Prof. Tyndall's Contributions to Molecular Physics. 533 



cules of the body occupy positions which are determined by their 

 attractive and repulsive forces ; so that if any one molecule be 

 forcibly moved from its position of equilibrium, it will of neces- 

 sity disturb its neighbours. In a system of molecules so related, 

 it is manifest that motion could be transmitted independently of 

 the aether which surrounds them. If we could imagine the aether 

 entirely away, the motion that we call heat would still be pro- 

 pagated from molecule to molecule through such a body. Con- 

 duction would manifest itself, while radiation would be absent 

 through want of a medium. 



In matter, however, as we have it, molecular motion is only 

 in part transmitted immediately from molecule to molecule, 

 being in part transmitted mediately by the aether. Now the 

 quantity of motion transmitted by the aether to our second stra- 

 tum of molecules cannot be the whole of that which the first or 

 superficial stratum imparted to the aether. The aether must 

 retard and indeed squander the internal molecular motion ; and 

 were the medium absent — were the cushion removed which 

 interferes with the direct propagation of motion from molecule 

 to molecule — conduction would be freer than at present; the 

 heat would penetrate further into the mass than when the aether 

 intervenes. 



The reasoning just employed leads to the inference that those 

 molecules which experience most resistance from the aether must 

 be the least competent to transfer the motion of heat from one 

 to the other. The direct power of communication is enfeebled 

 by the aether, and the motion obtained indirectly cannot make 

 good the loss. "We are thus led to the conclusion that the best 

 radiators ought to prove themselves the worst conductors. 



A broad consideration of the subject shows that the conclu- 

 sion is in general harmony with observed facts. Organic sub- 

 stances are all exceedingly imperfect conductors of heat, and 

 they are all excellent radiators. The moment we pass from the 

 metals to their compounds we pass from a series of good con- 

 ductors to bad ones, and from a series of bad radiators to 

 good ones *. 



* And we also pass, as a general rule, from a series of bodies which 

 vibrate in accord with the visible spectrum to a series which vibrate in dis- 

 cord with the spectrum. The lowering of the rate of vibration is a conse- 

 quence of chemical union. The comparative incompetence of compound 

 bodies to oscillate in visual periods has incessantly declared itself in these 

 researches. I would here refer to a most interesting illustration of the 

 same kind, derived from the experiments of MM. De la Provostaye and 

 Desains. These distinguished experimenters were the first to record the 

 important fact that the qualities of heat emitted by bodies at the same 

 temperature may be very unlike. Two experiments illustrate this fact. 

 The first is recorded in the Cornptes Rendus, vol. xxxiv. p. 951. One half 



