G. Chrystal, W. J. M. Rankine, J. Thomson 135 



with the summary of facts contained in a mathematical 

 formula, but require a definite picture of atoms and molecules, 

 whose dynamical interactions he tried to trace in their details. 

 He invented theories on the causes of elasticity, the constitu- 

 tion of gases, and the motion which constitutes heat. But 

 while most of these theories had to be abandoned, the use 

 which he made of them, and the consequences he drew from 

 them, remained, because they were founded on true dynamical 

 principles, and the results proved in many cases to be inde- 

 pendent of the particular hypothesis from which they happened 

 to be derived. Inspired by Joule and Kelvin, the dynamical 

 theory of heat occupied much of his attention, and he was 

 an early convert to the doctrine of the conservation of 

 energy. We owe to him the introduction of the term 

 " potential energy," one of the happy inspirations which, 

 furnishing an appropriate nomenclature, allowed the funda- 

 mental principle of the conservation of energy to be expressed 

 in a crisp and impressive form. Among his more technical 

 papers, the most important ones deal with stream lines, 

 the efficacy of propellers, and the construction of masonry 

 dams. Rankine was an accomplished musician, and occa- 

 sionally indulged in poetry. Some of the songs composed 

 and set to music by himself were published in a separate 

 volume. 



Rankine 's successor at Glasgow University, James Thom- 

 son, was a man of almost equal distinction. Like his brother, 

 Lord Kelvin, he never went to school. The two brothers 

 passed through the University together, and James took 

 his M.A. degree at the age of seventeen. He was for a time 

 apprenticed to Messrs. Fairbairn at Manchester, but bad 

 health obliged him to return home, where he occupied himself 

 with the invention of appliances for the better utilisation 

 of water power. At various periods of his life he returned 

 to the subject, and we owe to him several forms of water- 

 wheels, a centrifugal pump, and improvements in turbines. 

 At a meeting of the British Association in 1874 he described 

 a pump for drawing up water by the power of a jet, which 

 led to the construction of such pumps on a large scale. Among 

 his purely scientific contributions, that on the lowering of 

 the freezing point of water by pressure is the most important. 



