2 Ramsay, The Newly Discovered Elements. 



referred to equal weights, they are termed specific heats. 

 The term is due to Crawford, and the expression 

 " capacity for heat " dates back to the time when heat 

 was supposed to be an " imponderable form of matter " — 

 a view now long abandoned. Crawford, too, was the 

 first to attempt to measure the specific heats of gases, by 

 enclosing them in thin copper spheres, warming the vessels 

 with their contents, and plunging them into a calorimeter ; 

 a plan which, with some modifications, has in recent years 

 met with success in the hands of Dr. Joly. Your fellow- 

 townsman, Dalton,of immortal memory, in repeating Craw- 

 ford's experiments, began the investigation of a fact which 

 had been previously known, namely, that when a gas is 

 compressed, it grows warm. He showed, in a paper 

 before this Society in 1801, that the converse also holds 

 true ; that when a gas is allowed to expand against 

 pressure, it grows cold. Between that early date and the 

 year 1841, attempts were made, more or less successfully, 

 to measure the specific heats of gases ; and the idea was 

 rapidly gaining ground that heat was not an intangible 

 and imponderable substance, but a form of motion of the 

 ultimate parts of which all bodies are composed, their 

 molecules. This idea, even, was not new, but can be traced 

 back as far as Robert Boyle, who wrote about the 

 year 1660: — "Heat will appear the more likely to be 

 mechanically producible from considering the nature of it 

 And this seem-S principally to consist in that mechanical 

 property of matter we call motion ; which is subject to 

 three conditions. First, the agitation of the parts must 

 be vehement ; second, the determinations [or motions] 

 must be very various, and tend all manner of ways ; and 

 third, the agitated particles must be so minute as to be 

 singly insensible." 



And again : — " That vehement and tumultuous agita- 



