Memoir of Thomas Qraham. 

 however short the tube, the effects of fVictu)n ii 



While, however, it thus appears that the re^ult^ ol Cirahain s 

 investigation were in strict accordance with Daltou'b tlio()r\, it 

 must also be evident that Graham wiis the first to observe' the 

 exact numerical relation which obtains in this class of phenom- 

 ena, and that all-important circumstance entitles linn to be re- 

 garded as the discoverer of the law of DitluMon 1'1u'L.n\, 

 however, as first enunciated, was purely empirical. <uid (ml mi 

 himself says that something more must be a^sunud tli.m tli.a 

 gases are vacua to each other, in order to explain .ill tlu plu- 

 nomena observed; and according to his oriLiiiMl \h a tl ■- -^ p 

 of the process 



xpressing the final result Such has ] 

 Like other great men, Graham built 1 



the progress of physical science dunnu tli* 1 i-t t'\. ii^ \ 

 years, two principles have become nioie a'pl u <>i( , m,-i^ i i 'U-. 

 until at last they have completely rc\oluti(Mii -■< liii pi .'"- | i\^ 

 of Chemistry. In the first place it li.is app* nni th ii i * '>-t .)1 

 chemical as "well as of physical facts are o>oi dm. 'tu I li\ iiu .i- 

 sumption that all substances in the state of tias lia\ < tlu - mic 

 molecular volume, or, in other words contain the - !..< u .mi'i ■ 

 of molecules in a given space ; and m the second ["'ai' , ' ^' " 

 become evident that the phenomena of beat .lu -.'.[''. th' 

 manifestations of molecular motion. According to tin- \itu, 

 the temperature of a body is the vis vita of its luolec aU- . <md, 

 since all molecules at a given temperature ha\e tlu' .-aiue iis 

 viva, it follows that the molecules must nun e with \ elocities 

 which are inversely proportional to the square roots ol tlie 

 molecular weights. Moreover, since the molecu'ar \ ulnmes are 

 equal, aiwi the molecular weights therefore proportion.d to the 

 densities of the aeriform bodies in which the molecules ire the 

 active units, it also follows that the velocities of the molecules 

 in anv two gases are inversely proportional to the square roots 

 of their respective densities.'^ Thus the simple numerical rela- 

 tions first observed in the phenomena of diffusion are the direct 

 result of molecular motion, and it is now seen that Grahams 

 empirical law is included under the fundamental laws ol motion. 

 His investigation has become the basis of the new science ot 

 molecular mechanics, and his measurements of the rates ol Uit- 

 fusion prove to be the measures of molecular \ clooties 



From the study of diffusion, Graham passed 1)\ a natural 

 transition to the investigation of a class of phenomm.,. wln^h, 

 although closely allied to the first as to the etiects pi;>' ""''i» 

 differ wholly in their essential nature. Here also he followed 

 in the footsteps of Dalton. This distinguished chemist had 

 noticed that a bubble of air separated by a film of water from 



