Bacteria Developing at Different Temperatures 225 



obtained may have an important i)lace in judging the character of a 

 water by analytical means. In a large number of laboratories, where 

 the routine work consists in the control of the water supplies of large 

 cities or in the control of water filters, almost complete reliance is 

 placed on the results of bacteriological examinations in judging 

 whether such waters are of the required (quality. In such cases any 

 additional factors which will assist in a more accurate judgment, or 

 any change in the procedure which will enable such a judgment to be 

 arrived at more quickly than under present conditions, would be of 

 inestimable value in protecting the consumers from the effects of any 

 sudden change in the quality of such waters. 



Media for bacterial counts. — It is well known that gelatin and agar 

 do not show us the total bacterial content of a water, much higher 

 numbers being obtained when other and differently constituted media 

 are employed; and, furthermore, slight changes in the composition 

 of the culture media may cause considerable variations in the num- 

 bers of bacteria developing on those media. These points have been 

 very thoroughly investigated by Fuller,' Hesse and Nieder,^ Whipple,' 

 G. Hesse,'* the writer,^ and others, and their consideration need not 

 be entered upon at this time. Gelatin and agar continue to be the 

 media most commonly employed in quantitative bacterial determina- 

 tions, for several reasons, the principal ones being that the interpreta- 

 tion obtained by their use is well grounded, and that with the newer 

 media the time required to make a bacterial examination is lengthened 

 instead of shortened. It is largely true, moreover, that while gelatin 

 and agar do not show us the total bacterial content of a water, we are 

 able by their use to obtain a knowledge of a fairly representative sec- 

 tion of that bacterial content. The practice as to the use of gelatin 

 or agar varies widely in different laboratories. The principal advan- 

 tage obtained in the use of gelatin instead of agar is that a determina- 

 tion of the number of liquefying bacteria is possible — a factor of little 

 practical value, except when dealing with sewage and the effluents 

 from sewage disposal works, and which is more than offset by the 

 greater ease with which agar plates may be manipulated. 



During more than 15 years it has been the custom to employ 

 agar in routine work at Lawrence, the Lawrence agar being identical 

 in composition with that recommended by the Committee on Standard 



