
QUALITATIVE ANALYSIS OF MARKET MILK. G5om 
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after very vigorous shaking was diluted with sterilized water. A 
proper differentiation requires that the milk should be diluted 
so that each plate shall have about 200 or 300 colonies. If the 
number is higher than this, the differentiation is unsatisfactory; 
if the number per plate is less, the results are somewhat unre- 
liable, because the samples are too small. ‘The first difficulty 
_ of the experiments is therefore to obtain a proper dilution. To 
insure a really satisfactory differentiation of species, it would 
be necessary to make plates from several grades of dilution for 
each sample of milk. This would vastly increase the labor, 
and after some preliminary experiments we found it most sat- 
isfactory to use for each experiment two grades of dilution. 
In all of the experiments described we used dilutions of 100 
and 600, thus giving a double chance of obtaining satisfactory 
plates for study. Sometimes one and sometimes the other of 
these dilutions was the more useful. In a few cases where the 
number of bacteria proved to be very large, the dilution of 600 
diameters was insufficient for proper differentiation; and if this 
method were to be adopted in larger cities, where the numbers 
are commonly higher, it would be necessary to make the dilu- 
tion higher than 600. But with the exception of two or three 
experiments, the dilution of 600 has been quite sufficient for 
our market milk, and in many cases the dilution of 100 was 
more satisfactory. 
Before describing the results in detail it is necessary to de- 
scribe the types of bacteria found in the normal milk of this 
vicinity. ‘The number of species in the milk is of course quite 
large, but of the large number of species found a few seem to 
be present almost universally, while the others are present ina 
small number of the samples of milk, and when present are as 
a rule only few in number. We therefore may readily divide 
the milk bacteria which we have found into the normal forms 
and the unusual forms, with of course no sharp line separating - 
the two. ‘he unusual forms of bacteria are, for the present 
purposes, of little significance, although it may be that they 
will prove in the end to be of more significance than the normal 
types. Our experiments, however, were planned to determine 
the proportions of the normal bacteria in milk in different 
samples, 
