186 Profs. P. F. Frankland and Marshall Ward. 



to us, however, too narrow, as waters are surely to be condemned 

 for drinking purposes not only when they contain the germs of 

 zymotic disease at the time of analysis, but in all cases when they 

 are subject to contaminations which, like sewage, may at any time 

 convey such germs. 



Notwithstanding that great difficulties usually invest the discovery 

 of particular pathogenic forms in the presence of large numbers 

 of ordinary water microbes, the spirillum of Asiatic cholera was 

 early discovered in water by Koch, whilst the bacillus of typhoid 

 fever has more recently been on several occasions detected by various 

 investigators in drinking waters which had been suspected of com- 

 municating this disease. It is obvious that such discoveries are 

 rather of interest on account of their confirming the belief in the 

 communicability of these diseases through water than of special 

 hygienic importance in preventing outbreaks of zymotic disease. 



The bacteriological methods of examination have been also em- 

 ployed for a purpose of far greater importance than the mere quest 

 for pathogenic forms in particular water-supplies, viz., for direct ex- 

 periments on the vitality of known pathogenic organisms in waters of 

 different composition and under varied conditions of temperature and 

 the like. This investigation was also begun independently and 

 almost simultaneously both in this country and on the Continent, and 

 although the question appears at first sight a very simple one, it is in 

 reality surrounded with numerous pitfalls, which have, in some cases, 

 led to very discordant results. Perhaps the greatest difficulty which 

 attaches to this investigation consists in the very different degrees o£ 

 vitality which are exhibited by one and the same organism at dif- 

 ferent periods of its life, and according to the previous treatment 

 which the individual and its ancestors have received. On this 

 account a number of individuals of a particular species, on being in- 

 troduced into a given water, may conduct themselves quite differently 

 from a number of individuals of the same species, but taken from a 

 different source and having another ancestral history. This import- 

 ance of individuality and pedigree, with which we are so familiar in 

 dealing with the higher plants and animals, must also invariably be 

 taken into consideration in connexion with these lowly forms of 

 life* 



In spite of some conflicting results which have thus not unnaturally 

 been obtained in these investigations, it is sufficiently evident that 

 some pathogenic species, and notably those which form spores, are 

 capable of retaining their vitality in ordinary potable waters for a 



* Particular cases of the application of these methods to the examination of 

 food substances, brewing and distilling waters, the materials of clothing, dyeing 

 and other industries lie beyond our present enquiry ; their importance is obvious, 

 as is also the utility of the methods for the analyses of air, soils, &c. 



