254 IOWA ACADEMY OF SCIENCES. 



to count the number of colonies. For this dilution one-^ 

 tenth of a cubic centimeter of sewage is put into ten cubic 

 centimeters of sterilized water, and one-tenth c.c. of this 

 taken to make the culture. With the effluent no dilution 

 has been made. Two methods of counting the plates have 

 been employed. One is to divide the plates by means of a 

 dividing circle into tw^enty equal divisions, counting three 

 of these divisions, dividing by three to strike an average^ 

 and multiplying by twenty the number of divisions on the 

 plate, and by ten, the denominator of the fractional part 

 of a c.c. of sewage taken to make the culture. Of course, 

 when dilutions were made the above result was multiplied 

 by the denominator of the fractional part of a c.c. used, as 

 to illustrate, 21+18+12 -51^3-17X20 -340X10 3,400X 

 101^=343,400. The above sample being diluted by ten c.c. 

 of sterilized w^ater to 1-10 of a c.c. of sewage. 



The other method is practically the same. The plate is 

 divided into sixty square centimeters; three square centi- 

 meters are averaged and multiplied by the number of 

 square c.c. in the plate and the fraction of the denominator 

 of the dilution. 



In each method care was taken to obtain a good average 

 of the plate. As an illustration, if there was a spot where 

 the colonies were especially thick or thin, counts were^ 

 taken from them, and also from a spot containing about an 

 average number of bacteria, if possible. 



The pipettes, petri dishes, etc., used in the work, were- 

 sterilized by dry heat for one hour and kept away from 

 dust and moisture. 



The media used in these experiments has been, in the 

 main, ordinary agar agar, gelatine having been used on sev- 

 eral occasions to determine the variations between the 

 number of colonies produced by agar and gelatine cultures 

 respectively. It was found that on gelatine plates there is 

 usually a slight increase in the number of colonies, but on 

 account of the liquefying properties, it has not given as 

 much satisfaction as agar cultures. 



Another method employed for the determination of gas 

 producers is of special interest, as it can be show^i by 



