Mr Brown on Active Molecules: 43 



tides themselves, their unstable equilibrium in the fluid in which 

 they are suspended, their hygrometrical or capillary action, and 

 in some cases the disengagement of volatile matter, or of minute 

 air-bubbles, — have been considered by several writers as suffi- 

 ciently accounting for the appearances. Some of the alleged 

 causes here stated, with others which I have considered it un- 

 necessary to mention, are not likely to be overlooked or to de- 

 ceive observers of any experience in microscopical researches ; 

 and the insufficiency of the most important of those enumerated, 

 may, I think, be satisfactorily shown by means of a very simple 

 experiment. 



This experiment consists in reducing the drop of water con- 

 taining the particles to microscopic minuteness, and prolonging 

 its existence by immersing it in a transparent fluid of inferior 

 specific gravity, with which it is not miscible, and in which eva- 

 poration is extremely slow. If to almond-oil, which is a fluid 

 having these properties, a considerably smaller proportion of 

 water, duly impregnated with particles, be added, and the two 

 fluids shaken or triturated together, drops of water of various 

 sizes, from l-50th to l-2000dth of an inch in diameter, will be 

 immediately produced. Of these, the most minute necessarily 

 contain but few particles, and some may be occasionally obser- 

 vetl with one particle only. In this manner minute drops, 

 which if exposed to the air would be dissipated in less than a 

 minute, may be retained for more than an hour. But in all 

 the drops thus formed and protected, the motion of the particles 

 takes place widi undiminished activity, while the principal causes 

 assigned for that motion, namely, evaporation, and their mutual 

 attraction and repulsion, are either materially reduced or abso- 

 lutely null. 



It may here be remarked, that those currents from centre to 

 circumference, at first hardly perceptible, then more obvious, and 

 at last very rapid, which constantly exist in drops exposed to the 

 air, and disturb or entirely evercome the proper motion of the 

 particles, are wholly prevented in drops of small size immersed 

 in oil, — a fact which, however, is only apparent in those drops 

 that are flattened, in consequence of being nearly or absolutely 

 in contact with the stage of the microscope. 



U'hat the motion of the particles is not produced by any cause 

 acting on the burlace of the drop, may be proved by an inver- 



