or very thin

plates cut into narrow slips; these are twisted round with iron-wire,

which communicates the property of burning to the other metals.

Mercury is even difficultly oxydated in free air. In chemical

laboratories, this process is usually carried on in a matrass A, Pl. IV.

Fig. having a very flat body, and a very long neck BC, which vessel is

commonly called _Boyle's bell_. A quantity of mercury is introduced

sufficient to cover the bottom, and it is placed in a sand-bath, which

keeps up a constant heat approaching to that of boiling mercury. By

continuing this operation with five or six similar matrasses during

several months, and renewing the mercury from time to time, a few

ounces of red oxyd are at last obtained. The great slowness and

inconvenience of this apparatus arises from the air not being

sufficiently renewed; but if, on the other hand, too free a circulation

were given to the external air, it would carry off the mercury in

solution in the state of vapour, so that in a few days none would remain

in the vessel.

As, of all the experiments upon the oxydation of metals, those with

mercury are the most conclusive, it were much to be wished that a simple

apparatus could be contrived by which this oxydation and its results

might be demonstrated in public courses of chemistry. This might, in my

opinion, be accomplished by methods similar to those I have already

described for the combustion of charcoal and the oils; but, from other

pursuits, I have not been able hitherto to resume this kind of

experiment.

The oxyd of mercury revives without addition, by being heated to a

slightly red heat. In this degree of temperature, oxygen has greater

affinity to caloric than to mercury, and forms oxygen gas. This is

always mixed with a small portion of azotic gas, which indicates that

the mercury absorbs a small portion of this latter gas during oxydation.

It almost always contains a little carbonic acid gas, which must

undoubtedly be attributed to the foulnesses of the oxyd; these are

charred by the heat, and convert a part of the oxygen gas into carbonic

acid.