The four laws given by him are

66

"1. When two elastic fluids, denoted by A and B, are mixed together, there is no mutual repulsion amongst their particles; that is, the particles of A do not repel those of B, as they do one another. Consequently, the pressure or whole weight upon any one particle arises solely from those of its own kind.

"2. The force of steam from all liquids is the same, at equal distances above or below the several temperatures at which they boil in the open air; and that force is the same under any pressure of another elastic fluid as it is in vacuo. Thus the force of aqueous vapour of 212° is equal to 30 inches of mercury; at 30° below, or 182°, it is of half that force; and at 40° above, or 252°, it is of double the force; so likewise the vapour from sulphuric ether, which boils at 102°, then supporting 30 inches of mercury, at 30° below that temperature it has half the force, and at 40° above it, double the force; and so in other liquids. Moreover the force of aqueous vapour of 60° is nearly equal to inch of mercury, when admitted into a Torricellian vacuum; and water of the same temperature, confined with perfectly dry air, increases the elasticity to just the same amount.

"3. The quantity of any liquid evaporated in the open air is directly as the force of steam from such liquid at its temperature, all other circumstances being the same.

"4. All elastic fluids expand the same quantity by heat; and this expansion is very nearly in the same equable way as that of mercury; at least from 32° to 212°. It seems probable the expansion of each particle of the same fluid, or its sphere of influence, is directly as the quantity of heat combined with it; and consequently the expansion of the fluid as the cube of the temperature, reckoned from the point of total privation."

The first law accounts for a diffusion of gases to a great extent, but not entirely. It would result from it, if not qua

lified, that there would be a diminishing quantity of o which is the heaviest gas in the atmosphere, accord the height increased. This was Dalton's opinion, but not turned out to be the case. This law was much as and at the same time much misunderstood. The obj that vapour did not rise so rapidly in air as in a v seemed to him a strong one, which he did not quite get but considered it as presenting the same difficulty theories of the solution of water in air.

The law was stated too broadly, it did not even allow for the impenetrability of matter to have its due place many persons supposed it to mean that a space filled wit gas, might be filled with an equal quantity of another.

He subsequently stated these two propositions in following form, which he published in the second editi his "New system of chemistry," when, after many year reviewed himself and his reviewers. p. 191, Part I., 18

"1. The diffusion of gases through each other is eff by means of the repulsion belonging to the homogen particles; or to that principle which is always energet produce the dilatation of the gas.

"2. When any two or more mixed gases acquire an librium, the elastic energy of each against the surface of vessel or of any liquid, is precisely the same as if it were only gas present occupying the whole space, and all the were withdrawn."

There is no doubt that the law had been hastily expres explaining some points, it contradicted others. The ph menon of the mixing of gases is easily explained, if we a the constant intestine motion of the particles to be a necess condition of the existence of a body in a gaseous state. a paper "On the changes of temperature produced by rarefaction and condensation of air," by J. P. Joule. P Magaz., May, 1845.)

The second essay is on the force of steam or vapour.

gives a long table of the force of aqueous vapour at different temperatures, from 40° to 325°. Between 32° and 312° the

numbers are given from experiment; above and below these limits the numbers are from calculation. These tables were afterwards modified by himself, and others have also reduced them to greater accuracy. He objects to the tables from water and alcohol given by M. Betancourt in 1790, and to that in the Encyclopædia Britannica, because the authors had assumed the force of that from water, at 32°, to be nothing. This constituted one of the steps which the subject made in its rather retarded progress.

He gives a series of experiments on the power of vapour from liquids, supporting his conclusions by experiments on ether, alcohol, water of ammonia, solution of muriate of lime, mercury, and sulphuric acid, and says "That the variation of the force of vapour from all liquids is the same for the same variation of temperature, reckoning from vapour of any given force; thus assuming a force equal to 30 inches of mercury as the standard, it being the force of vapour from any liquid boiling in the open air, we find aqueous vapour loses half its force by a diminution of 30° of temperature; so does the vapour of any other liquid lose half its force by diminishing its temperature 30° below that in which it boils, and the like for any other increment or decrement of heat." p. 564.

When speaking of vapour of water in air, he says "the results of all agree in one general rule or principle, which is this; let 1 represent the space occupied by any kind of air of a given temperature, and free from moisture; p = the given pressure upon it in inches of mercury; =ƒ= the force of vapour from any liquid in that temperature in vacuo; then the liquid being admitted to the air, an expansion ensues, and the space occupied by the air becomes immediately and in a short time = 1+; or which is the same thing Ρ = Drs. Thus in water for instance, let p = 30 inches f 15 inches

to the given temperature 180°. Then y = 30 for the space; or the air becomes of twice the p. 572. "In short, in all cases the vapour arise: certain force according to temperature, and the air a the equilibrium by expanding and contracting as m required."

"The notion of a chemical affinity subsisting betwe gases and vapours of different kinds cannot at all be reco to these phenomena." p. 574.

This notion of chemical affinity holding the gas solution had begun to die out.

In essay third, "On evaporation," he concludes tha quantity of any liquid evaporated in the open air is di as the force of steam from such liquid at its temperatur other circumstances being the same. He adds a shewing the force of vapour, and the full evaporating of every degree of temperature from 20° to 85°, express grains of water that would be raised per minute fr vessel of six inches in diameter, supposing there wer vapour already in the atmosphere." p. 585. He obtained evaporation from a surface when the air was still and in motion. He adds also rules to find the amount of v that can be evaporated from a given surface when temperature of the air is given, and the condensing p and to find the force of the aqueous vapour.

The fourth essay on the expansion of elastic fluids by proves the law already stated.

The position of the question when he took up the sub may best be explained by himself, he says, p. 595, " principal occasion of this essay is another on the same sub by Messrs. de Morveau and du Vernois, in the first vol the Annales de Chimie. It appearing to them that the res of the experiments of De Luc, Col. Roy, de Saussu Priestley, Vandermonde, Berthollet, and Monge, did sufficiently accord with each other; and that it would be

importance to determine not only the whole expansion of each gas from two distant points, such as the freezing and boiling, but likewise whether that expansion be uniform in every part of the scale, they instituted a set of experiments expressly for those purposes. The result of which was; that betwixt the temperatures of 32° and 212°, the whole expansion of one gas differs much from that of another, it being in one instance about 4-10ths of the original, and in others, more than twelve times that expansion; and that the expansion is much more for a given number of degrees in the higher than in the lower part of the scale. These conclusions were so extremely discordant with and even contradictory to those of others, that I could not but suspect some great fallacy in them, and found it in reality to be the fact; I have no doubt it arose from the want of due care to keep the apparatus and materials free from moisture."

After giving his experiments on air, hydrogen, oxygen, carbonic and nitrous gas, in which "the small differences never exceeded six or eight parts, on the whole 345," he adds, "Upon the whole, therefore, I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat, and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher the temperature."

"This remarkable fact that all elastic fluids expand the same quantity in the same circumstances, plainly shews that the expansion depends solely upon heat; whereas the expansion in solid and liquid bodies seems to depend on an adjustment of the two opposite forces of heat and chemical affinity, the one a constant force in the same temperature, the other a variable one, according to the nature of the body; hence the unequal expansion of such bodies. It seems, therefore, that general laws respecting the absolute quantity and the nature of heat, are more likely to be derived from elastic fluids than

G