190 ESSAYS OF A BIOLOGIST 



gas, of water vapour. They are not all travelling at 

 uniform speeds; collisions are all the time occurring, 

 and the molecules are continuously changing their 

 rate of travel as they clash and bump. 



We have only to look down a microscope to con- 

 vince ourselves of the alteration in our experience 

 that it would mean if we were to become sufficiently 

 diminished. The tiniest solid particles in fluids can 

 be seen to be in a continuous state of agitation — in- 

 explicable until it was pointed out that this mysteri- 

 ous "Brownian" movement was the inevitable result 

 of impacts by the faster-moving molecules of the 

 fluid. Many living things that we can still see are 

 small enough to live permanently in such agitation; 

 the longest diameter of many bacteria is but half a 

 micron (a two-thousandth of a millimetre), and there 

 are many ultra-microscopic organisms which, owing 

 to their closer approximation to molecular dimen- 

 sions, must pass their lives in erratic excursions many 

 times more violent than any visible Brownian mo- 

 tion. 



If we could shrink, like Alice, at the persuasion of 

 some magic mushroom, the rain of particles on our 

 skin, now as unfelt as midges by a rhinoceros, would 

 at last begin to be perceptible. We should find our- 

 selves surrounded by an infinity of motes; titillated 

 by a dance of sand-grains; bruised by a rain of mar- 

 bles; pounded by flights of fives-balls. What is more, 

 the smaller we became, the more individuality and 

 apparent free-will should we detect in the surround- 



