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TRANSACTIONS OF THE CANADIAN INSTITUTE. [VOL. I. 



imperfectly conveys any idea of their number. Take the instance where 

 in heavy clay soil the autumn and spring rains have saturated it almost 

 to the surface. Here the small amount of air in the upper soil prevents 

 oxidation in large measure of the organic matters. The lack of oxygen 

 impeding the multiplication of bacteria is notably aided by the low 

 temperature of this soil induced by evaporation. Parkes indeed found a 

 difference of 12 ^ R in the temperature of the upper layers of two contig- 

 uous pieces of bog land, one of which was drained while the other was 

 not. Now, however, when the ground-water falls, the air follows, and 

 with moisture and warmth nitrification goes on. But this favorable con- 

 dition may be temporary. The heat of summer rapidly bakes such sur- 

 faces, air does not penetrate the deeper layers, and the moisture being 

 deficient the multiplication of bacteria is again limited. Clearly, how- 

 ever, the recession of ground-waters from the surface with increasing 

 temperature, as after the spring rains, means an increase in depth of the 

 area to which nitrification extends— or in other words the depth at which 

 bacterial infection of the soil is present. In fairly pervious soils the roots 

 of species of the clover and pea family have been found ten feet below the 

 surface, while the roots of trees following the area of perviousness and 

 nitrification, run laterally, and spreading out obstruct tile drains, and 

 reach into wells at a depth of twenty feet or more. Duclaux remarks, 

 the extension of germs downward in the soil ought theoretically to take 

 place by a general extension, by increase from point to point, by gradual 

 growth, cairsing invasion of the deeper layers, and in carrying life into 

 regions which as we know remain sterile. He then asks, Why this 

 sterility ? Fraenkel has pointed out that the temperature at two or three 

 metres below the surface is an absolute obstacle to the multiplication of 

 the bacilli of typhoid and cholera, yet inasmuch as microbes may through 

 cultivation acquire an ability to grow at lower temperatures than those 

 normal for them, Duclaux affirms that the low temperature is not suffi- 

 cient to explain the phenomenon. Further, the absence of nourishment 

 cannot be given as a sufficient explanation, since microbes can live and 

 even multiply in distilled water, living, as Pasteur says, on their own 

 tissues. A most potent cause exists, however, in the absence of oxygen, 

 which is more rare the deeper we go into the soil. Instead of oxygen, 

 we find in such layers carbonic acid relatively in excess, and Leone of 

 Munich, of Pettenkofer's laboratory, has by experiment found that 

 Munich waters, which at the place of origin contained 115 microbes per 

 centimetre contained 10,500 in 48 hours, and 500,000 after five days. At 

 the end, however, of another five days, he found that in this water, in 

 which the carbonic acid due to decomposition had so greatly increased, 

 the number or microbes had fallen off" to S7 per cent, centimetre. That 



