76 BACTERIA IN THE AIR 



Conditions of Bacterial Contamination of Air 



There are, speaking in a general way, four chief external conditions 

 affecting the occurrence of bacteria in air. They are as follow : — 



1. The presence of dust and air pollution. 



2. Dampness of surfaces. 



3. Gravity. 



4. Air currents. 



1 . Dust and Air Pollution. — 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. It is true that Helmholtz and Pasteur confirmed this, and 

 greatly added to our knowledge of the subject, but on the whole it 

 may be said that Schwann originated the germ-theory of air, and 

 Lister applied it in the treatment of wounds. Lister believed that 

 if he could surround wounds with filtered air free from dust and 

 particulate or germ matter, the result would be as good as if the 

 wounds 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 disease was not due to 

 " bad air " and " foul drains," but to germs conveyed in the air, was 

 a prophecy as daring as it was novel. Prom these and other like 

 investigations it came to be recognised that putrefaction begins as 

 soon as bacteria from the air gain an entrance to the putrefiable 

 substance, that it progresses in direct proportion to the multiplication 

 of these 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. The degree in either case will depend on air pollution 

 and on dust particles. Bacteria may be conducted on particles 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 composition, dust as a vehicle is not 

 really requisite. Nevertheless the estimation of the amount of dust 

 present in a sample of air may be 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 be 

 invisible. His ingenious experiments, reported in the Transactions 

 of the Royal Society of Edinhurgh (vol. xxxv.), have demonstrated 

 that by supersaturation of air the invisible dust particles may become 

 visible. As is now well known, Dr Aitken believes that fogs, mists, 

 * John Tyndall, F.R.S., Floating Matter of the Air, 1878. 



