34 LEADING AMERICAN MEN OF SCIENCE 



scope" or differential thermometer, consisting of a closed glass 

 tube with the bulbs at each end turned up. In the middle was a 

 drop of colored alcohol which moved in one direction or the other 

 when the bulbs were unequally heated. When he held a cylinder 

 filled with warm water and blackened on the bottom over one 

 bulb, and a cylinder with water at the same temperature and 

 bright on the bottom over the other, the drop of alcohol moved 

 instantly away from the blackened surface, showing that it emitted 

 heat more rapidly at the same temperature. By moving the cyl- 

 inder back and forth until the drop remained at rest, their relative 

 distances gave data for calculating their relative radiating power. 

 All metals, he found, gave off heat at the same rate, and he asks: 

 "Does not this afford a strong presumption that heat is in all 

 cases excited and communicated by means of radiations, or 

 undulations, as I should rather choose to call them ? " 



His theory of heat is so clearly expressed and anticipates in 

 so many respects our modern ideas, that it is worth quoting as an 

 example of the use of the scientific imagination. 



"No reasonable objection against this hypothesis (of the in- 

 cessant motions of the constituent particles of all bodies) founded 

 on a supposition that there is not room sufficient for these motions, 

 can be advanced; for we have abundant reason to conclude that 

 if there be in fact any indivisible solid particles of matter (which, 

 however, is very problematical) these particles must be so ex- 

 tremely small, compared to the spaces they occupy, that there 

 must be ample room for all kinds of motion among them. 



"And whatever the nature or directions of these internal mo- 

 tions may be, among the constituent particles of a solid body, as 

 long as these constituent particles, in their motions, do not break 

 loose from the systems to which they belong (and to which they 

 are attached by gravitation) and run wild in the vast void by 

 which each system is bounded (which, as long as the known laws 

 of nature exist, is no doubt impossible) the form or external ap- 

 pearance of a solid cannot be sensibly changed by them. 



"But if the motions of the constituent particles of any solid 

 body be either increased or diminished, in consequence of the 

 actions or radiations of other distant bodies, this event could not 

 happen without producing some visible change in the solid body. 



"If the motions of its constituent particles were diminished by 



