PHYSICAL SCIENCE 37 



more nearly in accordance with the knowledge of a 

 later age was useless, and useful advance was made 

 in the light of a theory which after times rejected. 

 When Black (1728-1799) discovered the phenomena 

 of specific and latent heat the different amounts of 

 heat needed to warm different substances, and the heat 

 required to melt solids and evaporate liquids he 

 found a better working hypothesis in the rival theory 

 of heat as an imponderable, invisible fluid. This con- 

 ception suggested at once the idea of heat as a constant 

 measurable quantity, and on it the science of calori- 

 metry, or the measurement of heat, was founded. 



However, observations accumulated rapidly which 

 pointed to another view of heat, and linked it with a 

 wider range of phenomena. Count Rumford (1753- 

 1814) by the boring of cannon, and Sir Humphry 

 Davy (1778-1829) by the friction of two pieces of 

 ice in a vacuum, showed that an unlimited amount 

 of heat could be obtained an amount roughly, at 

 all events, proportional to the work expended. 



But the fluid theory still persisted, and it was not 

 till Joule measured carefully the heat developed in 

 friction by known amounts of work that it was 

 generally relinquished. Joule found that the same 

 amount of work, whether mechanical or electrical, 

 and however expended, always developed exactly 

 the same amount of heat that, in effect, heat and 

 work were equivalent and interchangeable. This 

 result gave definiteness and point to a vague fore- 



