BACTERIA IN AIR, EARTH, AND WATER. 387 



stration of the different aerobic organisms present, another for the anaerobic 

 species, and others may be used to determine the mere numbers of these 

 bacteria that are present, there being sufficient material for a dozen or even 

 twenty analyses. If a large number of organisms be present it is necessary 

 to dissolve the sugar in a large quantity of water so as to obtain a sufficiently 

 dilute " solution," whilst if a small number only are expected to be present 

 a smaller quantity of water should be used. 



Water is one of the most convenient vehicles for the 

 distribution of micro-organisms. It has been noted that a 

 shower of rain diminishes the number of bacteria suspended 

 in the atmosphere in a most remarkable manner ; these 

 organisms being afterwards found in the puddles in the 

 road, in the pools in the sidewalks. In stagnant water and in 

 surface water of all kinds bacteria may be found in enormous 

 numbers, though the numbers and varieties in which they 

 are present may be very considerably modified by the amount 

 of organic material contained in the soil through which such 

 surface water comes, by the depth at which the water is 

 taken from the surface, and by the facilities for oxidation or 

 aeration that may be present. In the sluggish streams of 

 valleys, where there is a constant drainage from the surface 

 land, and where the water is so little disturbed that the air 

 of the atmosphere cannot obtain free access to the organic 

 matter suspended in the water, the number of micro-organisms 

 will, as a rule, be very considerable, whilst the rapidly -flowing 

 shallow streams that make their way over shingle or gravelly 

 beds will be found to lose their micro-organisms very rapidly 

 indeed ; if the organic matter has not already been left 

 behind in the sluggish reaches of the stream it is very 

 rapidly converted by oxidation into the ultimate products 

 of decomposition, and ordinary putrefactive micro-organisms 

 at any rate are no longer able to obtain any subsistence, 

 although, as Bolton maintains, the " water ' bacteria 

 can still flourish. They can even grow in distilled 

 water. In the water that comes from springs there may 

 be scarcely a single bacterium as the water rises to the 

 surface, but if such spring water be allowed to stand exposed 

 to dust and contamination of various kinds it very quickly 

 swarms with micro-organisms just as do the waters from 

 other sources. A very small quantity of sewage, which 

 really consists of water in which is suspended an enormous 

 quantity of organic matter, finding its way into a water 

 supply may contaminate it for a whole neighbourhood, such 



