284 THE BEGINNINGS OF IIFE. 



beneath the same bell-jar a narrow vessel containing 

 an organic infusion, and on a level with the surface 

 of this, a plate of glass smeared with glycerine covering 

 an equal area, we have two surfaces which are equally 

 exposed to the access of atmospheric germs ^ Yet, 

 in the course of a few days, the organic infusion will 

 swarm with infusoria, whilst the film of glycerine will, 

 in the majority of cases, show nothing more than a very 

 minute quantity of organic and inorganic debris, mixed 

 with a few particles or ^corpuscles,' which, when or- 

 ganic, seem for the most part to be nothing more than 

 mere dead particles, resulting from the disintegration 

 of organic matter. 



The briefest reflection upon the probabilities of the 

 case seems to suggest that this is most likely to be the 

 nature of the majority of ^particles' and '^globules' 

 which are encountered by all observers in this kind of 



^ If atmospheric germs are to fertilize the organic infusion, they must 

 be brought into contact with its surface either by gravitation or move- 

 ment of the air. The surface of glycerine v^^ould therefore be the very 

 best index as to the amount and nature of the particles which drop into 

 the infusion from the atmosphere. The inorganic particles and the 

 heavier organic particles have the greatest tendency to subside. Dust 

 which has been deposited from the atmosphere has been ascertained by 

 Dr. Percy and also by Mr. Tichborne (' Chemical News,' Oct. 1870) to 

 contain only from one-half to one-third per cent, of organic matter, though 

 amongst the finer particles, which remain longer suspended in still air, 

 the proportion of organic matter is probably much larger. We are all 

 familiar with the specks and motes which dance in the sunbeam, but 

 Prof. Tyndall has conclusively shown ('Nature,' No. 13, 1870) that the 

 electric light is a far more potent means of revealing the presence of 

 otherwise invisible impurities both in air and in water. 



