Mr Brown's Microscopical Observations. 343 



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 ap- 

 plication of plumbago, as employed by Dr Wollaston, but which in my 

 subject was inadmissible. 



The results so obtained can only be regarded as approximations, 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- 

 form size, though as existing in various substances and examined in cir- 

 cumstances more or less favourable, it is necessary to state that its diame- 

 ter appeared to vary from Tjo"oo tn t0 2oooo tn of an inch *. 



I shall not at present enter into additional details, nor shall I hazard any 

 conjectures whatever respecting these molecules, which appear to be of 

 such general existence in inorganic as well as in organic bodies ; and it is 

 only further necessary to mention the principal substances from which I 

 have not been able to obtain them. These are oil, resin, wax, and sulphur, 

 such of the metals as I could not reduce to that minute state of division 

 necessary for their separation, and finally, bodies soluble in water. 



In returning to the subject with which my investigation commenced, 

 and which was indeed the only object I originally had in view, I had still 

 to examine into the probable mode of action of the larger or peculiar par- 

 ticles of the pollen, which, though in many cases diminished in number 

 before the grain could possibly have been applied to the stigma, and par- 

 ticularly in Clarckia, the plant first examined, were yet in many other 

 plants found in less diminished proportion, and might in nearly all cases 

 be supposed to exist in sufficient quantity to form the essential agents in 

 the process of fecundation. 



I was now therefore to inquire, whether their action was confined to the 

 external organ, or whether it were possible to follow them to the nucleusof the 

 ovulum itself. My endeavours, however, to trace them through the tissue 

 of the style in plants well suited for this investigation, both from the size 

 and form of the particles, and the developement of the female parts, parti- 

 cularly Onagrariae, was not attended with success ; and neither in this nor 

 in any other tribe examined, have I ever been able to find them in any 

 part of the female organ, except the stigma. Even in those families in 

 which I have supposed the ovulum to be naked, namely, Cycadeae and 

 Coniferae, I am inclined to think that the direct action of these particles, 

 or of the pollen containing them, is exerted rather on the orifice of the 

 proper membrane than on the apex of the included nucleus ; an opinion 



* While this sheet was passing through the press, Mr Dollond, at my request, 

 obligingly examined the supposed pollen of Equisetum virgatum with his compound 

 achromatic microscope, having in its focus a glass divided into 10,000ths of an inch, 

 upon which the object was placed ; and although the greater number of particles or 

 molecules seen were about 2(Jo"oo> yet the smallest did not exceedjQ^oo tn oi an 

 inch. 



