440 BACTEEIOLiCMJY. 



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 bJLood. 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, sucli 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 organism is 

 present, gas will be given off and will accumulate in the closed tube. 

 The amount of carbonic acid in this^as can be roughly determined by 

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

 tents of the tube, the gas is brought into contact with the alkali. 

 The difference in the volume of the gas, 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 precipitaJ 

 tion of rain or «now mechanically drags down a considera- 



