Such were the notions which prevailed on the subject prior to the commencement of Dr. Black’s researches. No one advanced, or seemed to have any desire to advance, a step nearer to the truth: yet the whole was a mere question of fact, and a fact of the most obvious nature. In this we cannot fail to observe one of those instructive instances, which the history of science often brings before us, of the unaccountable blindness, even of inquiring minds, to truths constantly before their eyes, or, if perceived, to the importance of their being thoroughly examined. A very little consideration ought to have shown any observer, that the gain or loss of heat in the cases in question is by no means slight or trifling in amount: yet no one thought of this till Dr. Black pointed it out; and no one reasoned upon it, or perceived its bearing, till that philosopher showed the curious inference to be drawn from it. The case was simply this:—Two equal vessels, one full of water just at the freezing temperature, the other of actual ice, are brought into a warm room. In a short time the water acquires the temperature of the room. Exactly the same quantity of heat has been communicated to the vessel of ice; yet, at the end of the same time, it is found to retain precisely the same temperature as at first. A considerable part of it indeed has been melted, but it may take several hours more to melt the whole. Until that change is completed, the temperature does not vary a single degree. As soon as all the ice is liquefied, and not before, the temperature of the mass begins to rise, and proceeds to increase, from this time, as rapidly as that of the water in the other vessel did before, until it acquires the temperature of the room. What then, Dr. Black enquired, becomes of the heat which has been all along given to the vessel of ice? Heat has been communicated to it as well as to the other vessel; yet it has not been employed in raising the temperature, but in some way has been expended in converting the ice into water. It is but this simple fact otherwise stated, when we say that the heat so imparted has _disappeared_ as heat of temperature; but may it not have been destroyed or annihilated? To reply to this question we have only to consider that the same vessel of water, cooled nearly to the freezing point, and then exposed to a much greater degree of cold, must, by the same rule, continue parting with its excess of temperature above that of the colder bodies around it. Yet a thermometer immersed in it continues invariably at 32° till the whole has become ice; it then will sink to the lower temperature, but not before. Thus there must be within it a continued supply of _heat_ in order to keep it up to 32° all the time.