3io HISTORY OF SCIENCE. 



remarkable conclusion : " It appears to me to be extremely difficult, 

 if not quite impossible, to form any distinct idea of anything capable 

 of being excited and communicated in the manner the heat was excited 

 and communicated in these experiments, EXCEPT IT BE MOTION." 



These experiments and the author's conclusions from them did 

 not attract the attention they deserved, for men's minds were pre- 

 possessed with the notion of caloric. It was not, in fact, until more 

 than forty years after Rumford published his inquiry that the concep- 

 tion of heat as a mode of motion began to supersede the notion of 

 heat as the subtle fluid caloric. The caloric theory had indeed been 

 found useful as a means of expressing many facts, and of binding them 

 together by means of a common principle. Caloric, as we shall have 

 occasion more fully to discuss in another chapter, displaced phlogiston 

 in the interpretation of chemical phenomena, and easily explained a 

 considerable range of physical phenomena; but in explanation of 

 those adduced by Rumfard it utterly failed, and other experiments 

 were soon after devised to which the caloric theory was, if possible, 

 still more obviously inapplicable. For instance, Sir Humphry Davy 

 (whose great chemical discoveries will be mentioned in another chap- 

 ter) devised a very ingenious experiment in 1799. He arranged an 

 apparatus by which two pieces of ice could be made to rub against 

 each other in the exhausted receiver of an air-pump. In a few minutes 

 the pieces of ice were entirely converted into water, and this water 

 was collected and found to have a temperature of 35 F., although the 

 temperature of the atmosphere was lower than this. According to the 

 notions then prevalent, the capacity for heat had diminished; but, says 

 Davy, the capacity of water for heat is much. greater than that of ice, 

 and ice must have an absolute quantity of heat added to it before it can 

 be converted into water. Friction consequently does not diminish the 

 capacity of bodies for heat. The heat developed in this experiment 

 could not be derived from the air, for none was present, nor from the 

 parts of the apparatus, for the liquefaction took place only at the sur- 

 faces of the ice which Ribbed against each other. Davy came to the 

 conclusion that the immediate cause of heat is the motion of the in- 

 visible particles of bodies, and that it is by the ordinary laws of the 

 communications of motion that friction produces heat. He conceives 

 that the motion may consist of vibratory or undulatory movements of 

 the particles of bodies, or that these particles may revolve on their 

 axes or round each other. The particles of the hottest bodies, he 

 supposed, would move with the greatest velocity, and through a space 

 relatively greater. Temperature may be conceived to depend upon 

 the velocity of the vibrations ; increase of capacity upon the motions 

 being performed through greater spaces ; disappearance of sensible 

 heat in the conversion of solids into liquids, and these into gases, may 

 be explained by these states consisting in greater amplitude of the 

 vibrations of the particles. 



