runs from it is always at one temperature, that of 32° of Fahrenheit's scale. This and similar cases seem to have occupied the early thoughts of our philosopher; for his biographer informs us, that, in the oldest parcels of his notes, he found queries relating to this subject. How does it happen that, although heat is constantly flowing from surrounding bodies to the ice, its temperature is not increased? Water at 32°. when brought into a room at 60°. goes on increasing in temperature till it attains that of the circumambient air; but the ice, though exposed to exactly similar sources of heat, remains at 32°. Why, when water is cooled several degrees below its freezing point, does its temperature suddenly rise to that point, the instant that it congeals? or why is it, that, when a vessel of water is put upon the fire, a thermometer plunged into it continues to indicate increase of heat until it rises to 212°; and the water then boils, but does not become hotter, although it remains upon the fire, and has all its former opportunities of acquiring heat? Such were the queries which Dr Black has most happily resolved.

In regard to the liquefaction of ice, he has demonstrated that, when solids pass into the liquid state, the change is always accompanied with the absorption of heat, which is concealed or becomes latent in the liquid, and is not indicated by the thermometer, which instrument, therefore, is no measure of the absolute quantity of heat. A variety of interesting and curious experiments were undertaken with a view to ascertain the quantity of thermometric heat, which thus becomes latent during the conversion of ice into water. A pound of snow at 32° was mixed with a pound of water at 172°; the snow was melted, and the temperature of the mixture was only 32°; so that here 140o of thermometric heat had disappeared; their effect being, not to raise the temperature of the snow, but to convert it into water. We should say, therefore, from this experiment, that water at 32o is a compound of ice, and 140o of heat as indicated by the thermometer. If water, at the temperature of 32o, be mixed with an equal weight of warm water, suppose at 200°, the resulting temperature will be the mean; 232 ÷ 2 = 116; but if we use ice, the temperature will not be the mean, for 140° of heat must be subtracted from the warm water, which heat is consumed in liquefying the ice; the result, therefore, will be the same as if water at 32o and 60° were mixed, giving a mean of only 45°.

These experiments at once demonstrated the cause of many facts respecting the production of heat and cold, which, though long known, remained without any plausible explanation.

When solids become fluids, the production of cold is more or less evident, according to the rapidity of the change. Those saline bodies, for instance, which are very rapidly soluble in water, generate during their solution a considerable intensity of cold, for to become fluid they must absorb heat. When snow and salt are suddenly blended, there is an instant liquefaction, and the temperature of the substances being already low, a degree of cold equal to 0° of Fahrenheit is obtained. The production of cold by mixing snow and muriate of lime, a very soluble salt, is -40° Fahrenheit, and sufficient to freeze quicksilver even in a comparatively warm atmosphere. A mixture of 5 parts of sal ammoniac in powder, and 5 parts of nitre with 16 of water, sink the thermometer from 50° to 10°. Equal parts of nitrate of ammonia and water produce a more intense cold, and by a clever successive application of these freezing mixtures, the intense degree of cold of — 91°. Fahrenheit has been artificially exhibited. This is 123° below the freezing of water, and 40° below the greatest natural cold hitherto observed, which was at Hudson's Bay, where the spirit thermometer has been seen at 50o.

There are many counter illustrations of this doctrine of latent heat; in which heat is evolved during the conversion of liquids into solids. If oil of vitriol be poured upon magnesia, there is a sudden solidification of the acid by its union with the earth, and a considerable rise of temperature ensues. Water poured upon quicklime produces a similar phenomenon; and when water at perfect rest is exposed in a covered vessel to an intensely cold atmosphere, its temperature may be reduced to many degrees below its freezing point: A slight agitation causes it suddenly to become ice, and at that instant the temperature rises to 32°. A somewhat similar case is the sudden crystallization of saline solutions, during which their latent heat becomes sensible to the feeling, and is indicated by the thermometer.

In Dr Black's theory of latent heat, it is assumed that heat is matter; that it is a substance of excessive tenuity, existing in variable proportions in bodies; that when in a free state it affects our senses, and the thermometer, but that it occasionally enters into union with other substances, or is separated from them, consistent with the usual laws of chemical attraction. Thus, in fluids, it is combined or latent, but when they are converted into solids, it is separated in a free or sensible state. The other view of the question represents heat as the result of a vibrating motion among the particles of bodies; the vibrations being most rapid and extensive in the hottest bodies. In fluids the vibrations are accompanied by a motion of the particles round their own axes; and when solids pass into the fluid state, the vibratory motion or temperature is in part lost, by the communication of the rotatory motion to the particles.

Each of these hypotheses has had its able defenders and advocates; the ideas of Newton seem to have been favourable to the latter, and many facts may be adduced in its support. The strongest are the imponderability of heat, and its continuous extrication by friction. That we discover no increase of weight in a heated body may be attributed to the insufficiency of our instruments, but its unlimited production in a variety of cases, though consonant with the hypothesis of vibration, ill agrees with that of a specific form of matter.

If a soft iron nail be beaten upon the anvil, it becomes hot and brittle, and it cannot again be rendered malleable till it has been resoftened by exposure to the fire. By those who favour the notion of a matter of heat, this has been called an experimentum crucis. The matter of heat, say they, is squeezed out of the nail, as water out of a sponge, but it is reabsorbed in the fire. In this experiment, however, it must be recollected,that the mechanical arrangement of the particles of the iron is considerably altered; it is rendered very brittle; and hence, perhaps, its insusceptibility of becoming again hot till restored to its former state or texture by the expansive power of fire.

It was not until the publication of the researches which have just been considered, that a variety of curious circumstances concerning congelation were understood. The gradual progress in the freezing of large bodies of water has been shown to depend in some measure upon the remarkable anomaly respecting its maximum of density; but it is also materially connected with the phenomena of latent heat; for water, before it can become ice, must part with a quantity of heat, which, if suddenly evolved, would raise the thermometer 140°. It must also be obvious, that the process of thawing suffers a similar retardation, for ice requires for its conversion into water, the absorption of 140°. of sensible heat.

Thus we see that sudden congelation and sudden liquefaction are alike prevented; that the process must be gradual, and consequently productive of none of those evils which would result from a more rapid change.

One of the great advantages of irrigation, or meadow watering, is also explained by a reference to these principles. In an irrigated meadow, the surface of the water may be frozen; but as water at 40° is heavier than at 32°, the former will be its temperature in contact with the grass; and it is a temperature perfectly congenial to the functions of vegetable life. Sir Humphrey Davy examined the temperature in a water meadow near Hungerford, in Berkshire, by a very delicate thermometer. The temperature of the air, at seven in the morning, was 29°. The water was frozen above the grass; the temperature of the soil at the roots of the grass was 43°. Thus, by the peculiarity in the re

frigeration of water, by the defence afforded by the stratum of ice, and by the laws of congelation, the vegetables are not merely protected from the effects of an intensely cold atmosphere, but likewise from the injurious influence of sudden changes of temperature.

Congelation is to surrounding bodies a source of heat, and there is no inconsiderable mitigation of the extreme cold of air wafted over large bodies of water, by the transfer of latent to sensible heat, which must occur before they can freeze.

The theory of freezing mixtures has led to considerable improvements of their applications, and many new and curious discoveries have resulted in pursuing this inquiry. Indeed, whatever tends to disclose the laws of nature cannot ultimately fail of subjecting her more or less to the uses of life, and of manifesting more and more the wisdom of the Creator.

Having established the above facts respecting the cause of fluidity, Dr Black proceeded to the second part of his inquiry, relating to vaporisation, and pursued it with the same abilities and success. 1 Finding the thermometer to remain stationary at 212o in boiling water, he conceived the process of ebullition to be in some respects analogous to that of liquefaction, and that the heat which did not raise the temperature of the water, entered into union with it, and became latent in the steam. If this

1 "When we heat a large quantity of a fluid in a vessel, in the ordinary manner, by setting it on a fire, we have an opportunity of observing some other phenomena which are very instructive. The fluid is gradual-ly heated, and at last attains that temperature which it cannot pass without putting on the form of vapour. In these circumstances, we always observe, that it is thrown into the violent agitation which we call boiling. This agitation continues as long as we throw in more heat, or any of the fluid remains, and its violence is proportional to the celerity with which the heat is supplied.

"Another peculiarity attends this boiling of fluids, which, when first observed, was thought very surprising. However long and violently we boil a fluid, we cannot make it in the least hotter than when it began to boil. The thermometer always points at the same degree, namely, the vaporific point of that fluid. Hence the vaporific point of fluids is often called their Boiling point.

"When these facts and appearances were first observed, they seemed surprising, and different opinions were formed with respect to the causes upon which they depend. Some thought that this agitation was occasioned by that part of the heat, which was more than the water was capable of receiving, and which forced its way through, so as to occasion the agitation of boiling; others, again, imagined, that the agitation proceeded from air, which water is known to contain, and which is now expelled by the heat. Neither of these accounts, however, is just or satisfactory; the first is repugnant to all our experience in regard to heat we have never observed it in the form of an expansive fluid like air: it pervades all bodies, and cannot be confined by any vessel, or any sort of matter; whereas, the elastic matter of boiling water, can be confined by external pressure, as is evident in the experiments made with Papin's digester."

This quotation from Black's Lectures, (Vol. I. p. 153.), is inserted to show the state of the argument respecting the phenomena of ebullition previous to his researches.

were the case, it should be re-evolved during the condensation of steam; and thus a method was devised of ascertaining its thermometric quantity. Dr Black's experiments on this subject were very numerous. I shall allude to such as put the phenomenon in the clearest light, and are perfectly unconnected with hypothesis.

He noted the time consumed for raising a certain quantity of water to its boiling point, and then kept up the same heat till the whole was evaporated, and marked the time consumed by the process. It was thus easily computed what the temperature would have been, supposing the rise to have gone on above 212o in the same ratio as below it. The water was originally at 50°; it boiled in four minutes, and in twenty minutes was all evaporated. In four minutes, therefore, it had gained 162o for 50° + 162 = 212; and in twenty minutes would have gained 162 x 5 = 810°; which may, therefore, be considered as the equivalent thermometric expression of the latent heat of the steam. Another good illustration of the absorption of heat in the production of steam, is furnished by heating water under compression. It may then be raised many degrees above its ordinary boiling point; but, on removing the pressure, a portion of steam rushes out, and the remaining water has its temperature lowered to 2120, 1

Hence we learn, that the conversion of water into vapour is attended with a great loss of heat to the surrounding bodies; and although the perceptible temperature of water and steam are identical, the latter contains heat equivalent to between 800 and 900° of perceptible or thermometric temperature. When steam is reconverted into water, this large quantity of heat is again given out; and hence a small portion of steam is capable of heating a large body of water to its boiling point. The knowledge of this fact is of great economical importance; and in breweries and other manufactories, where large bodies of water are required to be boiled, the steam, instead of being suffered, as formerly, to pass off into the air, is conveyed by pipes into other vessels of water, which it heats during its condensation. In the same way, rooms and houses are warmed by the heat evolved during the condensation of steam, in iron or copper tubes which traverse the building, and the method is at once safe and effectual.

It is in consequence of the latent heat of steam, that, in the process of distillation, we are obliged to present so large a surface for condensation; and it is not difficult, by the help of a still, to calculate the latent heat of steam. If, for instance, one hun

1 See Black's Experiments, which prove the absorption of heat. Lectures, Vol. I. p. 157, &c.