162 Mr. Brown's Additional Remarks on Aciive Molecules. 



cules, — a supposition which, though professedly conjectural, 

 I regret having so much insisted on, especially as it may seem 

 connected with the opinion of the absolute identity of the 

 Molecules, from whatever source derived. 



On this latter subject, the only two points that I endea- 

 voured to ascertain, were their size and figure : and al- 

 though I was, upon the whole, inclined to think that in 

 these respects the Molecules were similar from whatever sub- 

 stances obtained, yet the evidence then adduced in support of 

 the sujDposition was far from satisfactory ; and I may add, 

 that I am still less satisfied now that such is the fact. But 

 even had the uniformity of the Molecules in those two points 

 been absolutely established, it did not necessarily follow, nor 

 have I any where stated, as has been imputed to me, that 

 they also agreed in all their other properties and functions. 



I have I'emarked, that certain substances, namely, sulphur, 

 resin, and wax, did not yield active particles, which, how- 

 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 pi'eviously 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. DoUond, and of which the three lenses that I have 

 genei'ally used, are ofa 4'Oth, 60th, and 70th of an inch focus. 



Many of the observations have been repeated and con- 

 firmed with other simple microscopes having lenses of similar 

 powers, and also with the best achromatic compound micro- 

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



The result of the inquiry at pi'esent 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 

 obtained 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 irre- 

 gularity and seeming independence resemble in a remarkable 

 degree the less rapid motions of some of the simplest animal- 

 cules of infusions. That the smallest moving particles ob- 

 served, 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 par- 

 ticles of considerably greater and various size, and either of 

 similar or of very different figure, also present analogous mo- 

 tions in like circumstances. 



I have 



