78 THE FLOATING-MATTER OF THE AIR. 



of extreme importance, which it will be useful to clear 

 up. ' Potential germs ' and ' hypothetical germs ' have 

 heen spoken of with scorn, because the evidence of the 

 microscope as to their existence was not forthcoming. 

 Sagacious writers had drawn from their experiments the 

 perfectly legitimate inference that in many cases the 

 germs exist, though the microscope fails to reveal them. 

 Such inferences, however, have been treated as the pure 

 work of the imagination, resting, it was alleged, on no 

 real basis of fact. But in the concentrated beam we 

 possess what is virtually a new instrument, exceeding 

 the microscope indefinitely in power. Directing it 

 upon media which refuse to give the coarser instrument 

 any information as to what they hold ia suspension, these 

 media declare themselves to be crowded with particles 

 — not hypothetical, not potential, but actual and myriad- 

 fold in number — showing the microscopist that there is 

 a world far beyond his range. 



In §§ 6 and 8 experiments on the infection of clear 

 infusions by others containing visible Bacteria are re- 

 ferred to. But for the infection to be sure it is not 

 necessary that the Bacteria should be visible. Over and 

 over again I have repeated the experiments of Dr. Bur- 

 don Sanderson on the effective power of ordinary distilled 

 water, in which the microscope fails to reveal a Bacte- 

 riuTYi. The water, for example, furnished to the Eoyal- 

 Institution laboratory by Messrs. Hopkin and Williams 

 is sensibly as infectious as an infusion swarming with 

 Bacteria. The vessels are the source of infection here. 



Perhaps the severest experiment of this kind ever 

 made was one executed by Dr. Sanderson with water 

 prepared by myself. In 1871 I sought anxiously and 

 assiduously for water free from suspended particles. The 

 liquid was obtained in various degrees of purity, but 

 never entirely pure. Knowing the wonderful power of 



