THE 



POPULAR SCIENCE 

 MONTHLY. 



APRIL, 1877. 



A COMBAT WITH AN INFECTIVE ATMOSPHERE. 1 



By Peofessoe JOHN TYNDALL, F. E. S. 



A 



YEAR ago I had the honor of bringing; before the members of 

 the Royal Institution some account of an investigation in which 

 an attempt was made to show that the power of atmospheric air to 

 develop life in organic infusions infusions, for instance, extracted 

 from meat or vegetables and its power to scatter light went hand-in- 

 hand. I then endeavored to show you that atmospheric air, when 

 left to itself, exercised a power of self-purification ; that the dust and 

 floating matter that we ordinarily see in it disappeared when the air 

 was left perfectly tranquil; and that, when the air had thus purified 

 itself, the power of scattering light and the power of generating life 

 had disappeared together. For the sake of reminding you of this 

 matter, we will now cause a beam of the lamp to pass through the 

 air. You see the track of the beam vividly in the air. You know 

 that the visibility of the track is not due to the air itself. If the 

 floating matter were removed from the air, you would not be able to 

 track the beam through the room at all. You see the track in conse- 

 quence of the floating dust suspended in the air. If the air be inclosed 

 in a place free from agitation the dust subsides, and then, as I en- 

 deavored to show you a year ago, the air possesses no power of gener- 

 ating life in organic infusions. The nature of the argument is this : 

 You see the dust as plainly as if it were placed upon your hand, and 

 you could feel it with your fingers. You found that the dust, when 

 it sowed itself in organic infusions, produced a definite crop in those 

 infusions ; and you are equally justified in inferring that the crop thus 

 produced is due to the germs in the dust, as a gardener would be in 

 believing that a certain crop is produced from the seeds which he 



1 A lecture delivered at the Royal Institution, on Friday, January 19, 187*7. 

 vol. x. 41 



