458 EEPORT— 1893. 



(2) That in the majority of cases they can retain their vitality and 

 virulence in potable vpaters for considerable periods of time — days, weeks, 

 and in the spore form for months or even years — but that their longevity 

 is almost invariably, and often very greatly, cui'tailed by the common 

 bacteria present in all natural waters. They are thus generally far more 

 persistent when introduced into sterilised than into nnsterilised water. 



(3) "With few exceptions the pathogenic bacteria which have been 

 experimented with do not undergo any extensive multiplication in potable 

 waters, although such multiplication is frequent in the case of foul waters 

 like sewage. 



Bacteriological examination, again, has greatly fortified the now 

 generally accepted views as to the communication of typhoid fever and 

 Asiatic cholera through the medium of drinking water by the actual dis- 

 covery of the typhoid and cholera bacilli in waters which had been 

 suspected of distributing these diseases. 



It is precisely in this particular of bacteriological water examination 

 that great advances have been recently made. The searching for patho- 

 genic bacteria in a potable water must always be very much like looking 

 for a needle in a haystack, and it has been abundantly shown that the 

 ordinary process of plate-cultivation is, excepting in rare cases, quite 

 inadequate for this quest, owing to the crowding out of the few ])atho- 

 genic by the overwhelming majority of non-pathogenic forms. It has, in 

 fact, become more and more evident that, in order to discover any 

 particular organism — pathogenic or otherwise, for the matter of that — 

 which is present in a very small minority, it is necessary to submit the 

 water or other material under examination to a preliminary treatment 

 before proceeding to plate-cultivation. This preliminary treatment must 

 be so conceived and executed as to foster the multiplication of the 

 particular organism of which we are in quest relatively to that of those 

 organisms which are of no interest, and thus secure a majority of the 

 former at the ensuing plate-cultivation, when no difficulty will arise in 

 detecting its presence. Such special methods of examination are now in 

 constant use for the detection of both the cholera and typhoid bacilli in 

 water. 



It will thus be seen that by combining chemical and bacteriological 

 methods of examination our knowledge of water hygiene has been very 

 greatly extended during recent years. 



In the matter of sewage treatment the most laborious investigations, 

 in which both chemical and bacteriological methods of examination were 

 simultaneously employed, have been made by the Massachusetts Board 

 of Health. The results of these investigations show, as would indeed be 

 expected, that intermittent filtration through soil, if properly carried out, 

 is the most efficient not only from a chemical but also from a bacterio- 

 logical point of view, and in the chemical precipitation of sewage there is, 

 in general, a much greater removal of micro-organisms than of organic 

 matter, or, in other words, the bacteriological efficiency of these precipita- 

 tion processes is generally much greater than their chemical efficiency. 



Bactericidal Action of Light. — Although I have not time to enter into 

 the subject of disinfection in its entirety — i.e., the destruction of bacteria 

 by chemicals and other agencies — there is a section of this subject on 

 which I should not omit to say a few words, viz., the disinfecting or 

 bactericidal action of light. 



Very soon after bacteria had been introduced to the general pubRc 



