in natural philosophy, is the dissipation of the parts of any substance either solid or fluid into the air, in the form of smoke, or otherwise, in an invisible manner, so that the substance evaporated then forms a fluid of equal subtilty and transparency with air itself.

When any substance is dissipated slowly and insensibly by simple exposure to the atmosphere, as water, camphor, &c. the evaporation is said to be spontaneous.

all cases, is greatly promoted by heat. Many substances may be made to evaporate by means of a strong fire, which otherwise appear absolutely fixed; and there are but few which can resist the violent action of the focus of a large burning mirror, without being in great part dissipated.

The degree of heat in which different bodies begin sensibly to evaporate is very different. When fluids of all kinds have been heated to a certain degree, their evaporation is attended with great intense motion called boiling; and as then the dissipation becomes much more sensible than before, this degree of heat is called their evaporative point; but improperly, for they begin sensibly to evaporate long before.

The boiling point of all fluids is by no means the same; and the degree of heat which would cause the most fusible metal to boil, is prodigiously greater than what would dissipate the most fixed and ponderous fluid with the utmost rapidity. Vitriolic, and especially nitrous ether, boil with a heat very little greater than that of our atmosphere in summer. Spirit of wine requires Fahrenheit's thermometer to be raised 175 degrees above 0°, before it boils; water requires 212 of the same degrees; oil of vitriol 550°, oil of turpentine 560°, quicksilver and linseed oil 600°, before they boil.

The quantity of any substance evaporated is found to depend so much on the degree of heat applied to it, that heat alone is generally reckoned to be the sole cause of evaporation. Many perplexing circumstances, however, occur, when we attempt to explain the manner in which a body, water for instance, naturally 800 times heavier than air, should become so much specifically lighter, as to be carried up by it to a very considerable height. One of the most generally received opinions concerning the formation of vapour is that adopted by Dr Halley. He supposes that a bubble, composed of a particle of air inclosed by a thin film of water is rarefied by heat to such a degree, that it becomes specifically lighter than common air; in consequence of which, such particles, whatever be their number, must ascend, and be suspended in the air when they arrive at such an height as to find the atmosphere precisely of the same specific gravity with themselves. But many objections may be made to such a theory. For though a great quantity of air is dispersed in water, it is not easy to account for the formation of a bubble distinct from the mass of water; nor, though we could account for this, ought it to be taken for granted without sufficient proof; and it remains to be explained by what means these bubbles would ascend into the air; because, by reason of their extreme small size, the cold of the external air would almost instantly reduce the rarefied air-bubble to the same specific gravity with itself. The bubble would then become specifically heavier than the atmosphere, and immediately fall down; so that vapour, instead of rising to the height of two or three miles, could scarce rise to the height of as many feet.

Another theory, from the expansion of water, is another by adopted by Dr Defaguliers, who reasons as follows:

"Water is expanded by heat; and supposing it to be expanded to a bulk more than 800 times greater than in its usual state, it becomes specifically lighter than common air, and must consequently rise in it till it meet with air above its own degree of rarefaction." To illustrate this, he observes, that boiling water, when it becomes vapour, is expanded to a bulk 14,000 times greater than when cold; and to account for evaporation in the ordinary heat of our atmosphere, he takes it for granted, that the degree of expansion is strictly proportioned to the degree of heat; from which he calculates thus. "In Sir Isaac Newton's scale, the heat of boiling water is 34; the mean heat of summer, 5; the mean heat of spring and autumn, 3; and the mean heat of winter, 2. Therefore, if the expansion of water by 34 degrees of heat be 14,000, the expansion by 5 degrees will be 2,058; by 3 degrees 1,235; and by 2, the mean heat of winter, it will be 823; which is sufficient to raise vapour, the usual density of which is to that of air as 800 to 1.

This theory is liable to the same objections with the former, and indeed to much greater: For if, even by the heat of winter, water is expanded so much that it becomes specifically lighter than air, by what force is it kept below, contrary to the laws of gravity? Or, supposing it once lodged in the air; how is it to be kept there; seeing the superior air must very soon restore it to the same degree of heat with itself; upon which it must immediately descend?

To avoid the difficulties to which the abovementioned theories are liable, another hath been published by lord Kaims and professor Hamilton. They account for evaporation on the principle of elective attraction, and then solve the natural phenomena in the following manner.

1. "By an elective attraction between air and water, there is always a quantity of air in water and of water in air."

2. When water is saturated with air, it will take up no more; and, in like manner, air saturated with water attracts no more.

3. This power of elective attraction is augmented by heat; for after air is saturated with water, or water with air, they will attract more by increasing their heat.

4. The air with its moisture being rarefied by heat, ascends to a higher region, giving place to purer air not yet saturated, which accordingly attracts more moisture, and water by that means is diffused thro' the air.

5. The grosser particles of water, swimming in the air, being accumulated by wind, especially contrary winds, become visible clouds, and fall down in rain by the force of gravity overcoming the elective attraction.

6. Cold also contributes its part, by condensing the watery particles, which fall down in snow when the cold is violent; and by these means a constant circulation of moisture is carried on."

This hypothesis is also liable to be overthrown: for, according to it, evaporation ought not to take place in vacuo; which is contrary to experience.—The answer given by lord Kaims to this objection is a denial of the fact. Professor Hamilton owns the dissipation of water by heat in vacuo; but denies that this is really evaporation, which he maintains to be a true solution of water in air; and as a proof of this, he put some water well purged of air under the exhausted receiver of an air-pump, and found that it lost only 2½ grains in the same time that an equal quantity exposed to the air lost 35 grains.

That water can be converted into steam in vacuo, cannot be denied without the greatest skepticism. It is even found much more easily dissipated in vacuo than when exposed to the open air, and boils with much less heat; 92 or 95 degrees being sufficient to make water boil in vacuo, whereas it requires 212° to make it boil when exposed to the air. Concerning this, Dr Black relates the following experiment.

Mr Watt of Glasgow, having formed a project of performing the evaporation of water in vacuo, with a view to convert it into steam with less expense of fuel, communicated his scheme to the doctor. The experiment was made with a small still resembling the body of a retort, with a vessel serving as a condenser. The whole apparatus was close, except a little hole at the extremity of the condensing vessel. Into this distilling vessel a quantity of water was put; it was then exhausted of air, by taking the condensing vessel, and holding it up, that the retort might be undermost. Being held over the fire in this position, the whole was soon filled with steam, which expelled the Evaporation air; and upon stopping the little hole, a pretty perfect vacuum was formed in the retort and condensing vessel. The distilling vessel being then set upon the fire, and the condensing vessel plunged in cold water, the water in the still began to boil, as might be known by the noise, with a degree of heat very little greater than that of the human body, and the steam came over, and was condensed in the refrigeratory; but Mr Watt observed that the evaporation was not quicker in vacuo, than when air was admitted.

Thus we see that water may be converted into steam by the effect of heat alone, without the assistance of air; and thus the hypothesis of lord Kaims falls to the ground. Professor Hamilton's experiment is evidently inconclusive, because it does not appear to have been fairly made. He put a certain quantity of water under an exhausted receiver for 24 hours, and exposed an equal quantity to the air in the same room; the first lost 2½ grains, the latter 35. From this we can conclude nothing but that the quantity of fluid, whatever it is, which remains after the receiver of an air-pump is exhausted, is capable of taking up 2½ grains of water. As no more of this fluid could be admitted to the water, than the precise quantity contained in the receiver, it is plain, that the other quantity of water which was exposed to the open air of a room, ought to have been included within a receiver of the same size with the first, from which the air was not exhausted: for in the manner wherein this experiment was conducted, an immense quantity of air had access to one vessel, and only as much as could be contained in the receiver of an air-pump, of the medium remaining after the air is exhausted, had access to the other; so that it would have been very absurd to expect the same result in both cases. Even circumstanced as the experiment is, it makes directly against that hypothesis which the professor is desirous of supporting; for, as the air-pump he made use of was capable of rarefying the air two and forty times, had the evaporation depended on the small quantity of air which was left in the receiver, only the two and fortieth part ought to have been dissipated of what would have been lost had the water been placed under a receiver full of air. Instead of this, a like quantity of water exposed to the air contained in a large room, lost only 14 times as much. Had the vacuum of the receiver been as large as the room, it is impossible to say how much would have been lost; and the only conclusion we can draw is, that the air, however necessary to the ascent of vapour, is in reality prejudicial to its first formation.

A very remarkable circumstance concerning the evaporation of fluids is, that some degree of cold is always produced in consequence of it; and by means of some of the more volatile ones, a great degree of cold is produced. If some vitriolic ether is put into a vial, let in a small vessel of water, and the whole placed under an air-pump, the ether boils violently as soon as the air is exhausted, while the water is frozen by the cold produced in consequence of its evaporation. This circumstance hath been made use of as a proof that evaporation is not occasioned by heat; and the sensible coldness of vapour has been thought a demonstration that heat could by no means be concerned in its formation. Dr Black, however, who treats particular- ly of the formation of vapour, hath accounted for this in a very satisfactory manner; and gives heat, in a certain degree, as the cause of evaporation in all cases whatever.

This gentleman is of opinion, that all fluid substances, besides the heat contained in them capable of affecting our senses or a thermometer, contain a certain quantity united to them in such a manner as not to be discoverable by either of these methods; which, however, on certain occasions, is apt to break forth, and shew its proper qualities as heat or fire. These two different states of this fluid, he distinguishes by the names of sensible and latent heat.—A proof that heat exists in this latent state, he gives from a mixture of ice with hot water. He observes, that, when two equal masses of the same matter heated to different degrees, are mixed together, the heat of the mixture ought to be an arithmetical mean betwixt the two extremes. But, on pouring hot water upon ice, he found the case to be considerably different, and that a quantity of heat was entirely lost; which he could account for in no other manner than by supposing, that it entered into the composition of the water, in such a manner as to be in a latent state, and the invisible cause of the fluidity of that element. (See Fluidity.)

What happened in this case, to a mixture of ice and water, the doctor thinks, always happens on the conversion of water into steam or vapour by means of heat: a great quantity of the heat thrown into the fluid enters into it when in the state of vapour, and forms a part of the fluid itself; in which state it is not discoverable either by the thermometer or by the touch, but yet is ready to appear again in its proper form when the vapour is condensed.

The most conclusive experiments, mentioned by Dr Black, as a proof of this theory, are, that hot water put under the exhausted receiver of an air-pump boils with great violence, and a part of it is suddenly dispersed in vapour. During this time the water itself cools remarkably fast, a part of the heat disappears, and is neither to be found in the steam nor in the water. What then has become of it? The doctor concludes it still exists in the steam, though in a latent state, and not to be discovered by the common methods.

In this manner, likewise, he accounts for the above-mentioned experiment of the water freezing under an exhausted receiver, in consequence of the evaporation of the ether.—This fluid being so extremely volatile that it went off in vapour when the temperature of the air was only $50^\circ$, absorbed the sensible heat of the surrounding water, which entered into the composition of the ethereal vapour, and there remained in a latent state. Certain it is, that, in this experiment, there was no perceptible increase of heat either in the ether or its vapour; the water indeed lost heat considerably, but neither of these gained it.

The element of fire, however, is so exceedingly subtle, and so much eludes our most diligent search, that we cannot conclude from its disappearance in one place, that it has imperceptibly occupied another; the doctor, therefore, has yet a stronger argument in favour of his theory. He maintains, that, in the condensation of steam by the refrigeratory of a common still, as much heat is communicated to the water in the refrigeratory as would be sufficient to heat the water which comes over to the heat of red-hot iron, were it all to exist in it in the form of sensible heat. Nevertheless, the steam is at no time hotter than the boiling water which emits it. The excess of heat, therefore, must have existed in the steam in a latent state, and become sensible during the condensation.

The method of calculating this quantity of heat is very easy. For, supposing the refrigeratory to contain 100 pounds of water, and that one pound has been distilled: If the water in the refrigeratory has received 10 degrees of heat during the distillation, we know that the one pound distilled has parted with 1000. If, in passing through the worm of the refrigeratory, it has been reduced to the temperature of $50^\circ$ Fahrenheit's, having been at $212^\circ$ when it entered the worm in form of steam, it has lost only $162^\circ$ of sensible heat; all the rest of that heat which it communicated to the refrigeratory, amounting to above $800^\circ$, was contained in it while in the form of steam, in such a state as not to be indicated by the thermometer.

By calculating in this manner, the doctor generally found, that the heat communicated by the steam was about $800$ degrees; which would have been sufficient to render a mass of iron equal in bulk to the water which came over, red-hot. In the experiment made by Mr Watt, no less than $1000$ degrees were communicated; though, all the time, the steam came over with a very gentle heat, but little superior to what the hand could bear.

This experiment, no doubt, unanswerably confirms the doctor's theory of heat. It is proper, however, to take notice, that a deception may very easily take place with such as repeat the experiment in a careless manner.—The upper part of the water in a refrigeratory grows very hot, while the under part is quite cold; and if a thermometer is plunged into it without stirring the water, a much greater degree of heat will be thought to be communicated than really there is. To avoid this mistake, it is necessary to stir the water well about, and then measure its degree of heat.

This theory of sensible and latent heat, so well established, cannot but be looked upon as a valuable discovery in natural philosophy; and will enable us to give a more satisfactory account of the formation of vapour when strong degrees of heat are applied, as well as in the common heat of our atmosphere, than any that hath hitherto been published. We shall begin with the conversion of any fluid, water, for instance, into vapour, when such a degree of heat is applied as to make it boil.

Here, the water has already received the utmost degree of heat which it is capable of containing. When a larger quantity continues still to be thrown in, it must either pass through the substance of the water and be dissipated in the air, or combine with the aqueous particles in the form of steam. That the extreme agility of heat causes great part of it to pass through the water and be lost, cannot be denied; but it is also evident, that a very considerable part combines with the substance of the water, and is converted into vapour. The action of boiling consists in the ascent of a great number of bubbles from the bottom of the vessel containing the water. These, growing continually larger as they ascend, break on the surface of the water, and... are found to be composed of steam or vapour*. As they continue till the evaporation of the very last drop of water, long after it has parted with all its air, this cannot be supposed to have any share in their formation. Indeed, Dr Boerhaave and others have proved, that there is no air contained in them; and both Dr Black and Professor Hamilton have shewn, that they are the very fluid which is dissipated in the form of smoke. If, therefore, steam exists, and appears in its own proper form, when the air has no access to it, which it has not till the bubble ascends to the top and breaks in the air, it is impossible that air can have a share in its formation; though by its superior gravity it is the sole cause of its ascent.

If we are inclined, then, to call vapour a solution of water in any thing, it must be in heat, or fire; seeing, according to Professor Hamilton himself, it appears in the proper form of vapour before it has had any connection with the air.

Upon the same principles we may easily account for the spontaneous or insensible evaporation of water when a degree of heat much less than that requisite to cause it boil is applied to it. From Dr Black's experiments it appears*, that a certain degree of heat is necessary to keep it in a fluid state; and whatsoever degree is applied to it superior to that absolutely necessary to keep it fluid, appears to be but loosely connected with it, so that the water will very readily part with this superfluous degree of heat to any colder body that comes in contact with it. Water in its fluid state, then, we may consider as a kind of compound, consisting of the pure element, and a certain quantity of heat so loosely combined with it as to affect the thermometer; but it is impossible that two substances having a tendency to mutual union can fail to be united in the closest manner of which they are capable when brought very near or in contact with each other. The water, therefore, having a constant tendency to absorb the sensible heat, and convert it into latent heat, must continue to do so more or less slowly according to the quantity contained in it. By this means there is a proportional quantity of vapour formed; for we must remember, that when heat and water are combined in the most intimate manner, they form a new substance totally different from water in its elementary state. The particles of vapour thus formed, must necessarily ascend to the surface of the water, and thence into the air, for the reasons already mentioned; and thus there will be a constant exhalation from the surface of water, when the atmosphere is of such a temperature as to keep it fluid.

It now remains to account for the evaporation of water from ice; when the atmosphere is of such a nature as to deprive the water of great part of its sensible heat, and reduce it to a solid form. From a very curious experiment*, Dr Black hath shewn, that, in the act of freezing, the latent heat of the water is called forth, and becomes sensible. The ice, therefore, during the process of congelation, is always a little warmer than the external air. In this case then, with regard to the external atmosphere, it may be considered as similar to water having a small fire under it, so as to make it a degree or two warmer than the surrounding atmosphere. The consequence of this would be, that the freezing water would communicate part of its sensible heat to the air, and another part would intimately combine with the aqueous particles, and form a vapour which would be carried up into the atmosphere. In like manner, the piece of ice, having a degree of sensible heat superior to that of the air, will communicate to it part of that heat; while another part, from its strong tendency to unite with the water, will undoubtedly do so in its passage from the ice into the air, and carry off part of the aqueous particles in an imperceptible vapour.

Thus, according to Dr Black's theory, whether the degree of heat contained in the atmosphere is greater or less than that of water, there must be a continual evaporation of that fluid. There is only one case, upon his principles, where the evaporation must be little or nothing; and that is, when a piece of ice is in the action of melting. Here the water indeed receives heat from the atmosphere; but as fast as it is received, it passes from a sensible into a latent state; and, till the water is saturated with latent heat, very little sensible heat can combine with the aqueous particles, so as to form them into steam. In this instance likewise, the Doctor's theory is verified by experience; for Professor Hamilton having included a piece of ice, while thawing, in an exhausted receiver, and likewise in one full of air, found that it lost nothing by evaporation in 24 hours.

From some experiments made by the Abbé Nollet, it appears, that evaporation is promoted by electricity. The consequences of his experiments are as follow:

1. Electricity augments the natural evaporation of fluids; all that were tried, except mercury and oil, were found to suffer a diminution greater than what could be ascribed to any other cause.

2. Electricity augments the evaporation of those fluids the most, which are most subject to evaporate of themselves; the volatile spirit of sal ammoniac, suffering a greater loss than spirit of wine or oil of turpentine, these two more than common water, and water more than vinegar or the solution of nitre.

3. The effects seemed always to be greater when the vessels containing the fluids were non-electric.

4. The increased evaporation was more considerable when the vessel which contained the liquor was more open; but the effects did not increase in proportion to the apertures.

5. Electricity was also found to increase the evaporation of moisture from solid bodies, and of consequence to increase the insensible perspiration of animals*.

As the electric fluid is generally thought to be the same with the element of fire, it cannot be thought that this discovery can be any objection to Dr Black's theory of the formation of vapour, but must rather tend to confirm it; as the phenomena of electricity present us with fire in a state wherein it has no sensible heat, and which, agreeable to the Doctor's mode of expression, may very properly be called its latent state. The very great readiness which this fluid shews to be converted into actual fire, so as even to fuse metals, may also be reckoned a kind of demonstration of the facility wherewith the sensible heat of any body may become latent, or the latent heat sensible.