482 ADDITIONAL REMARKS 



with otliei'Sj — 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 mat- 

 ter, 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 overlooked or to deceive 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 experi- 

 ment. 



This experiment consists in reducing the drop of water 

 containing the particles to microscopic minuteness, and pro- 

 longing its existence by immersing it in a transparent fluid 

 of inferior 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 

 4] smaller proportion of water^ duly impregnated with par- 

 ticles, 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 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 principal causes assigned for that motion, namely, 

 evaporation, and their mutual attraction and repulsion, are 

 either materially reduced or absolutely null. 



It may here be remarked, that those currents from centre 

 to circumference, at first hardly perceptible, then more ob- 

 vious, and at last very rapid, which constantly exist in drops 

 exposed to the air, and disturb or entirely overcome the 

 proper motion of the particles, are wholly prevented in 

 drops of small size immersed in oil, — a fact which, however. 



