32 STORRS AGRICULTURAL EXPERIMENT STATION. 
colonies is satisfactory only after this length of time. In a few 
instances, where the liquefiers were too abundant, it was im- 
possible to keep the plates so long, and the results obtained 
were unreliable; but in most of the experiments here recorded 
the plates were about six days old when studied, and differen- 
tiation was therefore as satisfactory as possible. Three and 
sometimes four plates were made at each test, and the results 
were averaged. 
In the study of the plates the total number of bacteria was 
determined, and then the total number of different types of 
colonies that could be differentiated from each other by the use 
of a hand lens and a low power microscope. This differentia- 
tion of the colonies on the plate is the crucial point in the series 
of experiments, and the value of the conclusions depends upon 
the reliability of the differentiations. It was found possible 
to distinguish without much difficulty about fifteen different 
groups of bacteria, by which is meant, of course, the number 
of types of distinguishable colonies in litmus gelatin. A con- 
siderably larger number can readily be distinguished, but out- 
side of these chief types the bacteria found are only isolated 
colonies, and play no special part in the problems under con- 
sideration. The groups tabulated are, as stated, separated from 
each other wholly by the character of their colonies upon litmus 
gelatin. We have made many further tests of these groups by 
isolating colonies of each type and carrying them through a 
long series of cultures, to determine whether our types repre- 
sented single species or groups of several celled species. These 
results are not yet ready for publication. In general we find 
that the types are commonly groups of several species; some, 
indeed, are quite complex. Some of these facts are briefly 
given in the following description of types. 
Group I. Bacterium lactis acidi (Leichman—Esten). This 
species, as shown by Hsten (An. Rep. Storrs Exp. Sta., 1896) 
is the lactic bacterium of our dairies par excellence, and needs 
no description. There seem to be several varieties of the 
species, some of which are capable of curdling milk, and others 
not. Our many tests of this species show us that a majority of 
the cultures of this type, isolated and tested, fail to curdle 
sterilized milk at 20°C. All produce acid, however, and all 
