440 BACTERIOLOGY. 



peritoneally into white rats or guinea-pigs by means of a 

 sterile syringe. The recovery of the animal indicates the 

 absence of pathogenic bacteria. 



If death occurs it may be due to the typhoid fever bacil- 

 lus, but as a rule, it is due to other pathogenic bacteria. It 

 is necessary, therefore, to identify, if possible, the noxious 

 organism. For this purpose, gelatin and agar plate culti- 

 vations are made from the heart blood. To still further 

 test the pathogenic action of the organism 1 c.c. of a pure 

 culture in bouillon is injected subcutaneously into guinea- 

 pigs. The colon bacillus is not fatal under these condi- 

 tions. If death does result it is frequently due to typhoid- 

 like bacteria and the presence of such organisms should at 

 once condemn the water. The addition of even a fraction 

 of a drop of a virulent bouillon typhoid culture to a liter of 

 water can be detected in this way. 



Aerogenic bacteria, such as the colon bacillus, can be readily de- 

 tected by employing" a fermentation tube. Several forms have been 

 devised but that of Einhorn will be found very convenient. The tube 

 is filled with glucose bouillon, sterilized and inoculated with the sus- 

 pected water. If the colon bacillus or other aerogenic org-anism is 

 present, g-as will be given off and will accumulate in the closed tube. 

 The amount of carbonic acid in this gas can be roug-hly determined by 

 filling the tube with 2 per cent. NaOH. On carefully shaking- the con- 

 tents of the tube, the g^as is brought into contact with the alkali. 

 The difference in the volume of the g^as, after absorption, is due to 

 carbonic acid. The residual gas can be tested, qualitatively, for hy- 

 drogen. For this purpose the tube is partially inverted to allow the 

 gas to pass into the bulb portion. A lighted match introduced into 

 the tube will cause a slight explosion (Smith). 



The bacteriological examination of snow or ice is of 

 scientific and, at times, of practical interest. The material 

 is melted in a sterile dish or flask and the water, thus ob- 

 tained, is examined as above. 



The air contains a large number of bacteria as dry, 

 finely divided, suspended matter or dust. The precipita- 

 tion of rain or snow mechanically drags down a considera- 



