II] OF MOLECULAK MAGNITUDES 45 



drop. Thus a drop one-tenth of that size (2-5 ^u.), the size, 

 apparently, of the drops of water in a Ught cloud, will fall a 

 hundred times slower, or say an inch a minute; and one again 

 a tenth of this diameter (say -25 jj,, or about twice as big, in linear 

 dimensions, as our micrococcus), will scarcely fall an inch in two 

 hours. By reason of this principle, not only do the smaller 

 bacteria fall very slowly through the air, but all minute bodies 

 meet with great proportionate resistance to their movements in 

 a fluid. Even such comparatively large organisms as the diatoms 

 and the foraminifera, laden though they are with a heavy shell 

 of flint or lime, seem to be poised in the water of the ocean, and 

 fall in it with exceeding slowness. 



The Brownian movement has also to be reckoned with, — that 

 remarkable phenomenon studied nearly a century ago (1827) by 

 Kobert Brown, facile frincefs botanicorum. It is one more of those 

 fundamental physical phenomena which the biologists have con- 

 tributed, or helped to contribute, to the science of physics. 



The quivering motion, accompanied by rotation, and even by 

 translation, manifested by the fine granular particles issuing from 

 a crushed pollen-grain, and which Robert Brown proved to have 

 no vital significance but to be manifested also by all minute 

 particles whatsoever, organic and inorganic, was for many years 

 unexplained. Nearly fifty years after Brown wrote, it was said 

 to be "due, either directly to some calorical changes continually 

 taking place in the fluid, or to some obscure chemical action 

 between the soUd particles and the fluid w^hich is indirectly 

 promoted by heat*." Very shortly after these last words were 

 written, it was ascribed by Wiener to molecular action, and we 

 now know that it is indeed due to the impact or bombardment of 

 molecules upon a body so small that these impacts do not for 

 the moment, as it were, "average out" to approximate equality 

 on all sides. The movement becomes manifest with particles of 

 somewhere about 20 jj. in diameter, it is admirably displayed by 

 particles of about 12/^ in diameter, and becomes more marked 

 the smaller the particles are. The bombardment causes our 

 particles to behave just Hke molecules of uncommon size, and this 



* Carpenter, The Microscope, erlit. 1862, p. 185. 



