Bache.] 1 ' ^ [April 20, 



last, because its discussion requires more than the brief space which 

 I have devoted to the previous individual results, and because it 

 leads directly to the conclusion that I have finally reached as to the 

 true cause of the movements. I started out with the conception, 

 which it seems is common to every one, that evaporation might be 

 accompanied with a series of minute explosions, which produce 

 shocks that manifest themselves through the mass of an aqueous 

 solution, in the form of minute movements of finely divided matter 

 held in unstable equilibrium by suspension in the fluid, and that 

 these, escaping cognizance from any ordinary observation, might 

 be visible as such, or in their effects, through the instrumentality of 

 high powers of the microscope. I had come to believe long before, 

 from observation and experiment, that no tremors from mechanical 

 agency or any other, except perhaps from evaporation, could pro- 

 duce the peculiar movements known as brownian, and finally it 

 remained to discover if this or any other intrinsic cause were at 

 work that would account for them. 



At this point I encountered an obstacle. My high powers of the 

 microscope were both water-immersion lenses. It seemed, there- 

 fore, that even when I had had the drop of liquid under observation, 

 sealed beneath a cover-glass, I might have included, by the use of 

 the water-immersion lens itself, an evaporating surface which might 

 have produced the optical illusion of the movement of the particles 

 in suspension. I proceeded, however, with my experiments, upon 

 the assumption that this, as the event proved to be the case, was 

 not true, and meanwhile procured from Vienna a one-fifteenth dry- 

 lens by Reichert, the highest power of dry-lens that he makes. 



I had already obtained for high-power lenses a film of liquid thin 

 enough to be observed through all its strata, free of air within the 

 cell, and protected from evaporation by being hermetically sealed. 

 Any ordinary manufactured cell is too deep, and with all precau- 

 tions taken contains a little air. On the other hand, the mere 

 cover-glass superposed on a glass slide contains too slight a 

 depth of fluid. I made a cell by using gum-shellac traced in a 

 circlet on a glass slide, which cell, when partially dried, is filled to 

 the brim with the liquid to be observed upon, whereupon the cover- 

 glass is pressed into the yielding gum, thereby expressing the con- 

 tained air with the superfluous liquid, when the product, dried over 

 night, is fit for use on the following evening. One slide, prepared 

 in this manner and filled with a slightly tinted solution of carmine 



