PHYSICS OF THE EIGHTEENTH CENTURY. 287 



education. Some academical controversies here directed his atten- 

 tion to certain unsolved problems in chemistry, and the result was the 

 masterly investigations by Black which will be subsequently described. 

 In 1756 Black was appointed the successor of Dr. Cullen in the chair 

 of anatomy and chemistry at Glasgow. The duties of the former, 

 however, he exchanged for the professorship of medicine, and he 

 soon acquired fame in his professorial capacity, and added a lucrative 

 medical practice to his other emoluments. It was between 1759 and 

 1762 that he completed those important researches concerning heat, 

 which laid the foundation of all our scientific knowledge of this subject, 

 and which enabled Watt soon afterwards to produce the steam engine, 

 surely the greatest present that Science ever made to Industry. Black 

 continued for thirty years to occupy his professorial chair in Glasgow 

 His lectures were so clear, and, from all absence of affectation, so 

 pleasing, that his class-rooms were always crowded. In his seventy- 

 first year Black died as he sat at meat, and so peacefully, without a 

 moment's illness, that his servant supposed he had fallen asleep with 

 the basin of milk in his hand unspilled. The manner of his departure 

 was, indeed, of the kind he had often wished it might be. The inves- 

 tigations which have made Black's name famous were completed 

 before he had reached his thirty-fourth year; yet he left it to others to 

 explore the field he had opened out, and devoted the rest of his life 

 to perfecting his courses of lectures as professor of chemistry in the 

 University of Glasgow. The researches of which we have to treat in 

 the present chapter were not published in a separate form, but were 

 incorporated into Black's professional lectures, and in this way be- 

 came known to the world. 



The thermometer, it should be observed, does not really directly 

 measure heat, but temperature. That which it actually indicates is the 

 change of volume which the mercury undergoes in changing its tempe- 

 rature ; or, to be quite accurate, it is the difference of the expansion 

 and contraction of the mercury and the glass which contains it. Al- 

 together apart from any theory of the nature of heat, it is obviously 

 true that a certain quantity of it must be required to raise the tempe- 

 rature of i Ib. of water from, let us say, 40 to 50, and that to raise 

 2 Ibs. of water from 40 to 50 twice 2& much heat will be required, and 

 to raise 3 Ibs. three times as much, and so on. We thus begin to get 

 the idea of heat as a quantity, and we may proceed to ask such a ques- 

 tion as this : Is the same quantity of heat required to raise the tempe- 

 rature of i Ib. of water from 50 to 60 as to raise it from 40 to 50? 

 There are many persons who, seeing that 10 is in both cases the range 

 of temperature through which the same weight of water has to be raised, 

 would confidently answer that the same quantity would be required. 

 But it is not the method of science to jump at conclusions in this way. 

 But when i Ib. of water at 60 is mixed with i Ib. of water at 40, it 

 is actually found that the temperature of the resulting mixture is 50. 



