1891.] lo7 [Bache. 



in the case of glycerin, the molecules of which under ordinary con- 

 ditions show scarcely any, if any movement at all, to exhibit it 

 clearly when warmed up to fifty degrees centigrade of temperature. 

 Exner also examined into the properties of fluids in which solid 

 molecules remain suspended. The results of his investigation re- 

 solve themselves into the following points : (i) The liveliness of 

 the molecular movement is heightened by light and heat, and by 

 radiant as well as by conducted heat ; (2) one of the consequences 

 of the molecular movement is, that the molecules, in a specifically 

 lighter fluid, not only do not sink to the bottom, but overcome the 

 force of gravitation to such a degree as to spread themselves equally 

 throughout the fluid ; (3) the velocity of this scattering is as the 

 intensity of the molecular movements influenced by light and heat. 

 It should be mentioned here that Fr. Schultze had already stated 

 that substances, when most finely divided, especially such as seemed 

 under the microscope to be amorphous, and exhibited the brownian 

 movements, remain suspended in pure water and in many other 

 fluids for days, weeks, and months at a time, so that the fluid con- 

 taining them presents a cloudy, or at least an opalescent appear- 

 ance." 



The account of the views of Herr Wiener in the Jahresbericht 

 makes an important omission. It devotes itself chiefly to reciting 

 what, in his view, does not cause the brownian movements, but 

 does not mention precisely to what he does ascribe them. Herr 

 Wiener says, in the last paragraph of his article, in Foggendorf s 

 Annakn, 1863: — "The weight of the preceding conclusion, that 

 one cannot ascribe the trembling movement to any exterior cause, 

 is very greatly added to by the ascertained fact, that the diameter 

 of the similarly moved water masses is so small that it nearly corre- 

 sponds with the wave-length of red light, and still more closely with 

 that of radiant heat." This passage gives the keynote to his views 

 on the brownian movements. After a most elaborate series of 

 experiments, in which he measured on a micrometrically divided 

 glass slide, with the addition of diagonal lines, and by watch, the 

 range and the time of the movements, he reached his most impor- 

 tant conclusion, that, because the dimensions of the aforesaid wave- 

 lengths of light and heat have a certain close correspondence with 

 the diameters of the minutest particles and water masses, they form 

 the moving impulse of the motions of the particles. He pictured 

 to himself that the aether surrounding the particles, being continu- 



PllOC. AMER. PHILOS. SCO. XXXIII. 14j. V. PRINTED MAY 9, 1894. 



