and temperature. The rules for these corrections, "which are sufficiently simple, I shall give in the Appendix.

Of experiments illustrative of the nature of gases in general, it may be proper to mention one or two that show the mode in which caloric exists in this class of bodies. In vapours, strictly so called, as the steam of water, caloric seems to be retained with but little force; for it quits the water when the vapour is merely exposed to a lower temperature. But, in gases, caloric is united by a very forcible affinity, and no diminution of temperature, that has ever yet been effected, can separate it from some of them. Thus the air of our atmosphere, in the most intense artificial or natural cold, still remains in the aëriform state. Hence is derived one character of gases, viz. that they remain aëriform under almost all variations of pressure and temperature; and in this class are also included those aërial bodies, which, being condensed by water, require confinement over mercury. The following experiment will show, that the caloric, contained in gases, is chemically combined.

Into a small retort (plate ii. fig. 26, b) put an ounce or two of well dried common salt, and about half its weight of sulphuric acid. By this process, a great quantity of gas is produced, which might be received and collected over mercury. But, to serve the purpose of this experiment, let it pass through a glass balloon, c, having three openings, into one of which the neck of the retort passes, while, from the other, a tube e proceeds, which ends in a vessel of water, f, of the temperature of the atmosphere. Before closing the apparatus, let a thermometer, d, be included in the balloon, to show the temperature of the gas. It will be found that the mercury, in this thermometer, will rise only a few degrees, whereas the water, in the vessel which receives the bent tube, will soon become boiling hot. In this instance, caloric flows from the lamp to the muriatic acid, and converts it into gas; but the heat, thus expended, is not appreciable by the thermometer. The caloric, however, is again evolved, when the gas is condensed by water. In this experiment, we trace caloric into a latent state, and again into the state of free or uncombined caloric.

A considerable part of the caloric, which exists in gases in a latent state, may be rendered sensible by rapid mechanical compression. Thus if air be suddenly compressed in the ball of an air-gun, the quantity of caloric liberated by the first stroke of the piston, is sufficient to set fire to a piece of the tinder called amadou*. A flash of light is said, also, to be perceptible at the moment of condensation. This fact has been applied to the construction of a portable instrument for lighting a candle. It consists of a common syringe, concealed in a walking stick. At the lower extremity, the syringe is furnished with a cap, which receives the substance intended to be fired, and and which is attached to the instrument by a male and female screw. The rapid depression of the piston

condenses the air, and evolves sufficient heat to set the tinder on fire*.

For demonstrating the influence of variations of atmospheric pressure on the formation of gases, better experiments cannot be devised than those of Lavoisiert. But as some students, who have the use of an air-pump, may not possess the apparatus described by Lavoisier (the glass bell and sliding wire), it may be proper to point out an easier mode of showing the same fact. This proof is furnished by the experiment already described, in which ether is made to assume alternately an aëriform and liquid state, by removing and restoring the pressure of the atmosphere.

Gases, when once formed, undergo a considerable change of bulk by variations of external pressure. The general law, which has been established on this subject is, that the volume of gases is inversely as the compressing force. If, for example, we have a quantity of gas occupying 60 cubic inches, under the common pressure of the atmosphere, it will fill the space of only 30 cubic inches, or one half, under a double pressure; of 20 inches, or one 3d, under a triple pressure; of 15 inches, or one 4th, under four times the pressure; and so on. When the pressure is sudden, considerable heat is evolved; and it appears, from Gay Lussac's experiments, that different gases, when equally compressed, give out different quantities of heat, bearing probably a proportion of their specific heats.

The law of the dilatibility of gases by heat has already been stated to be an enlargement of about 40th part of their bulk for each degree of Fahrenheit's scale, between the freezing and boiling points of water. At a temperature capable of rendering glass luminous (probably about 1035° Fahrenheit), 1 volume becomes about 2.5‡.

Before dismissing the consideration of the gases in general, there are a few properties, which it may be proper to notice, with the view of comparing the degree in which they belong to different individuals of the class.

I. The exact specific gravity of the different gases is a most important element, in calculating the proportion of the ingredients of compounds, into which they enter. Nothing, indeed, can show the importance of this object more strikingly, than the fact, that on the precise specific gravities of hydrogen and oxygen gases, depend the whole series of numbers, which are used to express the weights of the atoms of bodies on the Daltonian theory. The following Table exhibits the specific gravities of the most important of this class of bodies.

II. The determination of the specific heat of gases is a difficult and important problem, which has successively employed the labour and ingenuity of Crawford, Lavoisier and De la Place, Leslie, Gay Lussac, Dalton, and Delaroche and Berard. The results of the two last-mentioned philosophers, having been attained with the advantages of the improved state of the science, and of instruments of the greatest delicacy and refinement, are perhaps most entitled to confidence. The details of their experiments are given in the 85th volume of the Annales de Chimie, preceded by an historical review of the labours of their predecessors. The following Table contains the general results.

*

Gay Lussac's Table, which is more copious, but in which the numbers are not reduced to a mean of the barometer and thermometer, is copied into Thomson's Annals, ix. 16.; a Table by Professor Meinecke of Halle is inserted in the Journal of Science, &c. iii. 415.

III. All solid bodies, that possess a certain degree of porosity, are capable of absorbing gases. This was first observed in charcoal, the power of which to condense different gases will be fully described in the section on that substance. It has been found, also, by Saussure, jun. to belong to a stone called meerschaum, to adhesive slate, abestos, rock cork, and other minerals; and to raw silk and wool. The following general principles are deducible from the experiments of Saussure*.

1. It is necessary to deprive the solid of the air which it naturally contains. When of a nature not to be injured by heat, this is most effectually done by igniting the solid, and quenching it under mercury, where it is to be kept, till admitted to a given volume of the to be absorbed. Solids that are decomposible by heat may be deprived, though less effectually, of air, by placing them under a receiver, which must then be exhausted by the air pump.

gas

2. The same solid absorbs different quantities of different gases. Charcoal for instance condenses 90 times its bulk of ammoniacal gas, and not quite twice its bulk of hydrogen.

3. Solids; chemically the same, absorb different quantities of the same gas, according to their state of mechanical aggregation. Thus the dense charcoal of box-wood absorbed 7 volumes of air, while a light charcoal prepared from cork, did not absorb a sensible quan tity.

4. Different solids absorb different quantities of the same gas; the quantity of carbonic acid absorbed by charcoal being about seven times greater than that absorbed by meerschaum.

5. When the solid exerts no chemical action on the gas, the ab sorption is terminated in 24 or 36 hours.

6. The effect of moistening the solid is to retard the absorption and to diminish its amount; and when a gas has actually been absorbed, it is again driven out unchanged, partly by water of the ordinary temperature, and entirely by exposure to a boiling heat.

7. During the absorption of a gas by a solid, the temperature' of the latter rises several degrees, and bears a proportion to the absorbability of the gas, and the rapidity with which it is condensed.

8. Solids condense a greater number of volumes of the more absorbable gases under a rare than under a dense atmosphere; but if the * Thomson's Annals, vi, 241.

VOL. I.-Q.

absorption be reckoned by weight, it is most considerable under the latter state.

9. When a solid saturated with any one gas is introduced into an atmosphere of any other gas, a portion of the first is expelled, and a part of the second takes its place.

IV. Gases are absorbed by liquids. On this subject the following general principles may be laid down.

1. The same liquid absorbs different quantities of different gases. Thus water takes up its own bulk of carbonic acid, and not one fiftieth of its bulk of hydrogen gas.

2. Different liquids absorb different quantities of the same gas. Alcohol, for instance, absorbs almost twice as much carbonic acid, as is taken up by an equal volume of water.

3. The absorption is promoted by first freeing the liquid from air, either by long continued boiling in a vessel with a narrow neck, or by the air-pump. It requires, also, brisk and long continued agitation, especially with the less absorbable gases.

4. It does not appear that the gases are absorbed by all liquids in the same order. For example, of four gases naphtha absorbs most olefiant gas; oil of lavender most nitrous oxide; olive oil most carbonic acid; and solution of muriate of potash most carbonic oxide.

5. The vicidity of liquids, though it does not much influence the amount absorbed, occasions a longer time to be spent in effecting the absorption. On the other hand, the amount of any gas which is absorbed by water, is diminished by first dissolving in the water any saline substance.

6. In general the lightest liquids possess the greatest power of absorbing gases; whereas, when there is no evident chemical action, the heaviest gases are absorbed most copiously and rapidly by the same liquid.

7. The temperature of a liquid is raised by the absorption of a gas, in proportion to the amount and the rapidity of the absorption.

8. In all liquids the quantities of gases absorbed are directly as the pressure. For example, a liquid, which absorbs its own bulk of gas under the pressure of the atmosphere, will still absorb its own bulk of the same gas under double, triple, &c. pressure; but its own bulk of gas, twice compressed, is equal to double its bulk of gas ordinarily compressed, and so on.

9. When any liquid is agitated with a limited quantity of any mixture of two gases, it does not absorb one only to the exclusion of the other, but it absorbs both. In this case, the quantities, which takes up of each, are such, that the densities of the gases are the e in and out of the liquid, after the absorption is completed. s when 20 measures of pure carbonic acid are agitated with 10 mmon air, at least 10 measures of gas are absorbed. But from xture of 20 measures of carbonic acid with 10 of common air ds of 10 (-6.6) are absorbed by 10 measures of water; and s both in and out of the water, is two thirds carbonic acid and ird air, at the close of the experiment.