Report on the Bacteriology of Water. 395 



The ideal is practically realised in the case of a culture which, 

 once in position, remains without the necessity of any opening at all, 

 and this can frequently be carried out. In order to meet obvious 

 objections, however, I have in all cases made several cultures at each 

 temperature, and to make the results strictly comparable, I adopted 

 the following method of recording. 



1. The table of growths was prepared, giving the time of obser- 

 vation, temperature, and length of filament, &c., as in preceding 

 cases. 



2. Then the curve of growth was plotted ont on sectional paper, 

 care being taken that the squares, &c., were all equal, and that the 

 intervals between the observations were sufficiently short and 

 numerous to give good curves. Since the measurements were all 

 made with the same micrometer-scale and the same microscope, and 

 taking into account the proofs of accurate measurement already 

 given, no more need be said on that head. 



3. From bhese curves and tables I then measured the period 

 occupied by a rodlet or filament of any given length (to start with) 

 in growing to double its length, and call this the doubling period. 



4. The average doubling period for any temperature is then obtained 

 in the usual way, by taking the sum of the times and dividing by the 

 number of observations. Before saying more, however, it will be 

 best to examine the actual results, which now follow. 



I may add that I had already satisfied myself that the curves 

 obtained at widely different temperatures, are markedly different, 

 and less and less divergent as the temperatures of growth approach 

 one another, facts which are in accordance with experience with 

 other organisms, and which will be evident enough as we proceed. 



In what follows I select a number of representative curves from 

 larger series made to familiarise myself with the details ; and for the 

 sake of classification and with reference to some conclusions later 

 I present them arranged according to the food-materials employed. 

 The media chiefly used were four, viz. : (1) normal beef-broth ; (2) 

 the same with 1 per cent, of gelatine added to give a certain degree 

 of stiffness ; (3) a stiff 10 per cent, gelatine with mere traces of 

 broth (referred to as weak gelatine) ; and (4) normal 10 per cent, 

 broth-peptone gelatine. 



For various reasons I shall start with the cultures in weak 

 gelatine. 



Cultures in Weak Gelatine. 



On July 19 spores were sown in the stiff weak gelatine used 

 throughout July hitherto, and put into Sachs' box at 7 P.M. at 17 C. 

 The temperatures ran as follows : 



2 E 2 



