650 APPENDIX. 
to count on immunity for more than two years, and 
whenever we are liable to exposure it is well to be 
vaccinated. If it was unnecessary it will not be suc- 
cessful, while if it is successful we have reason to 
believe we were liable to at least a mild smallpox 
infection. 
Protective Substances Present in the Serum of Animals 
After Successful Vaccination. It has been repeatedly 
shown that the blood-serum of a calf several weeks 
after successful vaccination possesses feeble protective 
properties, so that the injection of one to two litres of 
it into a susceptible calf would prevent a successful 
vaccination. A further and more convincing fact has 
been demonstrated in the laboratory by Huddleston— 
namely, that when equal parts of a serum taken from 
a calf, two weeks after successful vaccination, and of 
an active vaccine are mixed together and inoculated 
in a susceptible calf the vaccine fails ‘‘to take.”’ 
The serum of an unvaccinated calf has no deleterious 
effect whatever when mixed with the vaccine. Serum 
taken even several years after vaccination, if the case 
is still immune, will inhibit very distinctly the action 
of fresh vaccine virus. 
The Form of Vaccine Virus Used. Vaccination is 
now usually performed with calf virus, as this is easier 
to obtain, is just as reliable, and practically eliminates 
the slight possibility of the transferrence of syphilis 
which existed in human vaccine. With active virus 
a portion of skin only one-sixteenth of an inch in 
diameter is scratched with the needle and the virus 
rubbed in. If preferred it may be inserted by a 
puncture. The vaccine is now usually mixed with 
glycerin and water and placed in capillary tubes. So 
