168 SCIENTIFIC THOUGHT. 



available energy : he introduced the word " motivity," the 

 conception of a quantity of a "possession the waste of 

 which is calkd dissipation." Whilst Thomson was thus 

 putting into scientific language and calculating an im- 

 portant and obvious property of nature — namely this, that 

 her processes mainly proceed in a certain definable direc- 

 tion — Rankine and Clausius were labouring independ- 

 ently at the mathematical wording, the analytical expres- 

 sion, of this remarkable discovery. Wherever a change in 

 a system of various elements, factors, or quantities takes 

 place mainly in a definite sense or direction, it is presum- 

 able that there exists a definite quantity which is always 

 growing or always decreasing. This quantity may not 

 be directly observable or measurable, as in mechanical 

 motion velocity or distance is directly measurable ; it 

 may be hidden — we may have no special sense with 

 which we can perceive it, as we possess a pressure sense, 

 a heat sense, a sound and light sense ; nevertheless, it 

 may be indirectly discoverable, being made up (a func- 

 tion) of definite observable quantities and factors (such 

 as heat, temperature, mass, volume, pressure, &c.) Now 

 Rankine and Clausius found that in all thermal changes 



mechanische Warmetheorie,' vol. i. 

 p. 387, and vol. ii. p. 324 sqq. A 

 great deal of this confusion would 

 have been avoided had Tait in 1868 

 introduced a really new term — viz., 

 that suggested later (1876) by 

 Thomson in a communication to 

 the Royal Society of Edinburgh, 

 and more fully explained in a 

 paper in the ' Phil. Mag. ,' May 

 1879, the term " Thermo-dynamic 

 Motivity." W^e should then have 

 two terms, inasmuch as the "con- 



sideration of the energy and 

 motivity, as two functions of all 

 the independent variables specify- 

 ing the condition of a body com- 

 pletely in respect to tempera- 

 ture, elasticity, capillary attraction, 

 electricity, and magnetism, leads 

 in the simplest and most direct 

 way to demonstrations of the theo- 

 rems regarding the thermo-dynamic 

 properties of matter" {loc. cit., 

 'Papers,' vol. i. p. 459). 



