a considerable evaporation from the earth and waters in the evening, which is condensed again in the cold air. I readily allow that this may take place; but it does not appear to me to militate against what has been advanced in the preceding Section. Evaporation may still be going on from below, while there is a precipitation from above; and thus we may account for the stratus not always resting on the ground, but frequently beginning at a small distance from it, or increasing in density for some feet upwards, there being a sort of shallow vapour plane preserved so long as the heat continued to be slowly transmitted from the earth.

M. De Luc asserts that clouds are not the constant result of evaporation from the earth. He accounts for them by supposing that the air is decomposed by the sun's rays, so as to deposit aqueous particles, which become clouds. If this be admitted to take place, it does not argue against the ascent of vapour: and whether the watery particles arise immediately from the ground, or are deposited by the air, they may equally be supposed capable of becoming cloud when operated upon by the nubific principle, which is believed to be electrical: indeed, these

two processes may co-operate to the production of clouds, so that, if this hypothesis be advanced against M. Howard's theory, it cannot be regarded as constituting a valid objection.*

*The following extract will, perhaps, furnish to the reader a more perfect idea of M. Howard's theory of the origin of clouds:

"On the remote and universal origin of clouds there can be but one opinion--that the water of which they consist has been carried into the atmosphere by evaporation. It is on the nature of this process, the state in which the vapour subsists for a time, and the means by which the water becomes again visible, that the greatest diversity of opinion has prevailed.

"The chemical philosopher, seduced by analogy, and accustomed more to the action of liquids on solids, naturally regards evaporation as a solution of water in the atmosphere, and the appearance of cloud as the first indication of its precipitation; which becoming afterwards (under favourable circumstances) more abundant, produces rain. The theory of Dr. Hutton goes a step further, assumes a certain rate of solution differing from that of the advance of temperature by which it is effected, and deduces a general explanation of clouds and rain from the precipitation which, according to his rule, should result from every mixture of different portions of saturated air. The fundamental principle of this theory has been disproved in an essay heretofore presented to the society, and which was written under the opinion, at present generally adopted by chemists, that evaporation depends on a solvent power in the atmosphere, and follows the general rules of chemical solution.

*

*See Phil. Mag. vol. xiv. p. 55.

It is said again, that nimbi have been observed to take place without the precurrence of other

"The author has since espoused a theory of evaporation which altogether excludes the abovementioned opinion (and consequently Dr. Hutton's also), and considers himself in a considerable degree indebted to it for the origin of the explanation he is about to offer. It will be proper, therefore, to state the fundamental propositions of this theory, with such other parts as appear immediately necessary, referring for mathematical demonstrations and detail of experiments to the work itself, which is entitled " Experimental Essays on the Constitution of mixed Gases; on the Force of Steam or Vapour from Water and other Liquids, in different Temperatures, both in a Torricellian Vacuum and in Air; on Evaporation; and on the expansion of Elastic Fluids by Heat. By John Dalton." See Memoirs of the Literary and Philosophical Society of Manchester, vol. v. part 2.-The propositions are as follow:

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 solely arises from those of its own kind.

2. The force of stream 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 other 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 like

modifications; but I have seen it no where proved, that after their formation rain has gone

wise 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 on in other liquids. Moreover, the force of aqueous vapour of 60° is nearly equal to half an inch of 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.

mercury

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

The following is a part of the essay on evaporation:

“When a liquid is exposed to the air, it becomes gradually dissipated in it; the process by which this effect is produced we call evaporation.

"Many philosophers concur in the theory of chemical solution: atmospheric air, it is said, has an affinity for water; it is a menstruum in which water is soluble to a certain degree. It is allowed, notwithstanding, by all, that each liquid is convertible into an elastic vapour in vacuo, which can subsist independently in any temperature: but as the utmost forces of these vapours are inferior to the pressure of the atmosphere in ordinary temperatures, they are supposed to be capable of existing in it in the same way as they do in a Torricellian vacuum: hence the notion of affinity is induced. According to this theory of evaporation, atmospheric air (and every other species of air for aught that appears) dissolves water, alcohol, ether, acids, and even metals. Water below 212° is chemically combined with the gases; above 212' it assumes a

on without a cirrose crown on the upper part of the nimbus. I have noticed the spon

new form, and becomes a distinct elastic fluid, called steam: whether water first chemically combined with air, and then, heated above 212°, is detached from the air or remains with it, the advocates of the theory have not determined. This theory has always been considered as complex, and attended with difficulties: so much that M. Pictet and others have rejected it, and adopted that which admits of distinct elastic vapours in the atmosphere at all temperatures, uncombined with either of the principal constituent gases, as being much more simple and easy of explication than the other; though they do not remove the grand objection to it, arising from atmospheric pressure."

On the Evaporation of Water below 212°.

"I have frequently tried the evaporation at all the temperatures below 212°: it would be tedious to enter into detail of all the experiments, but I shall give the results at some remarkable points. In all the high temperatures I used the vessel abovementioned, keeping a thermometer in it, by which I could secure a constant heat, or at least keep it oscillated within narrow limits.

*

"The evaporation from water of 180° was from 18 to 22 grains per minute, according to circumstances; or about one half of that at 212o.

"At 164° it was about one third of the quantity at the boiling temperature, or from 10 to 16 grains per minute.

* This refers to experiments on the evaporation of water at 212°, for which see the Essay.

D