366 Mr Brown on the Existence of Active Molecules 



were observed, consisting of fibrils having transverse contrac- 

 tions, corresponding in number, as I conjectured, with that of 

 the molecules composing them ; and those fibrils, when not con- 

 sisting of a greater number than four or five molecules, exhibit- 

 ed motion, resembling in kind and vivacity that of the mineral 

 fibrils already described, while longer fibrils of the same appa- 

 rent diameter were at rest. 



The substance found to yield these active fibrils in the largest 

 proportion and in the most vivid motion, was the mucous coat 

 interposed between the skin and muscles of the haddock, espe- 

 cially after coagulation by heat. 



The fine powder produced on the under surface of the fronds 

 of several ferns, particularly of Achrostichum calomelanos, and 

 the species nearly related to it, was found to be entirely com- 

 posed of simple molecules, and their primary fibre-like com- 

 pounds, both of them being evidently in motion. 



There are three points of great importance which I was an- 

 xious to ascertain respecting these molecules, namely, their form, 

 whether they are of uniform size, and their absolute magnitude. 

 I am not, however, entirely satisfied 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 reconcileable with their ap- 

 parent 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 slightly modified in this respect. 



My manner of estimating the absolute magnitude and unifor- 

 mity in size of the molecules, found in the various bodies sub-, 

 mitted to examination, was by placing them on a micrometer di- 

 vided 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 sub- 

 ject was inadmissible. 



