474 MICROSCOPICAL OBSERVATIONS 



with what I have been able to determine on any of these 

 points. 



As to form, I have stated the molecule to be spherical, 

 and this I have done with some confidence ; the apparent, 

 exceptions which occurred admitting, as it seems to me, of 

 being explained by supposing such particles to be compounds. 

 This supposition in some of the cases is indeed hardly re- 

 concileable with their apparent size, and requires for its 

 support the further admission that, in combination, the 

 figure of the molecule may be altered. In the particles 

 formerly considered as primary combinations of molecules, 

 a certain change of form must also be allowed ; and even 

 the simple molecule itself has sometimes appeared to me 

 when in motion to have been shghtly modified in this 

 respect. 



13] My manner of estimating the absolute magnitude and 

 imiformity in size of the molecules, found in the various 

 bodies submitted to examination, was by placing them on 

 a micrometer divided to five thousandths of an inch, the 

 lines of which were very distinct ; or more rarely on one 

 divided to ten thousandths, with fainter lines, not readily 

 visible without the application of plumbago, as employed 

 by Dr. Wollaston, but which in my subject was inad- 

 missible. 



The results so obtained can only be regarded as approxi- 

 mations, on which, perhaps, for an obvious reason, much 

 reliance will not be placed. From the number and degree 

 of accordance of my observations, however, I am upon the 

 whole disposed to believe the simple molecule to be of uni- 

 ' foiTO_ size, though as existing in various substances and 

 examined in circumstances more or less favorable, it is ne- 

 cessary to state that its diameter appeared to vary from 

 ys_!j55th to sg/oooth of an inch.^ 



I shall not at present enter into additional details, nor 



1 While this sheet was passing through the press, Mr. DoUond, at my re- 

 quest, obligingly examined the supposed pollen of Equisetum virgatum witli his 

 compound achromatic microscope, having in its focus a glass divided into 

 lO.OOOths of an inch, upon which the object was placed ; and although the 

 greater number of particles or molecules seen were about l-30,000th, yet the 

 smallest did not exceed l-30,000th of an inch. 



