Page:A biographical dictionary of eminent Scotsmen, vol 1.djvu/250

 him a most welcome visitor in every family. His principal friend at Glasgow was his associate Dr Adam Smith, professor of moral philosophy, with whom he had become intimate, when attending the university as a student A peculiar simplicity and sensibility, an incorruptible integrity, the strictest delicacy and correctness of manners, marked the character of each of the philosophers, and firmly bound them in the closest union.

"It seems to have been between the year 1759 and 1763, that his speculations concerning, which had long occupied his thoughts, were brought to maturity. And when it is considered by what simple experiments, by what familiar observations, Dr Black illustrated the laws of fluidity and evaporation, it appears wonderful that they had not long before been observed and demonstrated. They are, however, less obvious than might at first sight be imagined, and to have a distinct and clear conception of those seemingly simple processes or nature, required consideration and reflection. If a piece of wood, a piece of lead, and a piece of ice, are placed in a temperature much inferior to that of the body; and if we touch the piece of wood with the hand, it feels cold; if we touch the piece of lead, it feels colder still; but the piece of ice feels colder than either. Now, the first suggestion of sense is, that we receive cold from the wood; that we receive more from the lead; and most of all from the ice; and that the ice continues to be a source of cold till the whole be melted. But an inference precisely the contrary to all this is made by him, whose attention and reflection has been occupied with this subject. He infers that the wood takes a little heat from the hand, but is soon heated so much as to take no more. The lead takes more heat before it be as much satiated; and the ice continues to feel equally cold, and to carry off heat as fast as in the first moment, till the whole be melted. This, then, was the inference made by Dr Black.

"Boerhaave has recorded an interesting observation by Fahrenheit, namely, that water would sometimes grow considerably colder than melting snow without freezing, and would freeze in a moment when shaken or disturbed; and in the act of freezing give out many degrees of heat. Founded on this observation, it appears that Dr Black entertained some vague notion or conjecture, that the heat which was received by the ice, during its conversion into water, was not lest, but was still contained in the water. And he hoped to verify this conjecture, by making a comparison of the time required to raise a pound of water one degree in its temperature, with the time required to melt a pound of ice, both being supposed to receive the heat equally fast. And that he might ascertain how much heat was extricated during congelation, he thought of comparing the time required to depress the temperature of a pound of water one degree, with the time required for freezing it entirely. The plan of this series of experiments occurred to him during the summer season. But for want of ice, which he could not then procure, he had no opportunity of putting them to the test. He therefore waited impatiently for the winter. The winter arrived, and the decisive experiment was performed in the month of December 1761. From this experiment it appeared that as much heat was taken up by the ice, during its liquefaction, as would have raised the water 140 degrees in its temperature, and on the other hand, that exactly the same quantity of heat was given out during the congelation of the water. But this experiment, the result of which Dr Black eagerly longed for, only informed him how much heat was absorbed by the ice during liquefaction, was retained by the water while it remained fluid, and was again emitted by it in the process of freezing. But his mind was deeply impressed with the truth of the doctrine, by reflecting on the observations that