^M^ Mr Brown's Additional Remarks 



them, nor depended on that intestine motion which may be 

 supposed to accompany its evaporation. 



These causes of motion, however, either singly or combined 

 with others, — as, the attractions and repulsions among the 

 particles 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 sufficiently accounting for the appearances. Some 

 of the alleged causes here stated, with others which I have 

 considered it unnecessary to mention, are not likely to be over- 

 looked or to deceive observers of any experience in microscopi- 

 cal 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 prolong- 

 ing its existence by immersing it in a transparent fluid of in- 

 ferior specific gravity, with which it is not miscible, and in which 

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

 fluid having these properties, a considerably smaller propor- 

 tion 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 ne- 

 cessarily contain but few particles, and some may be occasion- 

 ally observed 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 with undiminished activity, while the prin- 

 cipal causes assigned for that motion, namely, evaporation and 

 their mutual attraction and repulsion, are either materially re- 

 duced or absolutely 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 overcome the proper mo- 

 tion of the particles, are wholly prevented in drops of small 



