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



media,* and it is well known what extremes of temperature, <fec, 

 spores can withstand. At the same time, since the rule is that a spore 

 germinates in even dilute solutions, when transferred thither, in 

 presence of oxygen and if the temperature rises, it may be regarded 

 as probable that, for aerobic bacteria at any rate, the changing con- 

 ditions in a river, &c, will prevent its remaining merely passive — 

 all available evidence is rather in favour of its either growing or 

 else dying if it cannot adapt itself to the circumstances, although 

 the death of spores may be delayed for many months and possibly 

 even longer. 



Indeed, recently, strong evidence has been produced, showing that 

 pathogenic microbes may sink to the bottom of lakes and rivers and 

 there remain in a living state, amongst the sediment or mud, for very 

 long periods of time, until in fact, some flood or other disturbance 

 * causes them to become once more suspended in the water, when they 

 may be carried by a stream or current to another place. It is obvious 

 that this hitherto but little recognised factor is of the very highest 

 importance in connexion with the supply of water from rivers subject 

 to objectionable pollution. f 



A third view is possible, viz., that the Schizomycete finds the new 

 environment at least not unsuited to its immediate requirements, 

 and that it grows and multiplies more or less successfully in the large 

 mass of water. 



This unquestionably happens with some forms, which, as we have 

 seen, are so well adapted for life in rivers, ponds, and even pipes, 

 that they have long been known as aquatic species. J As has been 

 stated, and will be seen more clearly shortly, however, this is also 

 true, to a limited extent, of many forms, including certain patho- 

 genic species, which are only met with in natural waters as in- 

 truders ; they are able to maintain themselves alive for variable 

 periods, and then usually succumb. 



Before passing to this part of the subject, we wish to remark upon 

 the method for a long time employed in the bacteriological examina- 

 tion of water, and on some of the general results obtained. 



Since 1881 it has been almost universally the custom to employ 

 the gelatine-plate cultures as devised by Koch. A measured small 

 quantity of the water to be examined is added to the nutrient gela- 



* Quoted by Koux ('Ann. de l'lnst. Pasteur,' vol. 1, 1887, p. 392). 



+ Lortefc and Despeignes, "Recherches sur les Microbes Patkogenes des Eaux 

 Potables distributes a la Yille de Lyon " (' Rev. d'Hygiene,' 12, 1890, No. 5) ; also 

 Lortet, " Die pathogenen Bakterien d. tiefen Sclilammes im Grenfer See " (' Cen- 

 tralbl. f. Bakter.,' 9, 1891, p. 709). 



% This term is, of course, not quite accurate, in view of the fact that all Schizo- 

 mycetes musr, have water to grow ; and are, indeed, descended from aquatic forms 

 — lower Algae. 



