Mr. Brown's Additional Remarks on Active Molecules. 163 



I have formerly stated my belief that these motions of the 

 particles neither ai'ose from cui-rents in the fluid containing 

 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 micro- 

 scopical researches : and the insufficiency of the most import- 

 ant of those enumerated, may, 1 think, be satisfactorily shown 

 by means of a very simple experiment. 



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 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 diame- 

 ter, will be immediately produced. Of these, the most minute 

 necessarily contain but lew particles, and some may be occa- 

 sionally 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, evapora- 

 tion, 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 drop^ 

 exposed to the air, and disturb or entirely oveiconie the proper 

 motion of the particles, are wholly prevented in (hops of small 

 size immersed in oil, — a fact which, liowever, is only apparent 

 in those drops that are flattened, in conse(iuence of being 

 nearly or absolutely in contact with the stage of the microscope. 



That the motion of the particles is not jModuccd by any 

 cause acting on the surface of the drop, may be proveil by an 



Y 2 inversion 



