mo 
te 
QUALITATIVE ANALYSIS OF MARKET MILK. OF o 
being small, and the differentiation not sharp. An acidity of 
I per cent. acid gave better results. There was very little dif- 
ference between the results obtained with r per cent. and 1.5 
per cent., although there seemed to be a small advantage in 
favor of 1.5 per cent. A medium 2 per cent. acid was again 
unsatisfactory. Hence the best grade of reaction was between 
I per cent. and 2 per cent., and we finally chose 1.5 per cent. 
as upon the whole a little more satisfactory than a lower grade. 
In all of our subsequent work this grade of reaction, 1.5 per 
cent. acid to the phenol-phthalein neutral point, has been 
chosen, and both the litmus solution and the gelatin medium 
have been brought to this point. 
DIFFERENT CULTURE MEDIA. 
Our next problem was to determine which of the media above 
mentioned is the best for the purpose of differentiation. ‘To 
test this a sample of milk was diluted to a proper extent, usually 
about 300 times, and then one cubic centimeter of the dilution 
was placed in several tubes of each of the three kinds of gelatin 
culture above described and also into mixtures of the different 
kinds. We have used fresh milk rather than older milk, inas- 
much asin the fresh milk the variety of bacteria is consider-_ 
ably greater than in samples of older milk, and consequently 
the differentiation is more difficult, and the test therefore a 
more rigid one. The three sets of tubes were poured into petri 
dishes, put aside at the ordinary room temperature, and allowed 
to develop until they were of a proper age for study. Hach 
plate was then studied by itself, and the results tabulated and 
‘compared with one another. 
The results of the comparative study of three series of plates 
cannot be satisfactorily expressed by tables. Although tables 
can give the total numbers of bacteria of each species distin- 
guished in the different plates, they cannot express at all the 
sharpness of differentiation of the different kinds of colonies — 
and do not give any idea as to which medium is best for this 
purpose. - This can be determined only by actual examination 
of the plates and by learning from practice which type of plate 
is easiest to study and most satisfactory. Three of these com- 
parative tables will be given below and will serve two purposes. 
They will illustrate the use of different culture media as 
