Bache.] -^^^ [April 20, 



to see, but one that I have mentioned, he did not rightly interpret. 

 It was a distinct lapse in his acuteness of observation, that involved 

 in his mention, without perception of its deepest significance, of 

 minute drops of oil, some of them not exceeding the solid particles 

 themselves in size, standing nearly or altogether at rest on the sur- 

 face of water ; the pointing of which fact is very clear, one which, 

 had it been known to me, would have induced me to try oil as well as 

 water among the first instead of among the last of my own experi- 

 ments, as actually happened in their sequence. 



To afford facility for the fullest comprehension of the subject, it 

 becomes proper here to resume the historical tenor of our way with 

 a brief account of some of the views of Herren Wiener, Exner, and 

 Schultze, more recent than those of Dr. Brown. In order to avoid 

 the responsibility for the necessary condensation, I prefer to quote, 

 as follows, from the summary of their views in the Jahresbericht of 

 1867: 



" Then Chr. Wiener, from whose account the preceding historical 

 remarks are quoted, instituted microscopic observations of these 

 movements, and came to the conclusion that this trembling, irregu- 

 lar, unsteady motion of solid molecules, which alter their direction 

 in the briefest fraction of time in their zigzag course, has for its 

 basis the continual movements which, by virtue of their molecular 

 constitution, belong to fluids. He learned through his investiga- 

 tions (i) that the movements are not those of infusoria ; (2) that the 

 movement is not communicated to the fluid; (3) that the trembling 

 movement is not in any way derived from the varying attraction 

 and collision of the various oscillating molecules with one another; 



(4) that the movement is not derived from changes of temperature ; 



(5) that, also, the movement is not derived from evaporation. 

 Consequently, there remained to him nothing to ascribe as the 

 cause of the peculiar movements but the property of the fluid itself. 

 This explanation received direct confirmation from Wiener's obser- 

 vation, that the amount of the movement has a certain relation to 

 the size of the molecule. Lately S. Exner has extended the investi- 

 gations of Wiener. Among the various influences which Exner 

 sought to test with reference to the molecular motion, was whether 

 either chemical causes or mechanical ones, such as pressure, 

 vibration, and so forth, could in any way produce an acceleration or 

 a retardation of the effect. Only by exposure to light and heat 

 did the motion become accelerated, and then in such a manner as, 



