ol 
hour, for the purpose of killing the remaining living bacteria; 0.5 per 
cent of carbolic acid was then added in order to complete the sterilization. 
After standing twenty-four hours the fluid was centrifugalized for five 
hours at a velocity of 4,000 revolutions a minute. The clear fluid above 
was then pipetted from the bacterial sediment. 
The liquid obtained by this method is perfectly clear and amber 
colored, and, if the process has been carried on with sufficient care, it is 
sterile. Intraperitoneal inoculations into small guinea pigs of as large 
a dose as 5 cubic centimeters has in these animals in several instances 
produced convulsions which lasted for an hour or more but from which 
the animals have always recovered. It seems very likely that these 
symptoms owed their origin rather to the small yet appreciable amount 
of carbolic acid present, rather than to the extracted substances of the 
bacilli. A number of experiments have been performed in which single 
injections of this fluid in doses of from 2 to 5 cubie centimeters in 
guinea pigs and from 1 to 5 cubic centimeters in monkeys have been 
made. Since the development of pest immunity takes place slowly, the 
animals were not tested with the virulent pest organism until fifteen 
days after the injection of the fluid. In my last series of inoculations 
with such an extract, comprising fifty-three animals, none of the guinea 
pigs ** and but 25 per cent of the monkeys, on subsequent inoculation 
with a virulent plague bacillus, proved to be immune. Therefore, the 
results in immunization with this extract of the plague bacillus in this 
series of experiments at least are not nearly as good as those obtained 
with the living organisms of sufficient attenuation to be used for human 
inoculation. Hence, immunization with the attenuated living plague 
bacillus must be acknowledged as being superior to that in which these 
extracts are employed, and therefore vaccination must still be considered 
“The method employed in testing the immunity of the guinea pigs in these 
experiments Was a severe one. The shaved abdomen of the animal was searified 
with a sharp knife, three parallel incisions being made through the dermis. The 
abdomen in this region was then thoroughly massaged with a portion of a 
spleen of a guinea pig which had died a few hours previously of plague infection 
produced with a plague organism which had passed successively (without growth 
on artificial media) through over 150 animals. Portions of the same spleen which 
were known to contain very large numbers of plague bacilli were used for all 
the inoculations. The abdomen was searified and the test thus made severe in 
order to demonstrate whether this method of immunization in which the extracted 
substances of the bacillus were employed was superior to that in which the living 
attenuated organism was used. However, these experiments are not to be consid- 
ered as final in relation to the value of immunization with the free receptors of 
the plague bacillus as a further study of this subject with certain modifications 
of method, is at present being pursued. Moreover, having failed to immunize 
guinea pigs with single injections of this extract, I shall naturally now endeavor 
to demonstrate whether it is possible to immunize these animals with repeated 
doses, both of it and of the animal exudates such as Hueppe and Kikuchi 
employed. 
