In preparing plates, such amounts of the water under examination shall be planted as will give from 25 to 250 colonies on a plate;[202] and the aim should be always to have at least two plates giving colonies between these limits. Where it is possible to obtain plates showing colonies within these limits, only such plates should be considered in recording results, except where the same amount of water has been planted in two or more plates, of which one gives colonies within these limits, while the others give less than 25 or more than 250. In such case, the result recorded should be the average of all the plates planted with this amount of water. Ordinarily it is not desirable to plant more than 1 cc. of water in a plate; therefore, when the total number of colonies developing from 1 cc. is less than 25, it is obviously necessary to record the results as observed, disregarding the general rule given above. Counting shall in all cases be done with a lens of 2½ diameter’s magnification, 3½X. The Engraver’s Lens No. 146 made by the Bausch & Lomb Optical Company fills the requirements, and is a convenient lens for the purpose. 8. THE TEST FOR THE PRESENCE OF MEMBERS OF THE B. COLI GROUP. It is recommended that the B. coli group be considered as including all non-spore-forming bacilli which ferment lactose with gas formation and grow aërobically on standard solid media. The formation of 10 per cent. or more of gas in a standard lactose broth fermentation tube within 24 hours at 37° C. is presumptive evidence of the presence of members of the B. coli group, since the majority of the bacteria which give such a reaction belong to this group. The appearance of aërobic lactose-splitting colonies on lactose-litmus-agar or Endo’s medium plates made from a lactose broth fermentation tube in which gas has formed confirms to a considerable extent the presumption that gas-formation in the fermentation tube was due to the presence of members of the B. coli group. To complete the demonstration of the presence of B. coli as above defined, it is necessary to show that one or more of these aërobic plate colonies consists of non-spore-forming bacilli which, when inoculated into a lactose broth fermentation tube, form gas. It is recommended that the standard tests for the B. coli group be either (A) the _Presumptive_, (B) the _Partially Confirmed_, or (C) the _Completed_ test as hereafter defined, each test being applicable under the circumstances specified. A. PRESUMPTIVE TEST. 1. Inoculate a series of fermentation tubes with appropriate graduated quantities of the water to be tested. In every fermentation tube there must always be at least three times as much medium as the amount of water to be tested. When necessary to examine larger amounts than 10 cc. as many tubes as necessary shall be inoculated with 10 cc. each.