Frost—Bacteriological Control of Public Milk Supplies. 1307 
B. Quantitative Methods. 
1. Media. The media used, agar and gelatine, were prepared 
according to the standards recommended by the Committee of 
the American Public Health Association.* The reaction varied 
at the time it was made and before sterilization from +0.7 to 
+ 1 . 
2. Dilution Methods. 
a) Water Blanks f. 
All laboratory workers know that “water blanks,” made for 
the purpose of diluting the sample to be analyzed, change in 
volume during the process of sterilization. The committee hav¬ 
ing in charge the preparation of standard methods of milk anal¬ 
ysis recognize this for they made the following statement: 
“In order that the bottles and tubes may contain 99 and 9 c. c. 
respectively after sterilization, they should be tilled a little above 
these marks ”4 No one, however, seems to have determined just 
what this loss amounts to or how it can be avoided. In fact, 
there seems to have been little or nothing written on this phase 
of the subject of quantitative analysis. It is, nevertheless, an 
important matter for the reason that this is one of the steps 
that ought to be precise. Certain factors are in the nature of 
the case indefinite, as for instance, inherent errors in sampling, 
the probability that certain colonies are descended from, a group 
of germs rather than a single one and the difficulty of measuring 
very small quantities of fluids with the pipettes used. For this 
reason it is very important that appreciable errors in technique 
should not be introduced where it is possible to avoid them. It 
seemed worth while, then, to determine the extent of, the error 
in this connection and how it could be avoided. 
I. Loss of Weight in Autoclaving. 
A-definite amount of water, accurately determined, was placed 
in bottles; these were plugged with cotton and autoclaved and 
then the water remaining was carefully determined. At first 
* Standard Methods for the Bacterial Examination of Milk. Am. 
Jonr. Pub. Health. 
Standard Methods of Water Analysis. Am. Jour. P. H., 1912. 
f This section read before the Soc. of Am. Bact. New York. 1913. 
% Proc. Assoc, of Am. Med. Milk Commissions, 1910, Vol. 4, p. 242. 
