PHILOSOPHICAL SOCIETY OF WASHINGTON. 141 



molecule of oxygen would undergo about seven thousand million 

 collisions in one second, a molecule of hydrogen among its fellows 

 would undergo about seventeen thousand million collisions per 

 second. It must be observed that the more violent the collisions of 

 the molecules, the less is their tendency toward the cohesion of the 

 liquid, or the adhesion of the solid form. 



Probable Size of Molecules. — From various considerations it has 

 been independently estimated by Joseph Loschmidt (1865), by G. 

 Johnstone Stoney (1868), by William Thomson (1870), and by J. 

 Clerk Maxwell (1873), that the effective size of the molecule is 

 probably not smaller than the thousand-millionth of an inch, nor 

 larger than three or four times this dimension ; which is about the 

 twenty-thousandth of a medium wave-length of light. Small as 

 this dimension is, we may reflect that by what may be called the 

 second power of our best microscopes, it would be easily visible, — 

 supposing that light-waves were capable of optical efficiency at this 

 degree of subdivision and amplification. 



These estimates of molecular distances and magnitudes are of 

 course but rough approximations ; but they indicate at least the 

 order of magnitude of very real things and agencies; and accepting 

 them as probable, we may " compare small things with great " by 

 saying that were the planet Venus brought within a distance from 

 our Earth about one aud a half times that of the Moon, this might 

 represent the relative mean distance of two molecules of our atmos- 

 phere ; at which separation ( about fifty times their own diameters), 

 they would probably count less than twenty million to the inch. 

 In like manner the distance of Venus from our Earth at conjunction 

 (as during the approaching transit of next Wednesday) would be 

 relatively comparable to the length of a mean excursion of the 

 molecules ; — some 3,000 times their diameter. While a few of their 

 longest free excursions would be comparable to the flight of the 

 the same planet if carried from the Earth to beyond the orbit of 

 Neptune. 



The Relation of Molecular and Atomic Motions. — Returning again 

 from this survey of molecular kinetics to the undulatory theory of 

 light and heat, we may say that the true physical relation of radia- 

 tion to conduction was first disclosed by the analytic spectrum, — 

 that marvellous instrumentality which physics has presented to her 



