Latent Heat.

Progress, Volume I, Issue 6, 2 April 1906, Page 152

Latent Heat.

What is termed latent heat is exhibited in the following manner .—lf,. — If, m consequence of the application of heat, a solid passes into a liquid, as ice, for instance, into water, the conversion occupies a longer time than could be explained by any theory which had been propounded down to the middle of the eighteenth century. Neither was it possible to explain how it is that ice never rises above the temperature of 32 0 until it is actually melted, no matter what the heat of the adjacent bodies may be. There were no means of accounting for these circumstances. And though practical men, being familiar with them did not wonder at them, they caused great astonishment among thinkers, who were accustomed to analyse events, and to seek a reason for common and every-day occurrences. Soon after the middle of the eighteenth century, Black, who was then one of the professors in the University of Glasgow, turned his attention to this subject He struck out a theory which, being eminently original, was violently attacked, but is now generally admitted. With a boldness and reach of thought not often equalled, he arrived at the conclusion that whenever a body loses some of its consistence, as in the case of ice becoming water, or water becoming steam, such body receives an amount of heat which our senses, though aided by the most delicate thermometer, can never detect. For this heat is absorbed ; we lose all sight of it, and it produces no palpable effect on the material world, but becomes as it were a hidden property. Black therefore, called it latent heat, because, though we conceive it as an idea, we cannot trace it as a fact. The body is, properly speaking, hotter ; and yet its temperature does not rise. Directly however the foregoing process is inverted, that is to say, directly the steam is condensed into water, or the water hardened into ice, the heat returns into the world of sense ; it ceases to be latent, and communicates itself to the surrounding objects. No new heat has been created ; it has indeed appeared and, disappeared so far as our senses are concerned ; but our senses were deceived, since there has in truth been neither addition nor diminution. That this remarkable theory paved the way for the doctrine of the indestructibility of force, will be obvious to whoever has examined the manner in which, m the history of the human mind, scientific conceptions are generated. The process is always so slow, that no single discovery has ever been made, except by the united labours of several successive generations. In estimating, therefore, what each man has done, we must judge him, not by the errors he commits, but by the truths he propounds. Most of his errors are not really his own He inherits them from predecessors and if he throws some of them oft we should be grateful, instead of being dissatisfied that he has not rejected all. Black, no doubt, fell into the error of regarding heat as a material substance, which obeys the laws of chemical composition. But this was merely a hypothesis which was bequeathed to him, and with which the existing state of thought forced him to encumber his theory. He inherited the hypothesis, and could not get rid of his troublesome possession. The real service which he rendered is that in spite of that hypothesis which clung to him to the last, he, far more than any of his contemporaries, contributed towards the great conception of idealising heat, and thus enabled his successors to admit it into the class of immaterial and supersensual forces.