82 THE FLOATING-MATTER OF THE AIR. 



wafted through the air. Cases may arise favourable 

 to the growth and dispersion of the full-grown or- 

 ganism. Whether, after desiccation, it retains the power 

 of reproduction is another question. But it ought, I 

 think, to be steadily borne in mind that the complete 

 Bacteria and the atmospheric matter from which they 

 spring are, in general, different things. I have care- 

 fully sought for atmospheric Bacteria, but have never 

 found them. They have never, to my knowledge, been 

 found by others ; and that they arise from matter which 

 has not yet assumed the Bacterial form is, as just shown, 

 capable of demonstration. An organic infusion, boiled 

 and shielded from atmospheric particles, will remain 

 clear for an indefinite period, while a fragment of glass 

 which has been exposed to the air, but on which no trace 

 of a Bacterium is to be found, will in two or three days 

 develop in the infusion a multitudinous crop of life. 



We have now to look a little more closely at these 

 particles, foreign to the atmosphere but floating in it, 

 and proved beyond doubt to be the origin of all the Bac- 

 terial life which our esperiments have thus far revealed. 

 We must also look at them as they exist in water, in 

 countless multitudes, being as foreign to this medium as 

 the floating atmospheric dust is to the air in which it 

 swims. The existence of the particles is quite as certain 

 as if they could be felt between the fingers, or seen by 

 the naked eye. Supposing them to augment in magni- 

 tude until they come, not only within range of the 

 microscope, but within range of the unaided senses. 

 Let it be assumed that our knowledge of them under 

 these circumstances remains as defective as it is now — 

 that we do not know whether tliey are germs, particles 

 of dead organic dust, or particles of mineral matter. 

 Suppose a vessel (say a flower-pot) to be at hand filled 

 with nutritious earth, with which we mix our unknown 



