278 THOMAS YOUNG 



was first introduced and defined. By it he was able to 

 avoid, and enable us to avoid, the confusion which had 

 crept into scientific literature by the incautious em- 

 ployment of the word force. Further, the theory now 

 known as the Young-Helmholtz theory, which refers all 

 the sensations of colour to three primary sensations 

 red, green, and violet was clearly enunciated by Young 

 in his thirty-seventh lecture, on 'Physical Optics.' His 

 views of the nature of heat were original and correct. 

 He regarded the generation of heat by friction as an 

 unanswerable confutation of the whole doctrine of 

 material caloric. He gave appropriate illustrations 

 of the manner in which he supposed the molecules of 

 bodies to be shaken asunder by heat. 'All these 

 analogies,' he says, 'are certainly favourable to the 

 opinion of the vibratory nature of heat, which has been 

 sufficiently sanctioned by the authority of the greatest 

 philosophers of past times and by the most sober 

 reasoners of the present.' In anticipation of Dr. Weils, 

 Young had observed and recorded the fact, that a cloud 

 passing over a clear sky sometimes causes the almost 

 instantaneous rise of a thermometer placed upon the 

 ground. The cloud he assumed acted as a vesture which 

 threw back the heat of the earth. Eadiant heat and light 

 are here placed in the same category. William Herschel 

 had already shown their kinship, by proving that the 

 most powerful rays of the sun were entirely non-lumi- 

 nous. Subsequent to this, the polarisation of heat, by 

 Principal James Forbes, rendered yeoman service in the 

 propagation of the true faith. 



Young's essay on the ' Cohesion of Fluids ' is to be 

 ranked amongst the most important and difficult of his 

 labours. It embraced his views and treatment of the 

 subject of capillary attraction. But as this topic is to 

 be treated here next week by a spirit kindred to that of 



