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XXXII. An Account of an Experiment on Heat . By George 
Fordyce, M. D. F. R. S. In a Letter to Sir Jofeph Banks, 
Bart . P. R . S . 
Read May 24, 1787. 
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S 1 R, 
H EAT changes the qualities and appearances of matter in 
various ways. It is alfo a powerful agent in many of 
the operations which mankind employ to fit matter for their 
ufe. Although the ancients performed many of thefe opera- 
tions with a coniiderable degree of accuracy, yet there are 
many which they were totally unacquainted with, and others 
they brought to little perfection. One principal caufe was 
their having no means of meafuring heat accurately. Van 
Helmont was the firft who found the mode of meafuring heat 
by expanlion. His meafure was an air thermometer, which is 
deferibed in his Differtation, named “ Aer”, cap . 12. Since his 
time, various improvements have been made on thermometers ; 
many are {till wanted. This inftrument is, however, the 
foundation of modern difeoveries on this fubjeft. The an- 
cients were acquainted with the manner of heating bodies by' 
communication, by friction, by burning fuel, by the fun, by 
fermentation, and the taking place of chemical combinations 
in other cafes. Boyle found, that melting a {olid body pro- 
duced cold (Experimental Hiftory of Cold, title I. chap: 18.) : 
3 Dr. 
