Description of the Instruments and Operations of Chemistry.
INTRODUCTION.
In the two former parts of this work I designedly avoided being
particular in describing the manual operations of chemistry, because I
had found from experience, that, in a work appropriated to reasoning,
minute descriptions of processes and of plates interrupt the chain of
ideas, and render the attention necessary both difficult and tedious to
the reader. On the other hand, if I had confined myself to the summary
descriptions hitherto given, beginners could have only acquired very
vague conceptions of practical chemistry from my work, and must have
wanted both confidence and interest in operations they could neither
repeat nor thoroughly comprehend. This want could not have been
supplied from books; for, besides that there are not any which describe
the modern instruments and experiments sufficiently at large, any work
that could have been consulted would have presented these things under a
very different order of arrangement, and in a different chemical
language, which must greatly tend to injure the main object of my
performance.
Influenced by these motives, I determined to reserve, for a third part
of my work, a summary description of all the instruments and
manipulations relative to elementary chemistry. I considered it as
better placed at the end, rather than at the beginning of the book,
because I must have been obliged to suppose the reader acquainted with
circumstances which a beginner cannot know, and must therefore have read
the elementary part to become acquainted with. The whole of this third
part may therefore be considered as resembling the explanations of
plates which are usually placed at the end of academic memoirs, that
they may not interrupt the connection of the text by lengthened
description. Though I have taken great pains to render this part clear
and methodical, and have not omitted any essential instrument or
apparatus, I am far from pretending by it to set aside the necessity of
attendance upon lectures and laboratories, for such as wish to acquire
accurate knowledge of the science of chemistry. These should familiarise
themselves to the employment of apparatus, and to the performance of
experiments by actual experience. _Nihil est in intellectu quod non
prius fuerit in sensu_, the motto which the celebrated Rouelle caused to
be painted in large characters in a conspicuous part of his laboratory,
is an important truth never to be lost sight of either by teachers or
students of chemistry.
Chemical operations may be naturally divided into several classes,
according to the purposes they are intended for performing. Some may be
considered as purely mechanical, such as the determination of the weight
and bulk of bodies, trituration, levigation, searching, washing,
filtration, &c. Others may be considered as real chemical operations,
because they are performed by means of chemical powers and agents; such
are solution, fusion, &c. Some of these are intended for separating the
elements of bodies from each other, some for reuniting these elements
together; and some, as combustion, produce both these effects during the
same process.
Without rigorously endeavouring to follow the above method, I mean to
give a detail of the chemical operations in such order of arrangement as
seemed best calculated for conveying instruction. I shall be more
particular in describing the apparatus connected with modern chemistry,
because these are hitherto little known by men who have devoted much of
their time to chemistry, and even by many professors of the science.
CHAP. I.
_Of the Instruments necessary for determining the Absolute and Specific
Gravities of Solid and Liquid Bodies._
The best method hitherto known for determining the quantities of
substances submitted to chemical experiment, or resulting from them, is
by means of an accurately constructed beam and scales, with properly
regulated weights, which well known operation is called _weighing_. The
denomination and quantity of the weights used as an unit or standard for
this purpose are extremely arbitrary, and vary not only in different
kingdoms, but even in different provinces of the same kingdom, and in
different cities of the same province. This variation is of infinite
consequence to be well understood in commerce and in the arts; but, in
chemistry, it is of no moment what particular denomination of weight be
employed, provided the results of experiments be expressed in convenient
fractions of the same denomination. For this purpose, until all the
weights used in society be reduced to the same standard, it will be
sufficient for chemists in different parts to use the common pound of
their own country as the unit or standard, and to express all its
fractional parts in decimals, instead of the arbitrary divisions now in
use. By this means the chemists of all countries will be thoroughly
understood by each other, as, although the absolute weights of the
ingredients and products cannot be known, they will readily, and without
calculation, be able to determine the relative proportions of these to
each other with the utmost accuracy; so that in this way we shall be
possessed of an universal language for this part of chemistry.
With this view I have long projected to have the pound divided into
decimal fractions, and I have of late succeeded through the assistance
of Mr Fourche balance-maker at Paris, who has executed it for me with
great accuracy and judgment. I recommend to all who carry on experiments
to procure similar divisions of the pound, which they will find both
easy and simple in its application, with a very small knowledge of
decimal fractions[54].
As the usefulness and accuracy of chemistry depends entirely upon the
determination of the weights of the ingredients and products both before
and after experiments, too much precision cannot be employed in this
part of the subject; and, for this purpose, we must be provided with
good instruments. As we are often obliged, in chemical processes, to
ascertain, within a grain or less, the tare or weight of large and heavy
instruments, we must have beams made with peculiar niceness by accurate
workmen, and these must always be kept apart from the laboratory in some
place where the vapours of acids, or other corrosive liquors, cannot
have access, otherwise the steel will rust, and the accuracy of the
balance be destroyed. I have three sets, of different sizes, made by Mr
Fontin with the utmost nicety, and, excepting those made by Mr Ramsden
of London, I do not think any can compare with them for precision and
sensibility. The largest of these is about three feet long in the beam
for large weights, up to fifteen or twenty pounds; the second, for
weights of eighteen or twenty ounces, is exact to a tenth part of a
grain; and the smallest, calculated only for weighing about one gros, is
sensibly affected by the five hundredth part of a grain.
Besides these nicer balances, which are only used for experiments of
research, we must have others of less value for the ordinary purposes
of the laboratory. A large iron balance, capable of weighing forty or
fifty pounds within half a dram, one of a middle size, which may
ascertain eight or ten pounds, within ten or twelve grains, and a small
one, by which about a pound may be determined, within one grain.
We must likewise be provided with weights divided into their several
fractions, both vulgar and decimal, with the utmost nicety, and verified
by means of repeated and accurate trials in the nicest scales; and it
requires some experience, and to be accurately acquainted with the
different weights, to be able to use them properly. The best way of
precisely ascertaining the weight of any particular substance is to
weigh it twice, once with the decimal divisions of the pound, and
another time with the common subdivisions or vulgar fractions, and, by
comparing these, we attain the utmost accuracy.
By the specific gravity of any substance is understood the quotient of
its absolute weight divided by its magnitude, or, what is the same, the
weight of a determinate bulk of any body. The weight of a determinate
magnitude of water has been generally assumed as unity for this purpose;
and we express the specific gravity of gold, sulphuric acid, &c. by
saying, that gold is nineteen times, and sulphuric acid twice the weight
of water, and so of other bodies.
It is the more convenient to assume water as unity in specific
gravities, that those substances whose specific gravity we wish to
determine, are most commonly weighed in water for that purpose. Thus, if
we wish to determine the specific gravity of gold flattened under the
hammer, and supposing the piece of gold to weigh 8 oz. 4 gros 2-1/2
grs. in the air[55], it is suspended by means of a fine metallic wire
under the scale of a hydrostatic balance, so as to be entirely immersed
in water, and again weighed. The piece of gold in Mr Brisson's
experiment lost by this means 3 gros 37 grs.; and, as it is evident
that the weight lost by a body weighed in water is precisely equal to
the weight of the water displaced, or to that of an equal volume of
water, we may conclude, that, in equal magnitudes, gold weighs 4893-1/2
grs. and water 253 grs. which, reduced to unity, gives 1.0000 as the
specific gravity of water, and 19.3617 for that of gold. We may operate
in the same manner with all solid substances. We have rarely any
occasion, in chemistry, to determine the specific gravity of solid
bodies, unless when operating upon alloys or metallic glasses; but we
have very frequent necessity to ascertain that of fluids, as it is often
the only means of judging of their purity or degree of concentration.
This object may be very fully accomplished with the hydrostatic balance,
by weighing a solid body; such, for example, as a little ball of rock
cristal suspended by a very fine gold wire, first in the air, and
afterwards in the fluid whose specific gravity we wish to discover. The
weight lost by the cristal, when weighed in the liquor, is equal to that
of an equal bulk of the liquid. By repeating this operation successively
in water and different fluids, we can very readily ascertain, by a
simple and easy calculation, the relative specific gravities of these
fluids, either with respect to each other or to water. This method is
not, however, sufficiently exact, or, at least, is rather troublesome,
from its extreme delicacy, when used for liquids differing but little in
specific gravity from water; such, for instance, as mineral waters, or
any other water containing very small portions of salt in solution.
In some operations of this nature, which have not hitherto been made
public, I employed an instrument of great sensibility for this purpose
with great advantage. It consists of a hollow cylinder, A b c f, Pl.
vii. fig. 6. of brass, or rather of silver, loaded at its bottom, b c f,
with tin, as represented swimming in a jug of water, l m n o. To the
upper part of the cylinder is attached a stalk of silver wire, not more
than three fourths of a line diameter, surmounted by a little cup d,
intended for containing weights; upon the stalk a mark is made at g,
the use of which we shall presently explain. This cylinder may be made
of any size; but, to be accurate, ought at least to displace four pounds
of water. The weight of tin with which this instrument is loaded ought
to be such as will make it remain almost in equilibrium in distilled
water, and should not require more than half a dram, or a dram at most,
to make it sink to g.
We must first determine, with great precision, the exact weight of the
instrument, and the number of additional grains requisite for making it
sink, in distilled water of a determinate temperature, to the mark: We
then perform the same experiment upon all the fluids of which we wish to
ascertain the specific gravity, and, by means of calculation, reduce the
observed differences to a common standard of cubic feet, pints or
pounds, or of decimal fractions, comparing them with water. This method,
joined to experiments with certain reagents[56], is one of the best for
determining the quality of waters, and is even capable of pointing out
differences which escape the most accurate chemical analysis. I shall,
at some future period, give an account of a very extensive set of
experiments which I have made upon this subject.
These metallic hydrometers are only to be used for determining the
specific gravities of such waters as contain only neutral salts or
alkaline substances; and they may be constructed with different degrees
of ballast for alkohol and other spiritous liquors. When the specific
gravities of acid liquors are to be ascertained, we must use a glass
hydrometer, as represented Pl. vii. fig. 14[57]. This consists of a
hollow cylinder of glass, a b c f, hermetically sealed at its lower
end, and drawn out at the upper into a capillary tube a, ending in the
little cup or bason d. This instrument is ballasted with more or less
mercury, at the bottom of the cylinder introduced through the tube, in
proportion to the weight of the liquor intended to be examined: We may
introduce a small graduated slip of paper into the tube a d; and,
though these degrees do not exactly correspond to the fractions of
grains in the different liquors, they may be rendered very useful in
calculation.
What is said in this chapter may suffice, without farther enlargement,
for indicating the means of ascertaining the absolute and specific
gravities of solids and fluids, as the necessary instruments are
generally known, and may easily be procured: But, as the instruments I
have used for measuring the gasses are not any where described, I shall
give a more detailed account of these in the following chapter.
FOOTNOTES:
[54] Mr Lavoisier gives, in this part of his work, very accurate
directions for reducing the common subdivisions of the French pound into
decimal fractions, and _vice versa_, by means of tables subjoined to
this 3d part. As these instructions, and the table, would be useless to
the British chemist, from the difference between the subdivisions of the
French and Troy pounds, I have omitted them, but have subjoined in the
appendix accurate rules for converting the one into the other.--E.
[55] Vide Mr Brisson's Essay upon Specific Gravity, p. 5.--A.
[56] For the use of these reagents see Bergman's excellent treatise upon
the analysis of mineral waters, in his Chemical and Physical Essays.--E.
[57] Three or four years ago, I have seen similar glass hydrometers,
made for Dr Black by B. Knie, a very ingenious artist of this city.--E.
CHAP. II.
_Of Gazometry, or the Measurement of the Weight and Volume of Aeriform
Substances._
SECT. I.
_Description of the Pneumato-chemical Apparatus._
The French chemists have of late applied the name of _pneumato-chemical
apparatus_ to the very simple and ingenious contrivance, invented by Dr
Priestley, which is now indispensibly necessary to every laboratory.
This consists of a wooden trough, of larger or smaller dimensions as is
thought convenient, lined with plate-lead or tinned copper, as
represented in perspective, Pl. V. In Fig. 1. the same trough or cistern
is supposed to have two of its sides cut away, to show its interior
construction more distinctly. In this apparatus, we distinguish between
the shelf ABCD Fig. 1. and 2. and the bottom or body of the cistern FGHI
Fig. 2. The jars or bell-glasses are filled with water in this deep
part, and, being turned with their mouths downwards, are afterwards set
upon the shelf ABCD, as shown Plate X. Fig. 1. F. The upper parts of the
sides of the cistern above the level of the shelf are called the _rim_
or _borders_.
The cistern ought to be filled with water, so as to stand at least an
inch and a half deep upon the shelf, and it should be of such dimensions
as to admit of at least one foot of water in every direction in the
well. This size is sufficient for ordinary occasions; but it is often
convenient, and even necessary, to have more room; I would therefore
advise such as intend to employ themselves usefully in chemical
experiments, to have this apparatus made of considerable magnitude,
where their place of operating will allow. The well of my principal
cistern holds four cubical feet of water, and its shelf has a surface of
fourteen square feet; yet, in spite of this size, which I at first
thought immoderate, I am often straitened for room.
In laboratories, where a considerable number of experiments are
performed, it is necessary to have several lesser cisterns, besides the
large one, which may be called the _general magazine_; and even some
portable ones, which may be moved when necessary, near a furnace, or
wherever they may be wanted. There are likewise some operations which
dirty the water of the apparatus, and therefore require to be carried
on in cisterns by themselves.
It were doubtless considerably cheaper to use cisterns, or iron-bound
tubs, of wood simply dove-tailed, instead of being lined with lead or
copper; and in my first experiments I used them made in that way; but I
soon discovered their inconvenience. If the water be not always kept at
the same level, such of the dovetails as are left dry shrink, and, when
more water is added, it escapes through the joints, and runs out.
We employ cristal jars or bell glasses, Pl. V. Fig. 9. A. for containing
the gasses in this apparatus; and, for transporting these, when full of
gas, from one cistern to another, or for keeping them in reserve when
the cistern is too full, we make use of a flat dish BC, surrounded by a
standing up rim or border, with two handles DE for carrying it by.
After several trials of different materials, I have found marble the
best substance for constructing the mercurial pneumato-chemical
apparatus, as it is perfectly impenetrable by mercury, and is not
liable, like wood, to separate at the junctures, or to allow the mercury
to escape through chinks; neither does it run the risk of breaking, like
glass, stone-ware, or porcelain. Take a block of marble BCDE, Plate V.
Fig. 3. and 4. about two feet long, 15 or 18 inches broad, and ten
inches thick, and cause it to be hollowed out as at m n Fig. 5. about
four inches deep, as a reservoir for the mercury; and, to be able more
conveniently to fill the jars, cut the gutter T V, Fig. 3. 4. and 5. at
least four inches deeper; and, as this trench may sometimes prove
troublesome, it is made capable of being covered at pleasure by thin
boards, which slip into the grooves x y, Fig. 5. I have two marble
cisterns upon this construction, of different sizes, by which I can
always employ one of them as a reservoir of mercury, which it preserves
with more safety than any other vessel, being neither subject to
overturn, nor to any other accident. We operate with mercury in this
apparatus exactly as with water in the one before described; but the
bell-glasses must be of smaller diameter, and much stronger; or we may
use glass tubes, having their mouths widened, as in Fig. 7.; these are
called _eudiometers_ by the glass-men who sell them. One of the
bell-glasses is represented Fig. 5. A. standing in its place, and what
is called a jar is engraved Fig. 6.
The mercurial pneumato-chemical apparatus is necessary in all
experiments wherein the disengaged gasses are capable of being absorbed
by water, as is frequently the case, especially in all combinations,
excepting those of metals, in fermentation, &c.
SECT. II.
_Of the Gazometer._
I give the name of _gazometer_ to an instrument which I invented, and
caused construct, for the purpose of a kind of bellows, which might
furnish an uniform and continued stream of oxygen gas in experiments of
fusion. Mr Meusnier and I have since made very considerable corrections
and additions, having converted it into what may be called an _universal
instrument_, without which it is hardly possible to perform most of the
very exact experiments. The name we have given the instrument indicates
its intention for measuring the volume or quantity of gas submitted to
it for examination.
It consists of a strong iron beam, DE, Pl. VIII. Fig. 1. three feet
long, having at each end, D and E, a segment of a circle, likewise
strongly constructed of iron, and very firmly joined. Instead of being
poised as in ordinary balances, this beam rests, by means of a
cylindrical axis of polished steel, F, Fig. 9. upon two large moveable
brass friction-wheels, by which the resistance to its motion from
friction is considerably diminished, being converted into friction of
the second order. As an additional precaution, the parts of these wheels
which support the axis of the beam are covered with plates of polished
rock-cristal. The whole of this machinery is fixed to the top of the
solid column of wood BC, Fig. 1. To one extremity D of the beam, a scale
P for holding weights is suspended by a flat chain, which applies to the
curvature of the arc nDo, in a groove made for the purpose. To the
other extremity E of the beam is applied another flat chain, i k m, so
constructed, as to be incapable of lengthening or shortening, by being
less or more charged with weight; to this chain, an iron trivet, with
three branches, a i, c i, and h i, is strongly fixed at i, and
these branches support a large inverted jar A, of hammered copper, of
about 18 inches diameter, and 20 inches deep. The whole of this machine
is represented in perspective, Pl. VIII. Fig. 1. and Pl. IX. Fig. 2. and
4. give perpendicular sections, which show its interior structure.
Round the bottom of the jar, on its outside, is fixed (Pl. IX. Fig. 2.)
a border divided into compartments 1, 2, 3, 4, &c. intended to receive
leaden weights separately represented 1, 2, 3, Fig. 3. These are
intended for increasing the weight of the jar when a considerable
pressure is requisite, as will be afterwards explained, though such
necessity seldom occurs. The cylindrical jar A is entirely open below,
de, Pl. IX. Fig. 4.; but is closed above with a copper lid, a b c,
open at b f, and capable of being shut by the cock g. This lid, as may
be seen by inspecting the figures, is placed a few inches within the top
of the jar to prevent the jar from being ever entirely immersed in the
water, and covered over. Were I to have this instrument made over again,
I should cause the lid to be considerably more flattened, so as to be
almost level. This jar or reservoir of air is contained in the
cylindrical copper vessel, LMNO, Pl. VIII. Fig. 1. filled with water.
In the middle of the cylindrical vessel LMNO, Pl. IX. Fig. 4. are placed
two tubes st, xy, which are made to approach each other at their upper
extremities t y; these are made of such a length as to rise a little
above the upper edge LM of the vessel LMNO, and when the jar abcde
touches the bottom NO, their upper ends enter about half an inch into
the conical hollow b, leading to the stop-cock g.
The bottom of the vessel LMNO is represented Pl. IX. Fig. 3. in the
middle of which a small hollow semispherical cap is soldered, which may
be considered as the broad end of a funnel reversed; the two tubes st,
xy, Fig. 4. are adapted to this cap at s and x, and by this means
communicate with the tubes mm, nn, oo, pp, Fig. 3. which are fixed
horizontally upon the bottom of the vessel, and all of which terminate
in, and are united by, the spherical cap sx. Three of these tubes are
continued out of the vessel, as in Pl. VIII. Fig. 1. The first marked in
that figure 1, 2, 3, is inserted at its extremity 3, by means of an
intermediate stop-cock 4, to the jar V. which stands upon the shelf of a
small pneumato-chemical apparatus GHIK, the inside of which is shown Pl.
IX. Fig. 1. The second tube is applied against the outside of the vessel
LMNO from 6 to 7, is continued at 8, 9, 10, and at 11 is engaged below
the jar V. The former of these tubes is intended for conveying gas into
the machine, and the latter for conducting small quantities for trials
under jars. The gas is made either to flow into or out of the machine,
according to the degree of pressure it receives; and this pressure is
varied at pleasure, by loading the scale P less or more, by means of
weights. When gas is to be introduced into the machine, the pressure is
taken off, or even rendered negative; but, when gas is to be expelled, a
pressure is made with such degree of force as is found necessary.
The third tube 12, 13, 14, 15, is intended for conveying air or gas to
any necessary place or apparatus for combustions, combinations, or any
other experiment in which it is required.
To explain the use of the fourth tube, I must enter into some
discussions. Suppose the vessel LMNO, Pl. VIII. Fig. 1. full of water,
and the jar A partly filled with gas, and partly with water; it is
evident that the weights in the bason P may be so adjusted, as to
occasion an exact equilibrium between the weight of the bason and of the
jar, so that the external air shall not tend to enter into the jar, nor
the gas to escape from it; and in this case the water will stand exactly
at the same level both within and without the jar. On the contrary, if
the weight in the bason P be diminished, the jar will then press
downwards from its own gravity, and the water will stand lower within
the jar than it does without; in this case, the included air or gas will
suffer a degree of compression above that experienced by the external
air, exactly proportioned to the weight of a column of water, equal to
the difference of the external and internal surfaces of the water. From
these reflections, Mr Meusnier contrived a method of determining the
exact degree of pressure to which the gas contained in the jar is at any
time exposed. For this purpose, he employs a double glass syphon 19, 20,
21, 22, 23, firmly cemented at 19 and 23. The extremity 19 of this
syphon communicates freely with the water in the external vessel of the
machine, and the extremity 23 communicates with the fourth tube at the
bottom of the cylindrical vessel, and consequently, by means of the
perpendicular tube st, Pl. IX. Fig. 4. with the air contained in the
jar. He likewise cements, at 16, Pl. VIII. Fig. 1. another glass tube
16, 17, 18, which communicates at 16 with the water in the exterior
vessel LMNO, and, at its upper end 18, is open to the external air.
By these several contrivances, it is evident that the water must stand
in the tube 16, 17, 18, at the same level with that in the cistern LMNO;
and, on the contrary, that, in the branch 19, 20, 21, it must stand
higher or lower, according as the air in the jar is subjected to a
greater or lesser pressure than the external air. To ascertain these
differences, a brass scale divided into inches and lines is fixed
between these two tubes. It is readily conceived that, as air, and all
other elastic fluids, must increase in weight by compression, it is
necessary to know their degree of condensation to be enabled to
calculate their quantities, and to convert the measure of their volumes
into correspondent weights; and this object is intended to be fulfilled
by the contrivance now described.
But, to determine the specific gravity of air or of gasses, and to
ascertain their weight in a known volume, it is necessary to know their
temperature, as well as the degree of pressure under which they subsist;
and this is accomplished by means of a small thermometer, strongly
cemented into a brass collet, which screws into the lid of the jar A.
This thermometer is represented separately, Pl. VIII. Fig. 10. and in
its place 24, 25, Fig. 1. and Pl. IX. Fig. 4. The bulb is in the inside
of the jar A, and its graduated stalk rises on the outside of the lid.
The practice of gazometry would still have laboured under great
difficulties, without farther precautions than those above described.
When the jar A sinks in the water of the cistern LMNO, it must lose a
weight equal to that of the water which it displaces; and consequently
the compression which it makes upon the contained air or gas must be
proportionally diminished. Hence the gas furnished, during experiments
from the machine, will not have the same density towards the end that it
had at the beginning, as its specific gravity is continually
diminishing. This difference may, it is true, be determined by
calculation; but this would have occasioned such mathematical
investigations as must have rendered the use of this apparatus both
troublesome and difficult. Mr Meusnier has remedied this inconvenience
by the following contrivance. A square rod of iron, 26, 27, Pl. VIII.
Fig. 1. is raised perpendicular to the middle of the beam DE. This rod
passes through a hollow box of brass 28, which opens, and may be filled
with lead; and this box is made to slide alongst the rod, by means of a
toothed pinion playing in a rack, so as to raise or lower the box, and
to fix it at such places as is judged proper.
When the lever or beam DE stands horizontal, this box gravitates to
neither side; but, when the jar A sinks into the cistern LMNO, so as to
make the beam incline to that side, it is evident the loaded box 28,
which then passes beyond the center of suspension, must gravitate to the
side of the jar, and augment its pressure upon the included air. This is
increased in proportion as the box is raised towards 27, because the
same weight exerts a greater power in proportion to the length of the
lever by which it acts. Hence, by moving the box 28 alongst the rod 26,
27, we can augment or diminish the correction it is intended to make
upon the pressure of the jar; and both experience and calculation show
that this may be made to compensate very exactly for the loss of weight
in the jar at all degrees of pressure.
I have not hitherto explained the most important part of the use of this
machine, which is the manner of employing it for ascertaining the
quantities of the air or gas furnished during experiments. To determine
this with the most rigorous precision, and likewise the quantity
supplied to the machine from experiments, we fixed to the arc which
terminates the arm of the beam E, Pl. VIII. Fig. 1. the brass sector l
m, divided into degrees and half degrees, which consequently moves in
common with the beam; and the lowering of this end of the beam is
measured by the fixed index 29, 30, which has a Nonius giving hundredth
parts of a degree at its extremity 30.
The whole particulars of the different parts of the above described
machine are represented in Plate VIII. as follow.
Fig. 2. Is the flat chain invented by Mr Vaucanson, and employed for
suspending the scale or bason P, Fig. 1; but, as this lengthens or
shortens according as it is more or less loaded, it would not have
answered for suspending the jar A, Fig. 1.
Fig. 5. Is the chain i k m, which in Fig. 1. sustains the jar A. This
is entirely formed of plates of polished iron interlaced into each
other, and held together by iron pins. This chain does not lengthen in
any sensible degree, by any weight it is capable of supporting.
Fig. 6. The trivet, or three branched stirrup, by which the jar A is
hung to the balance, with the screw by which it is fixed in an
accurately vertical position.
Fig. 3. The iron rod 26, 27, which is fixed perpendicular to the center
of the beam, with its box 28.
Fig. 7. & 8. The friction-wheels, with the plates of rock-cristal Z, as
points of contact by which the friction of the axis of the lever of the
balance is avoided.
Fig. 4. The piece of metal which supports the axis of the
friction-wheels.
Fig. 9. The middle of the lever or beam, with the axis upon which it
moves.
Fig. 10. The thermometer for determining the temperature of the air or
gas contained in the jar.
When this gazometer is to be used, the cistern or external vessel, LMNO,
Pl. VIII. Fig. 1. is to be filled with water to a determinate height,
which should be the same in all experiments. The level of the water
should be taken when the beam of the balance stands horizontal; this
level, when the jar is at the bottom of the cistern, is increased by all
the water which it displaces, and is diminished in proportion as the jar
rises to its highest elevation. We next endeavour, by repeated trials,
to discover at what elevation the box 28 must be fixed, to render the
pressure equal in all situations of the beam. I should have said nearly,
because this correction is not absolutely rigorous; and differences of a
quarter, or even of half a line, are not of any consequence. This height
of the box 28 is not the same for every degree of pressure, but varies
according as this is of one, two, three, or more inches. All these
should be registered with great order and precision.
We next take a bottle which holds eight or ten pints, the capacity of
which is very accurately determined by weighing the water it is capable
of containing. This bottle is turned bottom upwards, full of water, in
the cistern of the pneumato chemical apparatus GHIK, Fig. 1. and is set
on its mouth upon the shelf of the apparatus, instead of the glass jar
V, having the extremity 11 of the tube 7, 8, 9, 10, 11, inserted into
its mouth. The machine is fixed at zero of pressure, and the degree
marked by the index 30 upon the sector m l is accurately observed;
then, by opening the stop-cock 8, and pressing a little upon the jar A,
as much air is forced into the bottle as fills it entirely. The degree
marked by the index upon the sector is now observed, and we calculate
what number of cubical inches correspond to each degree. We then fill a
second and third bottle, and so on, in the same manner, with the same
precautions, and even repeat the operation several times with bottles of
different sizes, till at last, by accurate attention, we ascertain the
exact gage or capacity of the jar A, in all its parts; but it is better
to have it formed at first accurately cylindrical, by which we avoid
these calculations and estimates.
The instrument I have been describing was constructed with great
accuracy and uncommon skill by Mr Meignie junior, engineer and physical
instrument-maker. It is a most valuable instrument, from the great
number of purposes to which it is applicable; and, indeed, there are
many experiments which are almost impossible to be performed without it.
It becomes expensive, because, in many experiments, such as the
formation of water and of nitric acid, it is absolutely necessary to
employ two of the same machines. In the present advanced state of
chemistry, very expensive and complicated instruments are become
indispensibly necessary for ascertaining the analysis and synthesis of
bodies with the requisite precision as to quantity and proportion; it is
certainly proper to endeavour to simplify these, and to render them less
costly; but this ought by no means to be attempted at the expence of
their conveniency of application, and much less of their accuracy.
SECT. III.
_Some other methods of measuring the volume of Gasses._
The gazometer described in the foregoing section is too costly and too
complicated for being generally used in laboratories for measuring the
gasses, and is not even applicable to every circumstance of this kind.
In numerous series of experiments, more simple and more readily
applicable methods must be employed. For this purpose I shall describe
the means I used before I was in possession of a gazometer, and which I
still use in preference to it in the ordinary course of my experiments.
Suppose that, after an experiment, there is a residuum of gas, neither
absorbable by alkali nor water, contained in the upper part of the jar
AEF, Pl. IV. Fig. 3. standing on the shelf of a pneumato-chemical
apparatus, of which we wish to ascertain the quantity, we must first
mark the height to which the mercury or water rises in the jar with
great exactness, by means of slips of paper pasted in several parts
round the jar. If we have been operating in mercury, we begin by
displacing the mercury from the jar, by introducing water in its stead.
This is readily done by filling a bottle quite full of water; having
stopped it with your finger, turn it up, and introduce its mouth below
the edge of the jar; then, turning down its body again, the mercury, by
its gravity, falls into the bottle, and the water rises in the jar, and
takes the place occupied by the mercury. When this is accomplished, pour
so much water into the cistern ABCD as will stand about an inch over the
surface of the mercury; then pass the dish BC, Pl. V. Fig. 9. under the
jar, and carry it to the water cistern, Fig. 1. and 2. We here exchange
the gas into another jar, which has been previously graduated in the
manner to be afterwards described; and we thus judge of the quantity or
volume of the gas by means of the degrees which it occupies in the
graduated jar.
There is another method of determining the volume of gas, which may
either be substituted in place of the one above described, or may be
usefully employed as a correction or proof of that method. After the air
or gas is exchanged from the first jar, marked with slips of paper, into
the graduated jar, turn up the mouth of the marked jar, and fill it with
water exactly to the marks EF, Pl. IV. Fig. 3. and by weighing the water
we determine the volume of the air or gas it contained, allowing one
cubical foot, or 1728 cubical inches, of water for each 70 pounds,
French weight.
The manner of graduating jars for this purpose is very easy, and we
ought to be provided with several of different sizes, and even several
of each size, in case of accidents. Take a tall, narrow, and strong
glass jar, and, having filled it with water in the cistern, Pl. V. Fig.
1. place it upon the shelf ABCD; we ought always to use the same place
for this operation, that the level of the shelf may be always exactly
similar, by which almost the only error to which this process is liable
will be avoided. Then take a narrow mouthed phial which holds exactly 6
oz. 3 gros 61 grs. of water, which corresponds to 10 cubical
inches. If you have not one exactly of this dimension, choose one a
little larger, and diminish its capacity to the size requisite, by
dropping in a little melted wax and rosin. This bottle serves the
purpose of a standard for gaging the jars. Make the air contained in
this bottle pass into the jar, and mark exactly the place to which the
water has descended; add another measure of air, and again mark the
place of the water, and so on, till all the water be displaced. It is of
great consequence that, during the course of this operation, the bottle
and jar be kept at the same temperature with the water in the cistern;
and, for this reason, we must avoid keeping the hands upon either as
much as possible; or, if we suspect they have been heated, we must cool
them by means of the water in the cistern. The height of the barometer
and thermometer during this experiment is of no consequence.
When the marks have been thus ascertained upon the jar for every ten
cubical inches, we engrave a scale upon one of its sides, by means of a
diamond pencil. Glass tubes are graduated in the same manner for using
in the mercurial apparatus, only they must be divided into cubical
inches, and tenths of a cubical inch. The bottle used for gaging these
must hold 8 oz. 6 gros 25 grs. of mercury, which exactly
corresponds to a cubical inch of that metal.
The method of determining the volume of air or gas, by means of a
graduated jar, has the advantage of not requiring any correction for the
difference of height between the surface of the water within the jar,
and in the cistern; but it requires corrections with respect to the
height of the barometer and thermometer. But, when we ascertain the
volume of air by weighing the water which the jar is capable of
containing, up to the marks EF, it is necessary to make a farther
correction, for the difference between the surface of the water in the
cistern, and the height to which it rises within the jar. This will be
explained in the fifth section of this chapter.
SECT. IV.
_Of the method of Separating the different Gasses from each other._
As experiments often produce two, three, or more species of gas, it is
necessary to be able to separate these from each other, that we may
ascertain the quantity and species of each. Suppose that under the jar
A, Pl. IV. Fig. 3. is contained a quantity of different gasses mixed
together, and standing over mercury, we begin by marking with slips of
paper, as before directed, the height at which the mercury stands within
the glass; then introduce about a cubical inch of water into the jar,
which will swim over the surface of the mercury: If the mixture of gas
contains any muriatic or sulphurous acid gas, a rapid and considerable
absorption will instantly take place, from the strong tendency these two
gasses have, especially the former, to combine with, or be absorbed by
water. If the water only produces a slight absorption of gas hardly
equal to its own bulk, we conclude, that the mixture neither contains
muriatic acid, sulphuric acid, or ammoniacal gas, but that it contains
carbonic acid gas, of which water only absorbs about its own bulk. To
ascertain this conjecture, introduce some solution of caustic alkali,
and the carbonic acid gas will be gradually absorbed in the course of a
few hours; it combines with the caustic alkali or potash, and the
remaining gas is left almost perfectly free from any sensible residuum
of carbonic acid gas.
After each experiment of this kind, we must carefully mark the height at
which the mercury stands within the jar, by slips of paper pasted on,
and varnished over when dry, that they may not be washed off when placed
in the water apparatus. It is likewise necessary to register the
difference between the surface of the mercury in the cistern and that in
the jar, and the height of the barometer and thermometer, at the end of
each experiment.
When all the gas or gasses absorbable by water and potash are absorbed,
water is admitted into the jar to displace the mercury; and, as is
described in the preceding section, the mercury in the cistern is to be
covered by one or two inches of water. After this, the jar is to be
transported by means of the flat dish BC, Pl. V. Fig. 9. into the water
apparatus; and the quantity of gas remaining is to be ascertained by
changing it into a graduated jar. After this, small trials of it are to
be made by experiments in little jars, to ascertain nearly the nature of
the gas in question. For instance, into a small jar full of the gas,
Fig. 8. Pl. V. a lighted taper is introduced; if the taper is not
immediately extinguished, we conclude the gas to contain oxygen gas;
and, in proportion to the brightness of the flame, we may judge if it
contain less or more oxygen gas than atmospheric air contains. If, on
the contrary, the taper be instantly extinguished, we have strong reason
to presume that the residuum is chiefly composed of azotic gas. If, upon
the approach of the taper, the gas takes fire and burns quietly at the
surface with a white flame, we conclude it to be pure hydrogen gas; if
this flame is blue, we judge it consists of carbonated hydrogen gas;
and, if it takes fire with a sudden deflagration, that it is a mixture
of oxygen and hydrogen gas. If, again, upon mixing a portion of the
residuum with oxygen gas, red fumes are produced, we conclude that it
contains nitrous gas.
These preliminary trials give some general knowledge of the properties
of the gas, and nature of the mixture, but are not sufficient to
determine the proportions and quantities of the several gasses of which
it is composed. For this purpose all the methods of analysis must be
employed; and, to direct these properly, it is of great use to have a
previous approximation by the above methods. Suppose, for instance, we
know that the residuum consists of oxygen and azotic gas mixed together,
put a determinate quantity, 100 parts, into a graduated tube of ten or
twelve lines diameter, introduce a solution of sulphuret of potash in
contact with the gas, and leave them together for some days; the
sulphuret absorbs the whole oxygen gas, and leaves the azotic gas pure.
If it is known to contain hydrogen gas, a determinate quantity is
introduced into Volta's eudiometer alongst with a known proportion of
hydrogen gas; these are deflagrated together by means of the electrical
spark; fresh portions of oxygen gas are successively added, till no
farther deflagration takes place, and till the greatest possible
diminution is produced. By this process water is formed, which is
immediately absorbed by the water of the apparatus; but, if the hydrogen
gas contain charcoal, carbonic acid is formed at the same time, which is
not absorbed so quickly; the quantity of this is readily ascertained by
assisting its absorption, by means of agitation. If the residuum
contains nitrous gas, by adding oxygen gas, with which it combines into
nitric acid, we can very nearly ascertain its quantity, from the
diminution produced by this mixture.
I confine myself to these general examples, which are sufficient to give
an idea of this kind of operations; a whole volume would not serve to
explain every possible case. It is necessary to become familiar with the
analysis of gasses by long experience; we must even acknowledge that
they mostly possess such powerful affinities to each other, that we are
not always certain of having separated them completely. In these cases,
we must vary our experiments in every possible point of view, add new
agents to the combination, and keep out others, and continue our trials,
till we are certain of the truth and exactitude of our conclusions.
SECT. V.
_Of the necessary corrections upon the volume of the Gasses, according
to the pressure of the Atmosphere._
All elastic fluids are compressible or condensible in proportion to the
weight with which they are loaded. Perhaps this law, which is
ascertained by general experience, may suffer some irregularity when
these fluids are under a degree of condensation almost sufficient to
reduce them to the liquid state, or when either in a state of extreme
rarefaction or condensation; but we seldom approach either of these
limits with most of the gasses which we submit to our experiments. I
understand this proposition of gasses being compressible, in proportion
to their superincumbent weights, as follows:
A barometer, which is an instrument generally known, is, properly
speaking, a species of syphon, ABCD, Pl. XII. Fig. 16. whose leg AB is
filled with mercury, whilst the leg CD is full of air. If we suppose the
branch CD indefinitely continued till it equals the height of our
atmosphere, we can readily conceive that the barometer is, in reality, a
sort of balance, in which a column of mercury stands in equilibrium
with a column of air of the same weight. But it is unnecessary to
prolongate the branch CD to such a height, as it is evident that the
barometer being immersed in air, the column of mercury AB will be
equally in equilibrium with a column of air of the same diameter, though
the leg CD be cut off at C, and the part CD be taken away altogether.
The medium height of mercury in equilibrium with the weight of a column
of air, from the highest part of the atmosphere to the surface of the
earth is about twenty-eight French inches in the lower parts of the city
of Paris; or, in other words, the air at the surface of the earth at
Paris is usually pressed upon by a weight equal to that of a column of
mercury twenty-eight inches in height. I must be understood in this way
in the several parts of this publication when talking of the different
gasses, as, for instance, when the cubical foot of oxygen gas is said to
weigh 1 oz. 4 gros, under 28 inches pressure. The height of this
column of mercury, supported by the pressure of the air, diminishes in
proportion as we are elevated above the surface of the earth, or rather
above the level of the sea, because the mercury can only form an
equilibrium with the column of air which is above it, and is not in the
smallest degree affected by the air which is below its level.
In what ratio does the mercury in the barometer descend in proportion to
its elevation? or, what is the same thing, according to what law or
ratio do the several strata of the atmosphere decrease in density? This
question, which has exercised the ingenuity of natural philosophers
during last century, is considerably elucidated by the following
experiment.
If we take the glass syphon ABCDE, Pl. XII. Fig. 17. shut at E, and open
at A, and introduce a few drops of mercury, so as to intercept the
communication of air between the leg AB and the leg BE, it is evident
that the air contained in BCDE is pressed upon, in common with the whole
surrounding air, by a weight or column of air equal to 28 inches of
mercury. But, if we pour 28 inches of mercury into the leg AB, it is
plain the air in the branch BCDE will now be pressed upon by a weight
equal to twice 28 inches of mercury, or twice the weight of the
atmosphere; and experience shows, that, in this case, the included air,
instead of filling the tube from B to E, only occupies from C to E, or
exactly one half of the space it filled before. If to this first column
of mercury we add two other portions of 28 inches each, in the branch
AB, the air in the branch BCDE will be pressed upon by four times the
weight of the atmosphere, or four times the weight of 28 inches of
mercury, and it will then only fill the space from D to E, or exactly
one quarter of the space it occupied at the commencement of the
experiment. From these experiments, which may be infinitely varied, has
been deduced as a general law of nature, which seems applicable to all
permanently elastic fluids, that they diminish in volume in proportion
to the weights with which they are pressed upon; or, in other words,
"_the volume of all elastic fluids is in the inverse ratio of the weight
by which they are compressed_."
The experiments which have been made for measuring the heights of
mountains by means of the barometer, confirm the truth of these
deductions; and, even supposing them in some degree inaccurate, these
differences are so extremely small, that they may be reckoned as
nullities in chemical experiments. When this law of the compression of
elastic fluids is once well understood, it becomes easily applicable to
the corrections necessary in pneumato chemical experiments upon the
volume of gas, in relation to its pressure. These corrections are of two
kinds, the one relative to the variations of the barometer, and the
other for the column of water or mercury contained in the jars. I shall
endeavour to explain these by examples, beginning with the most simple
case.
Suppose that 100 cubical inches of oxygen gas are obtained at 10 deg.
(54.5 deg.) of the thermometer, and at 28 inches 6 lines of the barometer,
it is required to know what volume the 100 cubical inches of gas would
occupy, under the pressure of 28 inches[58], and what is the exact
weight of the 100 inches of oxygen gas? Let the unknown volume, or the
number of inches this gas would occupy at 28 inches of the barometer, be
expressed by x; and, since the volumes are in the inverse ratio of
their superincumbent weights, we have the following statement: 100
cubical inches is to x inversely as 28.5 inches of pressure is to 28.0
inches; or directly 28 : 28.5 :: 100 : x = 101.786--cubical inches, at
28 inches barometrical pressure; that is to say, the same gas or air
which at 28.5 inches of the barometer occupies 100 cubical inches of
volume, will occupy 101.786 cubical inches when the barometer is at 28
inches. It is equally easy to calculate the weight of this gas,
occupying 100 cubical inches, under 28.5 inches of barometrical
pressure; for, as it corresponds to 101.786 cubical inches at the
pressure of 28, and as, at this pressure, and at 10 deg. (54.5 deg.) of
temperature, each cubical inch of oxygen gas weighs half a grain, it
follows, that 100 cubical inches, under 28.5 barometrical pressure, must
weigh 50.893 grains. This conclusion might have been formed more
directly, as, since the volume of elastic fluids is in the inverse ratio
of their compression, their weights must be in the direct ratio of the
same compression: Hence, since 100 cubical inches weigh 50 grains, under
the pressure of 28 inches, we have the following statement to determine
the weight of 100 cubical inches of the same gas as 28.5 barometrical
pressure, 28 : 50 :: 28.5 : x, the unknown quantity, = 50.893.
The following case is more complicated: Suppose the jar A, Pl. XII. Fig.
18. to contain a quantity of gas in its upper part ACD, the rest of the
jar below CD being full of mercury, and the whole standing in the
mercurial bason or reservoir GHIK, filled with mercury up to EF, and
that the difference between the surface CD of the mercury in the jar,
and EF, that in the cistern, is six inches, while the barometer stands
at 27.5 inches. It is evident from these data, that the air contained in
ACD is pressed upon by the weight of the atmosphere, diminished by the
weight of the column of mercury CE, or by 27.5 - 6 = 21.5 inches of
barometrical pressure. This air is therefore less compressed than the
atmosphere at the mean height of the barometer, and consequently
occupies more space than it would occupy at the mean pressure, the
difference being exactly proportional to the difference between the
compressing weights. If, then, upon measuring the space ACD, it is found
to be 120 cubical inches, it must be reduced to the volume which it
would occupy under the mean pressure of 28 inches. This is done by the
following statement: 120 : x, the unknown volume, :: 21.5 : 28
inversely; this gives x = 120 x 21.5 / 28 = 92.143 cubical inches.
In these calculations we may either reduce the height of the mercury in
the barometer, and the difference of level in the jar and bason, into
lines or decimal fractions of the inch; but I prefer the latter, as it
is more readily calculated. As, in these operations, which frequently
recur, it is of great use to have means of abbreviation, I have given a
table in the appendix for reducing lines and fractions of lines into
decimal fractions of the inch.
In experiments performed in the water-apparatus, we must make similar
corrections to procure rigorously exact results, by taking into account,
and making allowances for the difference of height of the water within
the jar above the surface of the water in the cistern. But, as the
pressure of the atmosphere is expressed in inches and lines of the
mercurial barometer, and, as homogeneous quantities only can be
calculated together, we must reduce the observed inches and lines of
water into correspondent heights of the mercury. I have given a table in
the appendix for this conversion, upon the supposition that mercury is
13.5681 times heavier than water.
SECT. VI.
_Of Corrections relative to the Degrees of the Thermometer._
In ascertaining the weight of gasses, besides reducing them to a mean of
barometrical pressure, as directed in the preceding section, we must
likewise reduce them to a standard thermometrical temperature; because,
all elastic fluids being expanded by heat, and condensed by cold, their
weight in any determinate volume is thereby liable to considerable
alterations. As the temperature of 10 deg. (54.5 deg.) is a medium between
the heat of summer and the cold of winter, being the temperature of
subterraneous places, and that which is most easily approached to at all
seasons, I have chosen that degree as a mean to which I reduce air or
gas in this species of calculation.
Mr de Luc found that atmospheric air was increased 1/215 part of its
bulk, by each degree of a mercurial thermometer, divided into 81
degrees, between the freezing and boiling points; this gives 1/211 part
for each degree of Reaumur's thermometer, which is divided into 80
degrees between these two points. The experiments of Mr Monge seem to
make this dilatation less for hydrogen gas, which he thinks is only
dilated 1/180. We have not any exact experiments hitherto published
respecting the ratio of dilatation of the other gasses; but, from the
trials which have been made, their dilatation seems to differ little
from that of atmospheric air. Hence I may take for granted, till farther
experiments give us better information upon this subject, that
atmospherical air is dilated 1/210 part, and hydrogen gas 1/190 part for
each degree of the thermometer; but, as there is still great uncertainty
upon this point, we ought always to operate in a temperature as near as
possible to the standard of 10 deg., (54.5 deg.) by this means any errors
in correcting the weight or volume of gasses by reducing them to the
common standard, will become of little moment.
The calculation for this correction is extremely easy. Divide the
observed volume of air by 210, and multiply the quotient by the degrees
of temperature above or below 10 deg. (54.5 deg.). This correction is
negative when the actual temperature is above the standard, and positive
when below. By the use of logarithmical tables this calculation is much
facilitated[59].
SECT. VII.
_Example for calculating the Corrections relative to the Variations of
Pressure and Temperature._
CASE.
In the jar A, Pl. IV. Fig. 3. standing in a water apparatus, is
contained 353 cubical inches of air; the surface of the water within the
jar at EF is 4-1/2 inches above the water in the cistern, the barometer
is at 27 inches 9-1/2 lines, and the thermometer at 15 deg. (65.75 deg.).
Having burnt a quantity of phosphorus in the air, by which concrete
phosphoric acid is produced, the air after the combustion occupies 295
cubical inches, the water within the jar stands 7 inches above that in
the cistern, the barometer is at 27 inches 9-1/4 lines, and the
thermometer at 16 deg. (68 deg.). It is required from these data to
determine the actual volume of air before and after combustion, and
the quantity absorbed during the process.
_Calculation before Combustion._
The air in the jar before combustion was 353 cubical inches, but it was
only under a barometrical pressure of 27 inches 9-1/2 lines; which,
reduced to decimal fractions by Tab. I. of the Appendix, gives 27.79167
inches; and from this we must deduct the difference of 4-1/2 inches of
water, which, by Tab. II. corresponds to 0.33166 inches of the
barometer; hence the real pressure of the air in the jar is 27.46001. As
the volume of elastic fluids diminish in the inverse ratio of the
compressing weights, we have the following statement to reduce the 353
inches to the volume the air would occupy at 28 inches barometrical
pressure.
353 : x, the unknown volume, :: 27.46001 : 28. Hence, x = 353 x
27.46001 / 28 = 346.192 cubical inches, which is the volume the same
quantity of air would have occupied at 28 inches of the barometer.
The 210th part of this corrected volume is 1.65, which, for the five
degrees of temperature above the standard gives 8.255 cubical inches;
and, as this correction is subtractive, the real corrected volume of the
air before combustion is 337.942 inches.
_Calculation after Combustion._
By a similar calculation upon the volume of air after combustion, we
find its barometrical pressure 27.77083 - 0.51593 = 27.25490. Hence, to
have the volume of air under the pressure of 28 inches, 295 : x ::
27.77083 : 28 inversely; or, x = 295 x 27.25490 / 28 = 287.150. The
210th part of this corrected volume is 1.368, which, multiplied by 6
degrees of thermometrical difference, gives the subtractive correction
for temperature 8.208, leaving the actual corrected volume of air after
combustion 278.942 inches.
_Result._
The corrected volume before combustion 337.942
Ditto remaining after combustion 278.942
--------
Volume absorbed during combustion 59.000.
SECT. VIII.
_Method of determining the Absolute Gravity of the different Gasses._
Take a large balloon A, Pl. V. Fig. 10. capable of holding 17 or 18
pints, or about half a cubical foot, having the brass cap bcde
strongly cemented to its neck, and to which the tube and stop-cock f g
is fixed by a tight screw. This apparatus is connected by the double
screw represented separately at Fig. 12. to the jar BCD, Fig. 10. which
must be some pints larger in dimensions than the balloon. This jar is
open at top, and is furnished with the brass cap h i, and stop-cock l
m. One of these slop-cocks is represented separately at Fig. 11.
We first determine the exact capacity of the balloon by filling it with
water, and weighing it both full and empty. When emptied of water, it is
dried with a cloth introduced through its neck d e, and the last
remains of moisture are removed by exhausting it once or twice in an
air-pump.
When the weight of any gas is to be ascertained, this apparatus is used
as follows: Fix the balloon A to the plate of an air-pump by means of
the screw of the stop-cock f g, which is left open; the balloon is to
be exhausted as completely as possible, observing carefully the degree
of exhaustion by means of the barometer attached to the air-pump. When
the vacuum is formed, the stop-cock f g is shut, and the weight of the
balloon determined with the most scrupulous exactitude. It is then fixed
to the jar BCD, which we suppose placed in water in the shelf of the
pneumato chemical apparatus Fig. 1.; the jar is to be filled with the
gas we mean to weigh, and then, by opening the stop-cocks f g and l
m, the gas ascends into the balloon, whilst the water of the cistern
rises at the same time into the jar. To avoid very troublesome
corrections, it is necessary, during this first part of the operation,
to sink the jar in the cistern till the surfaces of the water within the
jar and without exactly correspond. The stop-cocks are again shut, and
the balloon being unscrewed from its connection with the jar, is to be
carefully weighed; the difference between this weight and that of the
exhausted balloon is the precise weight of the air or gas contained in
the balloon. Multiply this weight by 1728, the number of cubical inches
in a cubical foot, and divide the product by the number of cubical
inches contained in the balloon, the quotient is the weight of a cubical
foot of the gas or air submitted to experiment.
Exact account must be kept of the barometrical height and temperature of
the thermometer during the above experiment; and from these the
resulting weight of a cubical foot is easily corrected to the standard
of 28 inches and 10 deg., as directed in the preceding section. The small
portion of air remaining in the balloon after forming the vacuum must
likewise be attended to, which is easily determined by the barometer
attached to the air-pump. If that barometer, for instance, remains at
the hundredth part of the height it stood at before the vacuum was
formed, we conclude that one hundredth part of the air originally
contained remained in the balloon, and consequently that only 99/100 of
gas was introduced from the jar into the balloon.
FOOTNOTES:
[58] According to the proportion of 114 to 107, given between the French
and English foot, 28 inches of the French barometer are equal to 29.83
inches of the English. Directions will be found in the appendix for
converting all the French weights and measures used in this work into
corresponding English denominations.--E.
[59] When Fahrenheit's thermometer is employed, the dilatation by each
degree must be smaller, in the proportion of 1 to 2.25, because each
degree of Reaumur's scale contains 2.25 degrees of Fahrenheit; hence we
must divide by 472.5, and finish the rest of the calculation as
above.--E.
CHAP. III.
_Description of the Calorimeter, or Apparatus for measuring Caloric._
The calorimeter, or apparatus for measuring the relative quantities of
heat contained in bodies, was described by Mr de la Place and me in the
Memoirs of the Academy for 1780, p. 355. and from that Essay the
materials of this chapter are extracted.
If, after having cooled any body to the freezing point, it be exposed in
an atmosphere of 25 deg. (88.25 deg.), the body will gradually become
heated, from the surface inwards, till at last it acquire the same
temperature with the surrounding air. But, if a piece of ice be placed
in the same situation, the circumstances are quite different; it does
not approach in the smallest degree towards the temperature of the
circumambient air, but remains constantly at Zero (32 deg.), or the
temperature of melting ice, till the last portion of ice be completely
melted.
This phenomenon is readily explained; as, to melt ice, or reduce it to
water, it requires to be combined with a certain portion of caloric;
the whole caloric attracted from the surrounding bodies, is arrested or
fixed at the surface or external layer of ice which it is employed to
dissolve, and combines with it to form water; the next quantity of
caloric combines with the second layer to dissolve it into water, and so
on successively till the whole ice be dissolved or converted into water
by combination with caloric, the very last atom still remaining at its
former temperature, because the caloric has never penetrated so far as
long as any intermediate ice remained to melt.
Upon these principles, if we conceive a hollow sphere of ice at the
temperature of Zero (32 deg.) placed in an atmosphere 10 deg. (54.5 deg.),
and containing a substance at any degree of temperature above freezing, it
follows, 1st, That the heat of the external atmosphere cannot penetrate
into the internal hollow of the sphere of ice; 2dly, That the heat of
the body placed in the hollow of the sphere cannot penetrate outwards
beyond it, but will be stopped at the internal surface, and continually
employed to melt successive layers of ice, until the temperature of the
body be reduced to Zero (32 deg.), by having all its superabundant caloric
above that temperature carried off by the ice. If the whole water,
formed within the sphere of ice during the reduction of the temperature
of the included body to Zero, be carefully collected, the weight of the
water will be exactly proportional to the quantity of caloric lost by
the body in passing from its original temperature to that of melting
ice; for it is evident that a double quantity of caloric would have
melted twice the quantity of ice; hence the quantity of ice melted is a
very exact measure of the quantity of caloric employed to produce that
effect, and consequently of the quantity lost by the only substance that
could possibly have supplied it.
I have made this supposition of what would take place in a hollow sphere
of ice, for the purpose of more readily explaining the method used in
this species of experiment, which was first conceived by Mr de la Place.
It would be difficult to procure such spheres of ices and inconvenient
to make use of them when got; but, by means of the following apparatus,
we have remedied that defect. I acknowledge the name of Calorimeter,
which I have given it, as derived partly from Greek and partly from
Latin, is in some degree open to criticism; but, in matters of science,
a slight deviation from strict etymology, for the sake of giving
distinctness of idea, is excusable; and I could not derive the name
entirely from Greek without approaching too near to the names of known
instruments employed for other purposes.
The calorimeter is represented in Pl. VI. It is shown in perspective at
Fig. 1. and its interior structure is engraved in Fig. 2. and 3.; the
former being a horizontal, and the latter a perpendicular section. Its
capacity or cavity is divided into three parts, which, for better
distinction, I shall name the interior, middle, and external cavities.
The interior cavity f f f f, Fig. 4. into which the substances
submitted to experiment are put, is composed of a grating or cage of
iron wire, supported by several iron bars; its opening or mouth LM, is
covered by the lid HG, of the same materials. The middle cavity b b b
b, Fig. 2. and 3. is intended to contain the ice which surrounds the
interior cavity, and which is to be melted by the caloric of the
substance employed in the experiment. The ice is supported by the grate
m m at the bottom of the cavity, under which is placed the sieve n
n. These two are represented separately in Fig. 5. and 6.
In proportion as the ice contained in the middle cavity is melted, by
the caloric disengaged from the body placed in the interior cavity, the
water runs through the grate and sieve, and falls through the conical
funnel c c d, Fig. 3. and tube x y, into the receiver F, Fig. 1.
This water may be retained or let out at pleasure, by means of the
stop-cock u. The external cavity a a a a, Fig. 2. and 3. is filled
with ice, to prevent any effect upon the ice in the middle cavity from
the heat of the surrounding air, and the water produced from it is
carried off through the pipe ST, which shuts by means of the stop-cock
r. The whole machine is covered by the lid FF, Fig. 7. made of tin
painted with oil colour, to prevent rust.
When this machine is to be employed, the middle cavity b b b b, Fig.
2. and 3., the lid GH, Fig. 4. of the interior cavity, the external
cavity a a a a, Fig. 2. and 3. and the general lid FF, Fig. 7. are all
filled with pounded ice, well rammed, so that no void spaces remain, and
the ice of the middle cavity is allowed to drain. The machine is then
opened, and the substance submitted to experiment being placed in the
interior cavity, it is instantly closed. After waiting till the included
body is completely cooled to the freezing point, and the whole melted
ice has drained from the middle cavity, the water collected in the
vessel F, Fig. 1. is accurately weighed. The weight of the water
produced during the experiment is an exact measure of the caloric
disengaged during the cooling of the included body, as this substance is
evidently in a similar situation with the one formerly mentioned as
included in a hollow sphere of ice; the whole caloric disengaged is
stopped by the ice in the middle cavity, and that ice is preserved from
being affected by any other heat by means of the ice contained in the
general lid, Fig. 7. and in the external cavity. Experiments of this
kind last from fifteen to twenty hours; they are sometimes accelerated
by covering up the substance in the interior cavity with well drained
ice, which hastens its cooling.
The substances to be operated upon are placed in the thin iron bucket,
Fig. 8. the cover of which has an opening fitted with a cork, into which
a small thermometer is fixed. When we use acids, or other fluids capable
of injuring the metal of the instruments, they are contained in the
matras, Fig. 10. which has a similar thermometer in a cork fitted to its
mouth, and which stands in the interior cavity upon the small
cylindrical support RS, Fig. 10.
It is absolutely requisite that there be no communication between the
external and middle cavities of the calorimeter, otherwise the ice
melted by the influence of the surrounding air, in the external cavity,
would mix with the water produced from the ice of the middle cavity,
which would no longer be a measure of the caloric lost by the substance
submitted to experiment.
When the temperature of the atmosphere is only a few degrees above the
freezing point, its heat can hardly reach the middle cavity, being
arrested by the ice of the cover, Fig. 7. and of the external cavity;
but, if the temperature of the air be under the degree of freezing, it
might cool the ice contained in the middle cavity, by causing the ice
in the external cavity to fall, in the first place, below zero (32 deg.).
It is therefore essential that this experiment be carried on in a
temperature somewhat above freezing: Hence, in time of frost, the
calorimeter must be kept in an apartment carefully heated. It is
likewise necessary that the ice employed be not under zero (32 deg.); for
which purpose it must be pounded, and spread out thin for some time, in
a place of a higher temperature.
The ice of the interior cavity always retains a certain quantity of
water adhering to its surface, which may be supposed to belong to the
result of the experiment; but as, at the beginning of each experiment,
the ice is already saturated with as much water as it can contain, if
any of the water produced by the caloric should remain attached to the
ice, it is evident, that very nearly an equal quantity of what adhered
to it before the experiment must have run down into the vessel F in its
stead; for the inner surface of the ice in the middle cavity is very
little changed during the experiment.
By any contrivance that could be devised, we could not prevent the
access of the external air into the interior cavity when the atmosphere
was 9 deg. or 10 deg. (52 deg. or 54 deg.) above zero. The air confined
in the cavity being in that case specifically heavier than the external
air, escapes downwards through the pipe x y, Fig. 3, and is replaced by
the warmer external air, which, giving out its caloric to the ice,
becomes heavier, and sinks in its turn; thus a current of air is formed
through the machine, which is the more rapid in proportion as the
external air exceeds the internal in temperature. This current of warm
air must melt a part of the ice, and injure the accuracy of the
experiment: We may, in a great degree, guard against this source of
error by keeping the stop-cock u continually shut; but it is better to
operate only when the temperature of the external air does not exceed
3 deg., or at most 4 deg., (39 deg. to 41 deg.); for we have observed,
that, in this case, the melting of the interior ice by the atmospheric
air is perfectly insensible; so that we may answer for the accuracy of
our experiments upon the specific heat of bodies to a fortieth part.
We have caused make two of the above described machines; one, which is
intended for such experiments as do not require the interior air to be
renewed, is precisely formed according to the description here given;
the other, which answers for experiments upon combustion, respiration,
&c. in which fresh quantities of air are indispensibly necessary,
differs from the former in having two small tubes in the two lids, by
which a current of atmospheric air may be blown into the interior cavity
of the machine.
It is extremely easy, with this apparatus, to determine the phenomena
which occur in operations where caloric is either disengaged or
absorbed. If we wish, for instance, to ascertain the quantity of caloric
which is disengaged from a solid body in cooling a certain number of
degrees, let its temperature be raised to 80 deg. (212 deg.); it is then
placed in the interior cavity f f f f, Fig. 2. and 3. of the calorimeter,
and allowed to remain till we are certain that its temperature is reduced
to zero (32 deg.); the water produced by melting the ice during its
cooling is collected, and carefully weighed; and this weight, divided
by the volume of the body submitted to experiment, multiplied into the
degrees of temperature which it had above zero at the commencement of the
experiment, gives the proportion of what the English philosophers call
specific heat.
Fluids are contained in proper vessels, whose specific heat has been
previously ascertained, and operated upon in the machine in the same
manner as directed for solids, taking care to deduct, from the quantity
of water melted during the experiment, the proportion which belongs to
the containing vessel.
If the quantity of caloric disengaged during the combination of
different substances is to be determined, these substances are to be
previously reduced to the freezing degree by keeping them a sufficient
time surrounded with pounded ice; the mixture is then to be made in the
inner cavity of the calorimeter, in a proper vessel likewise reduced to
zero (32 deg.); and they are kept inclosed till the temperature of the
combination has returned to the same degree: The quantity of water
produced is a measure of the caloric disengaged during the combination.
To determine the quantity of caloric disengaged during combustion, and
during animal respiration, the combustible bodies are burnt, or the
animals are made to breathe in the interior cavity, and the water
produced is carefully collected. Guinea pigs, which resist the effects
of cold extremely well, are well adapted for this experiment. As the
continual renewal of air is absolutely necessary in such experiments, we
blow fresh air into the interior cavity of the calorimeter by means of a
pipe destined for that purpose, and allow it to escape through another
pipe of the same kind; and that the heat of this air may not produce
errors in the results of the experiments, the tube which conveys it into
the machine is made to pass through pounded ice, that it may be reduced
to zero (32 deg.) before it arrives at the calorimeter. The air which
escapes must likewise be made to pass through a tube surrounded with
ice, included in the interior cavity of the machine, and the water which
is produced must make a part of what is collected, because the caloric
disengaged from this air is part of the product of the experiment.
It is somewhat more difficult to determine the specific caloric
contained in the different gasses, on account of their small degree of
density; for, if they are only placed in the calorimeter in vessels like
other fluids, the quantity of ice melted is so small, that the result of
the experiment becomes at best very uncertain. For this species of
experiment we have contrived to make the air pass through two metallic
worms, or spiral tubes; one of these, through which the air passes, and
becomes heated in its way to the calorimeter, is contained in a vessel
full of boiling water, and the other, through which the air circulates
within the calorimeter to disengage its caloric, is placed in the
interior cavity, f f f f, of that machine. By means of a small
thermometer placed at one end of the second worm, the temperature of the
air, as it enters the calorimeter, is determined, and its temperature in
getting out of the interior cavity is found by another thermometer
placed at the other end of the worm. By this contrivance we are enabled
to ascertain the quantity of ice melted by determinate quantities of air
or gas, while losing a certain number of degrees of temperature, and,
consequently, to determine their several degrees of specific caloric.
The same apparatus, with some particular precautions, may be employed
to ascertain the quantity of caloric disengaged by the condensation of
the vapours of different liquids.
The various experiments which may be made with the calorimeter do not
afford absolute conclusions, but only give us the measure of relative
quantities; we have therefore to fix a unit, or standard point, from
whence to form a scale of the several results. The quantity of caloric
necessary to melt a pound of ice has been chosen as this unit; and, as
it requires a pound of water of the temperature of 60 deg. (167 deg.) to
melt a pound of ice, the quantity of caloric expressed by our unit or
standard point is what raises a pound of water from zero (32 deg.) to
60 deg. (167 deg.). When this unit is once determined, we have only to
express the quantities of caloric disengaged from different bodies by
cooling a certain number of degrees, in analogous values: The following
is an easy mode of calculation for this purpose, applied to one of our
earliest experiments.
We took 7 lib. 11 oz. 2 gros 36 grs. of plate-iron, cut into
narrow slips, and rolled up, or expressing the quantity in decimals,
7.7070319. These, being heated in a bath of boiling water to about 78 deg.
(207.5 deg.), were quickly introduced into the interior cavity of the
calorimeter: At the end of eleven hours, when the whole quantity of
water melted from the ice had thoroughly drained off, we found that
1.109795 pounds of ice were melted. Hence, the caloric disengaged from
the iron by cooling 78 deg. (175.5 deg.) having melted 1.109795 pounds
of ice, how much would have been melted by cooling 60 deg. (135 deg.)?
This question gives the following statement in direct proportion, 78 :
1.109795 :: 60 : x = 0.85369. Dividing this quantity by the weight of
the whole iron employed, viz. 7.7070319, the quotient 0.110770 is the
quantity of ice which would have been melted by one pound of iron whilst
cooling through 60 deg. (135 deg.) of temperature.
Fluid substances, such as sulphuric and nitric acids, &c. are contained
in a matras, Pl. VI. Fig. 9. having a thermometer adapted to the cork,
with its bulb immersed in the liquid. The matras is placed in a bath of
boiling water, and when, from the thermometer, we judge the liquid is
raised to a proper temperature, the matras is placed in the calorimeter.
The calculation of the products, to determine the specific caloric of
these fluids, is made as above directed, taking care to deduct from the
water obtained the quantity which would have been produced by the matras
alone, which must be ascertained by a previous experiment. The table of
the results obtained by these experiments is omitted, because not yet
sufficiently complete, different circumstances having occasioned the
series to be interrupted; it is not, however, lost sight of; and we are
less or more employed upon the subject every winter.
CHAP. IV.
_Of Mechanical Operations for Division of Bodies._
SECT. I.
_Of Trituration, Levigation, and Pulverization._
These are, properly speaking, only preliminary mechanical operations for
dividing and separating the particles of bodies, and reducing them into
very fine powder. These operations can never reduce substances into
their primary, or elementary and ultimate particles; they do not even
destroy the aggregation of bodies; for every particle, after the most
accurate trituration, forms a small whole, resembling the original mass
from which it was divided. The real chemical operations, on the
contrary, such as solution, destroy the aggregation of bodies, and
separate their constituent and integrant particles from each other.
Brittle substances are reduced to powder by means of pestles and
mortars. These are of brass or iron, Pl. I. Fig. 1.; of marble or
granite, Fig. 2.; of lignum vitae, Fig. 3.; of glass, Fig. 4.; of agate,
Fig. 5.; or of porcellain, Fig. 6. The pestles for each of these are
represented in the plate, immediately below the mortars to which they
respectively belong, and are made of hammered iron or brass, of wood,
glass, porcellain, marble, granite, or agate, according to the nature of
the substances they are intended to triturate. In every laboratory, it
is requisite to have an assortment of these utensils, of various sizes
and kinds: Those of porcellain and glass can only be used for rubbing
substances to powder, by a dexterous use of the pestle round the sides
of the mortar, as it would be easily broken by reiterated blows of the
pestle.
The bottom of mortars ought to be in the form of a hollow sphere, and
their sides should have such a degree of inclination as to make the
substances they contain fall back to the bottom when the pestle is
lifted, but not so perpendicular as to collect them too much together,
otherwise too large a quantity would get below the pestle, and prevent
its operation. For this reason, likewise, too large a quantity of the
substance to be powdered ought not to be put into the mortar at one
time; and we must from time to time get rid of the particles already
reduced to powder, by means of sieves to be afterwards described.
The most usual method of levigation is by means of a flat table ABCD,
Pl. 1. Fig. 7. of porphyry, or other stone of similar hardness, upon
which the substance to be reduced to powder is spread, and is then
bruised and rubbed by a muller M, of the same hard materials, the bottom
of which is made a small portion of a large sphere; and, as the muller
tends continually to drive the substances towards the sides of the
table, a thin flexible knife, or spatula of iron, horn, wood, or ivory,
is used for bringing them back to the middle of the stone.
In large works, this operation is performed by means of large rollers of
hard stone, which turn upon each other, either horizontally, in the way
of corn-mills, or by one vertical roller turning upon a flat stone. In
the above operations, it is often requisite to moisten the substances a
little, to prevent the fine powder from flying off.
There are many bodies which cannot be reduced to powder by any of the
foregoing methods; such are fibrous substances, as woods; such as are
tough and elastic, as the horns of animals, elastic gum, &c. and the
malleable metals which flatten under the pestle, instead of being
reduced to powder. For reducing the woods to powder, rasps, as Pl. I.
Fig. 8. are employed; files of a finer kind are used for horn, and still
finer, Pl. 1. Fig. 9. and 10. for metals.
Some of the metals, though not brittle enough to powder under the
pestle, are too soft to be filed, as they clog the file, and prevent its
operation. Zinc is one of these, but it may be powdered when hot in a
heated iron mortar, or it may be rendered brittle, by alloying it with a
small quantity of mercury. One or other of these methods is used by
fire-work makers for producing a blue flame by means of zinc. Metals may
be reduced into grains, by pouring them when melted into water, which
serves very well when they are not wanted in fine powder.
Fruits, potatoes, &c. of a pulpy and fibrous nature may be reduced to
pulp by means of the grater, Pl. 1. Fig. 11.
The choice of the different substances of which these instruments are
made is a matter of importance; brass or copper are unfit for operations
upon substances to be used as food or in pharmacy; and marble or
metallic instruments must not be used for acid substances; hence mortars
of very hard wood, and those of porcelain, granite, or glass, are of
great utility in many operations.
SECT. II.
_Of Sifting and Washing Powdered Substances._
None of the mechanical operations employed for reducing bodies to powder
is capable of producing it of an equal degree of fineness throughout;
the powder obtained by the longest and most accurate trituration being
still an assemblage of particles of various sizes. The coarser of these
are removed, so as only to leave the finer and more homogeneous
particles by means of sieves, Pl. I. Fig. 12. 13. 14. 15. of different
finenesses, adapted to the particular purposes they are intended for;
all the powdered matter which is larger than the intestices of the sieve
remains behind, and is again submitted to the pestle, while the finer
pass through. The sieve Fig. 12. is made of hair-cloth, or of silk
gauze; and the one represented Fig. 13. is of parchment pierced with
round holes of a proper size; this latter is employed in the manufacture
of gun-powder. When very subtile or valuable materials are to be sifted,
which are easily dispersed, or when the finer parts of the powder may be
hurtful, a compound sieve, Fig. 15. is made use of, which consists of
the sieve ABCD, with a lid EF, and receiver GH; these three parts are
represented as joined together for use, Fig. 14.
There is a method of procuring powders of an uniform fineness,
considerably more accurate than the sieve; but it can only be used with
such substances as are not acted upon by water. The powdered substance
is mixed and agitated with water, or other convenient fluid; the liquor
is allowed to settle for a few moments, and is then decanted off; the
coarsest powder remains at the bottom of the vessel, and the finer
passes over with the liquid. By repeated decantations in this manner,
various sediments are obtained of different degrees of fineness; the
last sediment, or that which remains longed suspended in the liquor,
being the finest. This process may likewise be used with advantage for
separating substances of different degrees of specific gravity, though
of the same fineness; this last is chiefly employed in mining, for
separating the heavier metallic ores from the lighter earthy matters
with which they are mixed.
In chemical laboratories, pans and jugs of glass or earthen ware are
employed for this operation; sometimes, for decanting the liquor without
disturbing the sediment, the glass syphon ABCHI, Pl. II. Fig. 11. is
used, which may be supported by means of the perforated board DE, at the
proper depth in the vessel FG, to draw off all the liquor required into
the receiver LM. The principles and application of this useful
instrument are so well known as to need no explanation.
SECT. III.
_Of Filtration._
A filtre is a species of very fine sieve, which is permeable to the
particles of fluids, but through which the particles of the finest
powdered solids are incapable of passing; hence its use in separating
fine powders from suspension in fluids. In pharmacy, very close and fine
woollen cloths are chiefly used for this operation; these are commonly
formed in a conical shape, Pl. II. Fig. 2. which has the advantage of
uniting all the liquor which drains through into a point A, where it may
be readily collected in a narrow mouthed vessel. In large pharmaceutical
laboratories, this filtring bag is streached upon a wooden stand, Pl.
II. Fig. 1.
For the purposes of chemistry, as it is requisite to have the filtres
perfectly clean, unsized paper is substituted instead of cloth or
flannel; through this substance, no solid body, however finely it be
powdered, can penetrate, and fluids percolate through it with the
greatest readiness. As paper breaks easily when wet, various methods of
supporting it are used according to circumstances. When a large quantity
of fluid is to be filtrated, the paper is supported by the frame of
wood, Pl. II. Fig. 3. ABCD, having a piece of coarse cloth stretched
over it, by means of iron-hooks. This cloth must be well cleaned each
time it is used, or even new cloth must be employed, if there is reason
to suspect its being impregnated with any thing which can injure the
subsequent operations. In ordinary operations, where moderate quantities
of fluid are to be filtrated, different kinds of glass funnels are used
for supporting the paper, as represented Pl. II. Fig. 5. 6. and 7. When
several filtrations must be carried on at once, the board or shelf AB,
Fig. 9. supported upon stands C and D, and pierced with round holes, is
very convenient for containing the funnels.
Some liquors are so thick and clammy, as not to be able to penetrate
through paper without some previous preparation, such as clarification
by means of white of eggs, which being mixed with the liquor, coagulates
when brought to boil, and, entangling the greater part of the impurities
of the liquor, rises with them to the surface in the state of scum.
Spiritous liquors may be clarified in the same manner by means of
isinglass dissolved in water, which coagulates by the action of the
alkohol without the assistance of heat.
As most of the acids are produced by distillation, and are consequently
clear, we have rarely any occasion to filtrate them; but if, at any
time, concentrated acids require this operation, it is impossible to
employ paper, which would be corroded and destroyed by the acid. For
this purpose, pounded glass, or rather quartz or rock-cristal, broke in
pieces and grossly powdered, answers very well; a few of the larger
pieces are put in the neck of the funnel; these are covered with the
smaller pieces, the finer powder is placed over all, and the acid is
poured on at top. For the ordinary purposes of society, river-water is
frequently filtrated by means of clean washed sand, to separate its
impurities.
SECT. IV.
_Of Decantation._
This operation is often substituted instead of filtration for separating
solid particles which are diffused through liquors. These are allowed to
settle in conical vessels, ABCDE, Pl. II. Fig. 10. the diffused matters
gradually subside, and the clear fluid is gently poured off. If the
sediment be extremely light, and apt to mix again with the fluid by the
slightest motion, the syphon, Fig. 11. is used, instead of decantation,
for drawing off the clear fluid.
In experiments, where the weight of the precipitate must be rigorously
ascertained, decantation is preferable to filtration, providing the
precipitate be several times washed in a considerable proportion of
water. The weight of the precipitate may indeed be ascertained, by
carefully weighing the filtre before and after the operation; but, when
the quantity of precipitate is small, the different proportions of
moisture retained by the paper, in a greater or lesser degree of
exsiccation, may prove a material source of error, which ought carefully
to be guarded against.
CHAP. V.
_Of Chemical Means for separating the Particles of Bodies from each
other; without Decomposition, and for uniting them again._
I have already shown that there are two methods of dividing the
particles of bodies, the _mechanical_ and _chemical_. The former only
separates a solid mass into a great number of smaller masses; and for
these purposes various species of forces are employed, according to
circumstances, such as the strength of man or of animals, the weight of
water applied through the means of hydraulic engines, the expansive
power of steam, the force of the wind, &c. By all these mechanical
powers, we can never reduce substances into powder beyond a certain
degree of fineness; and the smallest particle produced in this way,
though it seems very minute to our organs, is still in fact a mountain,
when compared with the ultimate elementary particles of the pulverized
substance.
The chemical agents, on the contrary, divide bodies into their primitive
particles. If, for instance, a neutral salt be acted upon by these, it
is divided, as far as is possible, without ceasing to be a neutral salt.
In this Chapter, I mean to give examples of this kind of division of
bodies, to which I shall add some account of the relative operations.
SECT. I.
_Of the Solution of Salts._
In chemical language, the terms of _solution_ and _dissolution_ have
long been confounded, and have very improperly been indiscriminately
employed for expressing both the division of the particles of a salt in
a fluid, such as water, and the division of a metal in an acid. A few
reflections upon the effects of these two operations will suffice to
show that they ought not to be confounded together. In the solution of
salts, the saline particles are only separated from each other, whilst
neither the salt nor the water are at all decomposed; we are able to
recover both the one and the other in the same quantity as before the
operation. The same thing takes place in the solution of resins in
alkohol. During metallic dissolutions, on the contrary, a decomposition,
either of the acid, or of the water which dilutes it, always takes
place; the metal combines with oxygen, and is changed into an oxyd, and
a gasseous substance is disengaged; so that in reality none of the
substances employed remain, after the operation, in the same state they
were in before. This article is entirely confined to the consideration
of solution.
To understand properly what takes place during the solution of salts, it
is necessary to know, that, in most of these operations, two distinct
effects are complicated together, viz. solution by water, and solution
by caloric; and, as the explanation of most of the phenomena of solution
depends upon the distinction of these two circumstances, I shall enlarge
a little upon their nature.