ON ACTIVE MOLECULES. 481 



ever, 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 

 microscope mentioned in the Pamphlet as having been made 

 for me by Mr. Dollond, and of which the three lenses that 

 I have generally used, are of a 40th, 60th, and 70th of an 

 inch focus. 



Many of the observations have been repeated and con- 

 firmed with other simple microscopes having lenses of simi- 

 lar powers, and also with the best achromatic compound 

 microscopes, either in my own possession or belonging to 

 my friends. ■ ^ . . " 



The result of the inquiry(at present essentially agrees with . 

 that which may be collected from my printed account, ca " 

 an(p may be here briefly stated in the following terms ; 

 namely. 



That extremely minute particles of solid matter, whether 

 obtained from organic or inorganic substances, when sus- 

 pended in piu'e water, or in some other aqueous fluids, 

 exhibit motions for which I am unable to account, and 

 which from their irregularity 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 very different 

 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 con- 

 taining them, nor depended on that intestine motion Avhich 

 may be supposed to accompany its evaporation. 



These causes of motion, however, either singly or combined 



31 



