42 Mr Brown (m, Active Molecules. 



not necessarily follow, nor have I any where stated, as has been 

 imputed to me, that they also agree in all their other properties 

 and functions. 



I have remarked, that certain substances, namely, sulphur, 

 resin, and wax, did not yield active particles, which, however, 

 proceeded merely from defective manipulation ; for I have since 

 readily obtained them from all these bodies : at the same time I 

 ought to notice that their existence in sulphur was previously 

 mentioned to me by my friend Mr Lister. 



In prosecuting the inquiry subsequent to the publication of 

 my observations, I have chiefly employed the simple micro- 

 scope mentioned in the pamphlet, as having been made for me 

 by Mr Dollond, and of which the three lenses that I have gene- 

 rally used, are of a 40th, 60th, and 70th of an inch focus. 



Many of the observations have been repeated and confirmed 

 with other simple microscopes having lenses of similar powers, 

 and also with the best achromatic compound microscopes, 

 either in my own possession, or belonging to my friends. 



The result of the inquii-y at present essentially agrees with 

 that which may be collected from my printed account, and may 

 be here briefly stated in the following terms : namely, 



That extremely minute particles of solid matter, whether ob- 

 tained from organic or inorganic substances, when suspended in 

 pure water, or in some other aqueous fluids, exhibit motions for 

 which I am unable to account, and which, from their irregulari- 

 ty and seeming independence, resemble in a remarkable degree 

 the less rapid motions of some of the simplest animalcules of 

 infusions ; that the smallest moving particles observed, and 

 which I have termed Active Molecules, appear to be spherical 

 or nearly so, and to be between l-20,000dth and l-30,000dth 

 of an inch in diameter ; and that other particles of considerably 

 greater and various size, and either of similar or of veiy dif- 

 ferent figure, also present analogous motions in like circum- 

 stances. 



I 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 evajwration. 



These causes of motion, however, either singly or combined 

 with otherb, — as, tlic attractions and repulsions among the par- 



