PHILOSOPHICAL SOCIETY OF WASHINGTON. 149 



may call it, perfect purity of optical tone is to be observed. With 

 any considerable compression of a gas, that is, with any great 

 crowding together of the molecules and shortening of their mean 

 free excursions, whereby the increased frequency of collision is con- 

 stantly disturbing the atomic orbits before their motions can be fully 

 absorbed by the sether, there will result a momentary hastening or 

 retarding of the normal periods, giving to the spectral lines an in- 

 creased breadth or wider range of refrangibility. And when the 

 condensation reaches that of the "liquid" or "solid" condition, 

 preventing all free excursion, the incessant agitation of the atoms 

 results in a universal clang or optical "noise," in which all uni- 

 formity of period seems lost, and perturbations of all possible degrees 

 present us with the discord and confusion of a perfectly continuous 

 spectrum.* 



The Chemist has taught us that in numerous cases the normal 

 molecule is divided into sub-molecules. Thus the relations of the 

 compounds of arsenic, as well as of those of phosphorus, indicate 

 the composition by half molecules of these elements ; the ratios of 

 the so-called " sesqui-salts " point to the same result; the allotropic 

 condition of oxygen — called ozone — is formulated as having the 

 equivalency of one and a half molecules ; one molecule of aqueous 

 vapor (and therefore of water) consists of one molecule of hydro- 

 gen and a half molecule of oxygen ; two molecules of ammonia 

 are resolved into three equal molecules of h)'drogen and one of 

 nitrogen ; and a single molecule of hydrogen united with a single 

 one of chlorine will form two molecules of hydrochloric acid, — each 

 containing an equal division of the two constituents. Although 

 this dichotomy of the molecule is suggestive of binary systems in 

 some way specially linked together and at the same time susceptible 

 of various re-arrangements, yet the fact remains that these divided 

 molecules are still extremely complex physical systems, — apparently 

 identical in constitution and construction, and therefore uudistiu- 

 guishable from each other. The Chemist however adhering too 

 literally to the phrase of Dalton, has neglected the obvious import 



* J. Clerk Maxwell has felicitously compared the atomic oscillations 

 producing a continuous spectrum, to the clang of a bell "on which innu- 

 merable hammers are continually plying theirstrokes all out of time, [when] 

 the sound will become a mere noise in which no musical note can be dis- 

 tinguished." (Encyclopced. Brit. 1875: art. "Atom:" vol. II, p. 43.) 



