816 PEOGRESS IN PHYSICS. 



centuiy" in the domain of physics one must not attempt more than 

 an outline of greater events, and it will be convenient to arrange them 

 under the several principal subdivisions of the science according- to 

 the usually accepted classification. 



HEAT. 



Although more than one philosopher of the seventeenth and eight- 

 eenth centuries suggested the identity of heat and molecular motion, 

 the impression made was not lasting, and up to ver}' near the begin- 

 ning of the nineteenth century the caloric theory was accepted almost 

 without dispute. This theory implied that heat was a subtile fluid, 

 definite quantities of which were added to or subtracted from material 

 substances Avhen they became hot or cold. As carefully conducted 

 experiments seemed to show that a body weighed no more or no less 

 when hot than when cold, it was necessary to attribute to this fluid 

 called caloric the mysterious property of "imponderability;"'' that is, 

 unlike all forms of ordinary matter, it possessed no weight. To avoid 

 calling it matter it was b}^ many classed, with light, electricity, and 

 magnetism, as one of the imponderable agents. Various other prop- 

 erties were attributed to caloric, necessary to the reasonable explana- 

 tion of a steadily increasing array of experimental facts. It was 

 declared to be elastic, its particles being mutually self-repellant. It 

 was thought to attract ordinary matter, and an ingenious theory of 

 caloric was constructed, modeled upon Newton's famous but erroneous 

 corpuscular theory of light. During the latter part of the eight- 

 eenth century Joseph Black, professor in the universities of Glasgow 

 and Edinburgh, developed his theory of latent heat, Avhich, although 

 founded upon a false notion of the nature of heat, was a most impor- 

 tant contribution to science. The downfall of the caloric theory umst 

 be largely credited to the work of a famous American who published 

 the results of his experiments just at the close of the eighteenth 

 centurv. 



Benjamin Thompson, known as Count Rumford, was born in the 

 town of Woburn, Mass., in 1753. His inclination toward physical 

 experimentation was strong in his early youth, and he received much 

 instruction and inspiration from the lectures of Prof. John Winthrop, 

 of Harvard College, some of which he was enabled to attend imder 

 trying conditions. Having received special official consideration by 

 appointment to office under one of the colonial governors, he was 

 accused at the breaking out of the Revolutionary war of a leaning 

 toward Toryism, and was thus prevented from making his career 

 among his own people. At the age of 22 years he fled to England, 

 returning to America only for a brief period in command of a British 

 regiment. In England he soon became eminent as an experimental 

 philosopher, and in 1778 became a Fellow of the Royal Society. He 



