48 SCIENCE AND IMMORTALITY 



out : " a single grain of indigo, the weight, say, 

 of a small pin, gives a distinct coloration to a ton 

 of water. In order to produce this reaction, the 

 indigo must have been divided up into enough 

 parts to be present in large numbers in every drop 

 of water ; it must have been broken up into 

 millions and millions of parts." 



Even more remarkable is the ubiquitous emigra- 

 tions of the molecules of the substance known as 

 fluorescein. Fluorescein added to water causes the 

 water to fluoresce, and a single grain will cause 

 fluorescence in a hundred tons of water. 



Now, these minute atoms and molecules are 

 not passive, inert things ; they are in constant 

 movement. The molecules of the air of the room 

 in which I sit are darting hither and thither with 

 incredible speed. The nitrogen molecules are 

 dashing about at a rate of nearly a quarter a mile 

 a second, the oxygen molecules are rather slower, 

 and the heavier carbonic acid molecules are slower 

 still. If hydrogen be present, its molecules out- 

 strip all the rest, for it darts at the rate of more 

 than a mile a second. It is a case, however, of 

 the more haste the less speed, for the molecules 

 constantly collide, so constantly indeed that a 

 molecule of hydrogen can proceed only ^^q^qqq 

 of an inch without collision, i,e. coUides about 

 1 6,000,000,000 times a second ; while if it be 



