BACTERIA IN AIR, EARTH, AND WATER. 393 



considerable number of species, there must be several centres 

 from which these must be derived, each additional source, of 

 course, bringing in an additional element of danger. After ex- 

 amining 400 springs, wells, and streams, W. Migula concluded 

 that when there are more than ten species of bacteria in any 

 sample of water, especially when these are species not ordin- 

 arily met with, the water should not be used for drinking 

 purposes. In only 59 out of 400 was such a number of species 

 obtained, whilst 169 contained more than 1,000 individuals 

 per cc., 66 of these having over 10,000, and 40 over 50,000. 

 He found in all 28 species, and observed that the number of 

 colonies does not by any means correspond with the number 

 of species, though in some cases it undoubtedly does. 

 Ordinary putrefactive bacteria are almost invariably absent 

 from spring water, but they are usually found where the 

 number of colonies is between 1,000 and 10,000 per cc., but 

 they also occur where the number of germs is below 50 cc., 

 but very seldom where the number is over 10,000. Of 

 course the only perfect method is to examine each separate 

 species by itself and to examine carefully any organisms that 

 bear the slightest resemblance to any of the pathogenic 

 species. It is a good rule to observe that all water taken 

 from near the surface, or spring water that has been allowed 

 to come to the surface and remain there for some time before 

 use, should be filtered through sand or through porcelain 

 filters. If in the process of filtration pure air can be mixed 

 with the water so much the better ; the best of all filters for 

 this purpose being the Pasteur-Chamberland, which may be 

 readily applied to every household water supply. In large 

 water-purification works gravel and sand are by far the best 

 filters, especially if these are frequently renewed, the old 

 filtering medium being burned before being again used. 



For the examination of soil the first method used was simply to sprinkle a 

 little of the earth on a plate of nutrient gelatine, and then to examine the 

 organisms that grew on it. This, of course, was an exceedingly imperfect 

 method. The next step was to mix a small quantity of the earth with sterilized 

 nutrient gelatine in a test tube, and then to make a plate cultivation either 

 on a tube or on a flat surface. Now, however, that it has been found that 

 there are so many organisms on the surface of the earth, the mass of earth 

 to be examined has been diluted by adding a considerable quantity of 

 sterilized distilled water. This is allowed to stand for a considerable time 

 in order that the two may be thoroughly incorporated into a thin brown 

 liquid with as little sediment as possible, and from it plate cultivations 



