XIII, A, 1 Heise and Behrman: Water Analysis in Field 17 



ployed. The upper limit will be determined by the fact that 

 1 per cent agar is the weakest that solidifies on cooling to the 

 temperatures ordinarily encountered (25° to 30° C). 



Incubation is at the ordinary temperature. No special ap- 

 paratus is, therefore, required. 



Colony counts are made both at the end of twenty-four and 

 forty-eight hours, using a lens magnifying at least 5 diameters. 

 The average of the two counts is the recorded value. 



When the number of colonies is high, the plate is marked 

 into sectors of convenient size, and the total number of colonies 

 is estimated, or else the number on representative areas of 1 

 square centimeter is determined (a small card with openings 

 of appropriate size and shape has been found very convenient 

 for field work), and the necessary calculation for the total area 

 is jnade. The presence of red colonies is noted. 



The presence of the organisms of the colon group is indicated 

 by the formation of gas in the tube cultures and by the for- 

 mation of acid, as shown by the change of litmus from blue 

 to red. 



Summing up the whole question of the value of field methods, 

 it might not be out of place to quote from the introduction of 

 Leighton's paper: 



To the methods hereinafter proposed the term "assay" readily lends 

 itself. There is no attempt at water analysis. The plan contemplates the 

 determination of ingredients which give to water certain well-known charac- 

 teristics. The methods have been found to be more nearly accurate than 

 was at first anticipated, though this fact, it is believed, has not greatly 

 increased their usefulness for the purposes in view. By their use, combined 

 with a fair amount of common sense, the essential characteristics of waters 

 can be ascertained at small expense. In almost every situation in which 

 such determinations are significant they will afford sufficiently satisfactory 

 data. In the case of finely balanced considerations of a purely physical, 

 chemical, or geologic nature, however, they are practically useless. They 

 are intended for practical purposes and have no place in pure science. 



In the Philippines field methods have shown themselves to 

 be both accurate and efficient. They have enabled the differen- 

 tiation between good and bad waters used for domestic pur- 

 poses, the selection of proper water for municipal supplies, the 

 condemnation of dangerous sources of infection in cholera- 

 infested districts, and the rapid evaluation of waters desired 

 for industrial purposes. 



151772 2 



