200 Scientific Reviews. 
*¢ T have formerly stated my belief that these motions of the particles neither 
arose from currents 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 aoe 
as, the attractions and repulsions among the particles themselves, their unstable 
equilibrium in the fluid in which they are suspended, their hygrometrical or ca- 
pillary action, and in some cases the disengagement of volatile matter, or of mi- 
nute air bubbles,—have been considered by several writers as sufficiently account- 
ing 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 microscopical researches : and 
the insufficiency 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 containing the par- 
ticles 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 evaporation is extremely slow. If to almond-oil, which is a fluid hay- 
ing these properties, a considerably smaller proportion of water, duly impregnat- 
ed with particles, be added, and the two fluids shaken or triturated together, 
drops of water of various sizes, from 1-50th to 1-2000dth 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 undimi- 
nished activity, while the principal causes assigned for that motion, namely, eva- 
poration and their mutual attraction and repulsion, are either materially reduced 
or absolutely null.” 
Mr. Brown concludes these supplementary remarks by notes, 
where he considers these observations to have been anticipated, and 
quotes the works of Leeuwenhoeck, , Stephen, Gray, Needham, Buf- 
fon, Spallanzani, Gleichen, Wrisberg, Muller, Dr. Drummond, and 
Mr. Bywater of Liverpool, who all, he states, include active mo- 
lecules with true animalcules. 
Wethink, from the historical sketch which we have given of those 
modern observations and discoveries which have excited so much 
interest, both from the high scientific attainments of the observers, 
and from the extensive application of which they seemed to be capa- 
ble, that, amidst such perplexing evidence, if we have not satisfied 
our readers of what is the actual truth, we have at least laid before 
them, as far as has been in our power, those statements on which. 
the discussion has hitherto hung. 
The elements of the question are thus at the disposal of obser- 
vers, and every one may at his leisure verify the assertions which 
have been made on each side, satisfying himself on a question 
which presents nothing positive, and every thing to be contested. 
