BACTERIOLOGICAL INVESTIGATION OF WATER. 249 



milling the disease, and she is known to live for long periods 

 after sucking the blood, it is claimed that this viability consti- 

 tutes the xole reason why only this species is infectious. Other 

 species of Culicidce deposit their eggs and die usually before 

 twelve days have elapsed since their time of feeding, too short 

 a time for the development of the infection. 



SCARLET FEVER. 



Various etiological factors have been proposed as responsible 

 for scarlet fever. Mallory's intracellular "bodies" have been 

 unsubstantiated. It has been assumed by some writers that 

 streptococci, possibly Streptococci pyogenes, are responsible. 



About this subject, however, nothing has been brought for- 

 ward the claims of which justify consideration now. 



QUESTIONS. 



What is rabies? What is its period of incubation ? 



Give Pasteur's treatment in suspected or positive cases of rabies. 



CHAPTER XXVII. 



BACTERIOLOGICAL EXAMINATIONS OF WATER, AIR, 



AND SOIL. 



THE BACTERIOLOGICAL INVESTIGATION OF WATER. 



BOTH qualitative and quantitative bacteriological examina- 

 tions of water are often resorted to in order to test the adapta- 

 bility of this substance for human and animal consumption. 

 By the quantitative test the number of bacteria present in 

 tlio water is ascertained without any reference to their patho- 

 genic character, and when this number exceeds 500 bacteria 

 per c.c. the water is condemned. This is evidently not a 

 very fair test, but it is not without value when it is consid- 

 ered that in a number of instances the virulence of some 

 pathogenic microorganisms is increased when they are inocu- 

 lated together with saprophytic germs ; and again, that the 



