071 Active Molecules. 317 



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

 in those drops that are flattened, in consequence of being 

 nearly or absolutely in contact with the stage of the micro- 

 scope. 



That the motion of the particles is not produced by any 

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

 inversion of the experiment ; for by mixing a very small pro- 

 portion of oil with the water containing the particles, micro- 

 scopic drops of oil of extreme minuteness, some of them not 

 exceeding in size the particles themselves, will be found on the 

 surface of the drop of water, and nearly or altogether at rest; 

 while the particles in the centre or towards the bottom of the 

 drop continue to move with their usual degree of activity. 



By means of the contrivance now described for reducing the 

 size and prolonging the existence of the drops containing the 

 particles, which, simple as it is, did not till very lately occur 

 to me, a greater command of the subject is obtained, sufficient 

 perhaps to enable us to ascertain the real cause of the motions 

 in question. 



Of the few experiments which I have made since this man- 

 ner of observing was adopted, some appear to me so curious, 

 that I do not venture to state them until they are verified by 

 frequent and careful repetition. . 



I shall conclude these supplementary remarks to my former 

 Observations, by noticing the degree in which I consider those 

 observations to have been anticipated. 



That molecular was sometimes confounded with animalcular 

 motion by several of the earlier microscopical observers, ap- 

 pears extremely probable from various passages in the writings 

 of Leeuwenhoek, as well as from a remarkable Paper by Ste- 

 phen Gray, pubhshed in the 19th volume of the Philosophical 

 Transactions. 



Needham also, and BufFon, with whom the hypothesis of 

 organic particles originated, seem to have not unfrequently 

 fallen into the same mistake. And I am inclined to believe 

 that Spallanzani, notwithstanding one of his statements re- 

 specting them, has under the head of Animaletti (Tultimo o/v 



