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



cient quantity on water containing bacteria in suspension, and organic 

 substances in solution, but it is not probable that this forms an impor- 

 tant factor in the case of natural waters which cannot be subjected 

 in their entirety to direct insolation. 



(6.) The evidence is still less conclusive as regards mechanical 

 disturbances in the water, so far as they directly affect the living cells 

 of the micro-organisms, but it is at least highly probable that every 

 wind-raised wave, every tumble over a fall or weir, and every pause in 

 a backwater or lake, must affect the matter, if only in so far as it 

 alters the gaseous contents of the water, or the relative distances 

 between the individual micro-organisms. 



Enough has been said to show that the bacteriologist who attacks 

 the question before us must at least bear these facts in mind.* 



Now, as to the questions of distilled as opposed to non-distilled 

 water, and of sterilised as opposed to non-sterile waters. 



It will be conceded forthwith that distilled, and we will assume 

 pure, water offers little scope for practical enquiry. Such water is 

 unknown in nature, except momentarily or in inaccessible forms, and 

 the only lessons to be expected from its action on bacteria are of 

 purely scientific and philosophical interest ; distilled water, therefore, 

 should be used in check experiments, and the results compared with 

 those obtained with other waters, not forgetting that " distilled 

 water " is not a constant medium. 



As to experiments with sterilised water, the matter is very different, 

 for most observers are unanimous as to the longer vitality of patho- 

 genic forms in sterilised water than in the same water before sterilisa- 

 tion ; the experiments in sterilised water may thus furnish us with the 

 ultimate limits of vitality, and will, therefore, act as valuable guides. 



It must, however, be remembered that there are two ways of sterilis- 

 ing a water,t (1) by heat and (2) by filtration. In both cases the con- 

 stitution of the water may be altered. Where heat is employed the 

 gases are driven off, in whole or in part ; soluble products may be 

 rendered insoluble, e.g., carbonates precipitated ; the proteids, &c, of 

 the killed micro-organisms are placed more or less at the disposal 

 of the living ones which follow ; and the solution (which a natural 

 water really is) becomes more concentrated. J Moreover, many meta- 



* We have not thought it necessary to discuss the question as to the action of 

 electricity on bacteria : the results hitherto are negative, excepting in so far as 

 electrical currents alter the chemical constitution of the medium. For literature 

 and criticism see Duclaux (" Action de l'Electricite sur les Microbes," ' Ann. de 

 l'lnst. Past.,' vol. 4, 1890, pp. 677—680). 



f It is obviously unnecessary to discuss sterilisation by means of antiseptic and 

 poisonous substances, although this is, of course, of great importance in connexion 

 with the treatment of sewage, &c. 



t Though, of course, there is not necessarily diminution in volume in the process 

 of sterilisation. 



