. I TBK DYNAMICAL EVIDENCE OF THE 



tUMonoo of molecules, would be far greater than we have any reason to believe 



it to be. 



A much more difficult investigation, which has hardly yet been attempted, 

 to the electric properties of gases. No one has yet explained why dense 

 are such good insulators, and why, when rarefied or heated, they permit 

 the discharge of electricity, whereas a perfect vacuum is the best of all insulators. 



It is true that the diffusion of molecules goes on faster in a rarefied gas, 

 & e mean path of a molecule is inversely as the density. But the 

 difference between dense and rare gas appears to be too great to be 

 accounted for in this way. 



But while I think it right to point out the hitherto uncouquered difficulties 

 of this molecular theory, I must not forget to remind you of the numerous 

 farta which it satisfactorily explains. We have already mentioned the gaseous 

 laws, as they are called, which express the relations between volume, pressure, 

 and temperature, and Gay Lussac's very important law of equivalent volumes. 

 The explanation of these may be regarded as complete. The law of molecular 

 specific heats is less accurately verified by experiment, and its full explanation 

 depends on a more perfect knowledge of the internal structure of a molecule 

 than we as yet possess. 



But the most important result of these inquiries is a more distinct con- 

 ception of thermal phenomena. In the first place, the temperature of the 

 medium is measured by the average kinetic energy of translation of a single 

 molecule of the medium. In two media placed in thermal communication, the 

 temperature as thus measured tends to become equal. 



In the next place, we learn how to distinguish that kind of motion which 

 we call heat from other kinds of motion. The peculiarity of the motion called 

 heat is that it is perfectly irregular ; that is to say, that the direction and 

 magnitude of the velocity of a molecule at a given time cannot be expressed as 

 depending on the present position of the molecule and the time. 



In the visible motion of a body, on the other hand, the velocity of the 

 re of mass of all the molecules in any visible portion of the body is the 

 observed velocity of that portion, though the molecules may have also an irregular 

 depending agitation on account of the body being hot. 



In the transmission of sound, too, the different portions of the body have 

 a motion which is generally too minute and too rapidly alternating to be directly 

 observed. But in the motion which constitutes the physical phenomenon of sound, 



