less remarkable, and delighted the best performers. Watt solved an important part of the problem; he found out the temperament assigned by a master of the art, by help of the phenomena of the beats of imperfect consonances, then very ill understood, and of which he could have gained no knowledge except from the profound, but very obscure work of Dr. Robert Smith of Cambridge.*

HISTORY OF THE STEAM-ENGINE.

I HAVE now come to the most brilliant period of Watt's life, and also, I am afraid, to the most difficult part of my undertaking. As to the immense importance of the inventions which are now to be the subject of my discourse, no doubt can possibly be entertained; but I fear that I cannot make them be appreciated as they deserve, without going into very minute numerical comparisons. In order that these comparisons, if indispensable, may be readily understood, I shall now lay before you, as briefly as possible, the nice doctrines of Natural Philosophy on which we shall have to rest them.

* See the article TEMPERAMENT in the Encyclopædia Britannica, which is given in Brewster's edition of Robison's Mechanical Philosophy, vol. iv. p. 412. The title of Dr. Smith's book is, "Harmonics, or the Philosophy of Musical Sounds, by Robert Smith, D.D., F.R.S., and Master of Trinity College, in the University of Cambridge." The first edition of this work was published at Cambridge in 1749; the second, much improved and augmented, at London in 1759.—Tr.

By the effect of a mere change of temperature, water may exist in any one of three states, quite distinct from each other. These are the solid, the liquid, and the gaseous or vapour state. Below zero in the scale of the centigrade thermometer, [32° of Fahrenheit] water becomes ice; at 100°, [212° Fahrenheit] it turns rapidly into vapour; at all the intermediate degrees it is liquid.

A close observation of the points of transition from one of these states to another, leads to discoveries of the utmost importance, which form the key to the economical appreciation of steam-engines.

Water is not necessarily warmer than all kinds of ice; water may remain at a temperature of zero [centigr.] without freezing; ice may remain at zero without melting; but there is great difficulty in believing that this water and this ice, both at the same degree of temperature, both at zero, differ only in their physical qualities; or that no element, other than water, properly so called, distinguishes the solid from the liquid water. A very simple experiment will throw light on this mystery. Mix a kilogram * of water at zero, with a kilogram of water at 75°, centigrade. The two kilograms of mixture will have a temperature of 37 degrees; that is to say, the mean temperature of the two component liquids. The hot water is thus found to have retained 37° of its former tem

* A kilogram is 2.679514 lbs. troy, or 2.204857 lbs. avoirdupois.-TR.

perature ;—it has communicated the other 37° to the cold water. All this is quite natural, and might have been anticipated.*

Let us, however, repeat the experiment with one single difference. In place of the kilogram of water at zero, let us take a kilogram of ice at the same temperature of zero. From the mixture of this kilogram of ice with the kilogram of water at 75°, will result two kilograms of liquid water, because the ice, steeped in hot water, will certainly be dissolved, and will continue of the same weight as before. But were you to attribute to the mixture, as in the former instance, a temperature of 374°, you would be deceived; the temperature here will only be zero, there will remain no trace of the 75° of heat which the kilogram of water possessed; these 75° will have separated the atoms of ice from each other, and will have blended with them, but without heating them in any way whatsoever.†

I have no hesitation in offering this experiment of Black as one of the most remarkable in modern

* "When hot and cold water are mixed together, the excess of heat contained in the hot water is equally distributed in an instant through the whole mixture, and raises the temperature of it according to the greatness of the excess of temperature, and the proportion which the hot water bore to the cold. If the quantities of hot and cold water are equal, the temperature is the middle degree between that of the hot and that of the cold." See Dr. Black's Lectures on Chemistry, vol. i. p. 122. Edit. 1803.—Tr.

+ The fact is stated by Dr. Black in these words: “ I have, in the same manner, put a lump of ice into an equal quantity of water, heated to the temperature of 176°, and the result was that the fluid was no hotter than water just ready to freeze." Black, vol. i. p. 125.-TR.

B

physics. The following are, in fact, its consequences :—

Water and ice, both at zero, [centigr.] differ intimately in their composition. The liquid contains more than the solid does, 75° of an imponderable body, which is called heat. These 75° are so well concealed in the watery mixture,-I had almost said in the watery alloy,—that the most delicate thermometer does not indicate their presence. Heat, not perceptible by our senses, not perceptible by the most sensitive instruments; in a word, LATENT HEAT,-for this is the name it has received, -is, therefore, one of the constituent elements of bodies.

A comparison of boiling water, or water at 100° [centigr.] with the steam which issues from it, and the temperature of which is also 100°, leads, but on a far more extensive scale, to analogous results. At the instant of forming itself into the state of vapour at 100°, water, at the same temperature of 100°, is impregnated, under a latent form, under a form which is not perceptible by the thermometer, with an enormous quantity of heat. When the steam returns to a liquid state, this compositionheat is disengaged, and communicates itself to any thing in its way, which is capable of absorbing it. If, for instance, a single kilogram of steam, at 100°, be made to pass through 5·35 kilograms of water at zero, this steam becomes altogether liquid. The 6.35 kilograms, which are the result of the mixture, stand at 100° temperature. There enters, then, into the intimate composition of a kilogram

of steam, a quantity of latent heat capable of raising a kilogram of water (if prevented from evaporating), from 0 up to 535 centigrade degrees. This result will, no doubt, appear enormous, but it is certain; water, as steam, exists only in this condition; wherever a kilogram of water at zero is naturally or artificially converted into steam, it ought to gain, in order to its transformation, and in fact it does gain from surrounding bodies, 535° of heat. These degrees, as can never be too often repeated, are restored integrally by the steam to the surfaces of all sorts on which its ulterior liquefaction takes place. Here we have, as I may observe in passing, the whole secret of heating by steam. This ingenious process is very imperfectly understood, when it is imagined that the aqueous gas conveys to a distance, in the tubes through which it circulates, only sensible or thermometric heat; the chief effects produced are owing to the composition-heat, the concealed heat, the latent heat, which is disengaged at the instant that the contact of cold surfaces reconverts the steam from the gaseous to the liquid state.

We may now, then, rank heat among the constituent elements of steam. To obtain heat, we must burn some kind of fuel; steam has, then, a commercial value greater than that of the liquid, by the whole price of the combustible employed in the act of conversion into steam. If the difference between these two values is very great, it must be attributed principally to latent heat; ther