254 EXAMINATIONS OF WATER, AIR, AND SOIL. 



though of great utility, it does not give definite information 

 as to the poisonous organisms of the water. For this pur- 

 pose a qualitative examination for pathogenic germs is of 

 much more value. This is not, however, as easily per- 

 formed, and in the great majority of cases gives negative 

 results. 



The bacteria most often sought in this way are the bacillus 

 of cholera and the bacillus of typhoid fever, both of which 

 are short-lived in water, and have only rarely been demon- 

 strated, partly perhaps on account of their low vitality in 

 water, and partly also because they are looked for at a date 

 considerably later than that at which they were originally 

 contained in the water. From the very nature of these 

 bacteria it is easy to understand why they should be short- 

 lived. The presence in the water of a number of ordinary 

 saprophytes interferes with the growth of pathogenic bacteria, 

 and either destroys them or consumes the pabulum neces- 

 sary for their growth. Sedimentation of the water, which is 

 constantly taking place, carries along with insoluble inorganic 

 matter the bacteria to the bottom ; and in running water 

 the oxygen of the air and direct sunlight act as efficient 

 germicides. 



In making qualitative examinations of water for typhoid 

 and cholera germs, what has been said in the special chapters 

 on those germs should be borne in mind. It is advan- 

 tageous, for instance, to mix the water with three times its 

 volume of sterile bouillon, and to incubate this mixture before 

 making the plates. 



For cholera, after six hours in the incubator the plates may 

 be made, taking for this purpose the fluid from the upper 

 portion of the mixture, as this germ grows rapidly, and 

 chiefly on the surface of the fluid. 



For typhoid fever the incubation should be continued for 

 two or three days before making the plates. By this incu- 

 bation the ordinary saprophytes are retarded in growth, as 

 they have less tendency to thrive at the incubator tempera- 

 ture, whereas the reverse is the case for pathogenic germs, 

 which grow much more rapidly at 37 C., and are thus in 



