MICHIGAN ACADEMY OF SCIENCE. 
201 
pennyroyal, etc.; when exposed to the direct action of these, however, they 
rarely recovered, except from Ammonia gas and the vapors of carbon disul- 
phide. Hydrocyanic acid gas is extremely poisonous even when quite 
diffused; the only objection to it being, the fact that it is equally poisonous 
to human beings. All insects, however, are not so resistant to the action of 
vapors and gases. 
The chemistry of germicides is on a much better basis than that of insecti- 
cides. The chemist and the bacteriologist working together have been able 
to determine the constituents of coal tar and the conditions necessary to pro- 
duce any possible germicidal value, while a carbolic acid coefficient has been 
assigned to most of the well known chemicals: so that in many cases a chem- 
ical analysis is sufficient to determine the germicidal value, even of coal tar 
products, a value which checks up very closely with that determined by a 
bacteriological test. We have found the B. pyocyaneous, the most satis- 
factory test organism; the advantages of which are threefold: its high resis- 
tance to germicides, its easy growth on beef bouillon or other media, and its 
characteristic formation of green pigment. These properties make it easy 
to conclude as to the dilutions which are efficient while its high resistance 
assures one that any other germ under like conditions will be destroyed in 
dilutions of the disinfectant much greater than are required for the B. pyo- 
cyaneous. There is this advantage to the use of it alone, as the test organism 
that its carbolic acid coefficient is much smaller than if almost any other 
germ were used. 
In the table and chart , the germicidal value of each agent used is expressed 
in terms of carbolic acid using B. pyocj^aneous as the test organism, but 
to illustrate the selective action that certain germicidal products may have 
toward certain organisms, it might be stated that two coal tar oils derived 
from coal distilled under different conditions, can have the same carbolic 
acid coefficient based on B. pyocyaneous, while when based on B. typhosus, 
the value of one is nearly twice as great. These results are not inconsistent 
but they make the value of any comparative tests of germicides depend 
on a complete account of the method followed. In fact, so much depends 
on the minutia of details that an absolute agreement between two bacterio- 
logists, working independently, is not to be expected. The method followed 
in our experiments is in some of its features, very similar to that adopted 
by the Royal Commission of London for the standardization of disinfectants. 
A 24 hour growth of the B. pyocyaneous in bouillon culture medium, is 
filtered to break up or exclude clumps of bacteria difficult of penetration. 
Six drops of this culture containing millions of bacteria are mixed with 
5 cc. of a dilution of the germicide. After one minute’s contact with this, a 
subculture is made by removing a loop full of the solution, which, if properly 
mixed, will contain a proportional number of the bacteria originally intro- 
duced. This loop full is planted in a tube containing 5 or 6 cc. of sterile 
bouillon which is then placed in the incubator for 24 to 72 hours, during 
which time any organism not destroyed will have multiplied sufficiently to be 
readily recognized. Subcultures are made in a similar manner after 2, 3, 4 and 
5 minutes contact between bacteria and germicide, and changes are made 
in the dilution of the latter until such a dilution is obtained which will allow 
growth after two or three minutes contact, but less than five minutes. The 
degree of dilution which will give this result for each preparation can be used 
in comparison with that dilution of Carbolic Acid which is equally efficient 
to obtain what is known as the Carbolic Acid Coefficient — a very satisfactory 
method of readily showing the relative values of different germicides under 
26 
