120 ROBERT BROWN 



He seems to have been pretty sure that something more than 

 the mere " aura " of older writers was concerned in the matter, 

 and while looking into the evidence for the existence or trans- 

 mission of material substance, he observed that in the fovilla 

 of the pollen there were vast crowds of minute particles which 

 were in a continual state of dancing motion. He hoped that it 

 might be possible to identify these bodies along their track into 

 the ovule, and so to settle the more urgent questions as to the 

 mode of fertilisation. He states that he made his observations 

 with a simple microscope, the focal length of the lens of which 

 was -%". Later on he used a much more powerful pocket 

 microscope made by Dollond with power up to -fa" F.L. He 

 got Dollond to check the results with a compound achromatic 

 microscope, and estimated the size of the particles to be ^~ to 



^ ". Brown was fully aware that he was not the first observer 



oOjOOO 



who had seen these moving particles. They had been already 

 noticed by Needham and by Gleichen, but these writers had 

 paid no special attention to them. Brown's great merit in this 

 matter lies in the admirable way in which he conducted the 

 investigation. At first he thought he had lighted upon some- 

 thing which was essentially a peculiarity of the male elements ; 

 then, extending his observations, he had to expand his first 

 idea and admit the " active molecules " to represent a state or 

 condition of living matter generally. As he still further widened 

 the sphere of his investigations, he proved that the same move- 

 ments occurred in dead tissues, and further that inorganic bodies 

 also exhibited the phenomenon. Later on he found that the 

 movements depended on the minuteness of the particles. He 

 excluded the effect of evaporation, currents and other disturbing 

 influences, and, indeed, the whole investigation shews him to us 

 in the character of an accomplished experimenter as well as a 

 brilliant observer. The complete explanation of these " active 

 molecules," which are in the state generally described as 

 "Brownian movement," still constitutes an unsolved problem, 

 and one finds that it even now continues to occupy the attention 

 of the physicist. 



Any attempt adequately to review the whole of Brown's life 

 work is impossible within the limits necessarily imposed by the 



