BACTERIA IN THE AIR lOI 



ture. In a day or two the colonies appear, and may be 

 examined. 



Micro-organisms in the Air. Schwann was one of the first 

 to point out that when a decoction of meat is effectually 

 screened from the air, or supplied solely with calcined air, 

 putrefaction does not set in. Helmholtz and Pasteur con- 

 firmed this, but it may be said with some truth that Schwann 

 originated the germ theory, and Lister applied it in the 

 treatment of wounds. Lister beheved that if he could sur- 

 round wounds with filtered air the results would be as good 

 as if they were shut off from the air altogether. 



It was Tyndall ' who first laid down the general principles 

 upon which our knowledge of organisms in the air is based. 

 That the dust in the air was mainly organic matter, living 

 or dead, was a comparatively new truth; that epidemic dis- 

 ease was not due to " bad air '* and ** foul drains," but to 

 germs conveyed in the air, was a prophecy as daring as it 

 was correct. From these and other like investigations it 

 came to be recognised that putrefaction begins as soon as 

 bacteria gain an entrance to the putrefiable substance, that 

 it progresses in direct proportion to the multiplication of 

 bacteria, and that it is retarded when they diminish or lose 

 vitality. 



Tyndall made it clear that both as regards quantity and 

 quality of micro-organisms in the air there neither is nor 

 can be any uniformity. They may be conducted on par- 

 ticles of dust^ — '' the raft theory" — but being themselves 

 endowed with a power of flotation commensurate with their 

 extreme smallness and the specific lightness of their com- 

 position, dust as a vehicle is not really requisite. Never- 

 theless the estimation of the amount of dust present in a 

 sample of air is a very good index of danger. It is to Dr. 

 Aitken that we are indebted for devising a method by which 

 we can measure dust particles in the air, even though they 



' John Tyndall, F.R.S., Floating Matter of the Air, 



