Proceedings. 21 



rather strongly and have a dancing or Brownian movement, 

 a movement which has been compared to that of a balance- 

 wheel of a watch. These particles, when seen singly or on 

 end, are round, but as they are commonly grouped in twos 

 they present the form of a figure of 8. They are the 

 Bacterium Termo of Cohn, and that they are the cause of 

 putrefaction and associated with the dust of the air is 

 abundantly proved in various ways. 



Prof. Tyndall made an air-tight box with glass sides, and 

 moistened its inner surface with glycerine. When this box 

 was first made, a ray of electric light passed through it showed 

 the air to be full of floating particles. After the box had been 

 left for three days, these particles had fallen and become 

 entangled in the glycerine. The electric light now showed 

 the air to be pure or free from floatmg particles of dust. It 

 was also found to be incapable of exciting putrefaction, for 

 when tubes containing putrescible matter were introduced 

 into it they underwent no such change. 



In another experiment Prof. Tyndall prepared fifty flasks 

 of putrescible but sterilised infusions. Twenty-five of these 

 he opened in a hay-loft, and the other twenty-five on the 

 edge of an Alpine chff. The hay-loft series became full of 

 organisms, and passed to a state of decomposition. The 

 Alpine series, which had been opened in pure air, remained 

 pure as before. Air may also be rendered incapable of 

 producing putrefaction by passing it through red-hot tubes, 

 by which the organic germs are destroyed ; or by straining 

 it through cotton-wool, by which they are filtered out ; or by 

 keeping up a freezing temperature (20°), at which they will 

 not grow. Flesh and other putrescible substances may also 

 be preserved by thoroughly drying them, or by j)lacing them 

 in or covering them by something which proves destructive to 

 all germs. 



Bacteria exist not only in the air, but also in the soil of the 

 earth's surface, and as dust they pass from one to the other. 

 In the soil they conduce much to its fertility, as they serve to 

 reduce dead animal and vegetable substances to a condition 

 in which they are available as nourishment for plants ; this 

 they do by breaking up the complex combinations of which 



