ON ACTIVE MOLECULES. 483 



is only apparent in those drops that are flattened, in con- 

 sequence of being nearly or absolutely in contact with the 

 stage of the microscope. 



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 

 proportion of oil with the water .containing the particles, 

 microscopic 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 contain- 

 ing the particles, which, simple as it is, did not til] 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 

 manner of observing was adopted, some appear to me so 

 curious, that I do not venture to state them until they are [s 

 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 animal- 

 cular motion by several of the earlier microscopical obser- 

 vers, appears extremely probable from various passages in 

 the writings of Leeuwenhoek, as well as from a very inter- 

 esting Paper by Stephen Gray, published in the 19th 

 volume of the Philosophical Transactions. 



Needham also, and Buffon, with whom the hypothesis 

 of organic particles originated, seem to have not unfre- 

 quentiy fallen into the same mistake. And I am inclined 

 to believe that Spallanzani, notwithstanding one of his 

 statements respecting them, has under the head of Anima- 



