ACQUIRED RESISTANCE. 259 
the herds from the severe and fatal infection. But to induce a 
mild attack was not easy. It would do no good to inoculate an 
animal with a small number of the bacilli which produce the disease, 
for these, by multiplying, would soon become so numerous as to 
bring about a severe attack of the disease. It was evident that 
this end could be reached only by weakening the power of the dis- 
ease-producing bacilli. After- continued experimenting he finally 
accomplished his purpose by cultivating the bacilli at a temperature 
somewhat above that at which they make their best growth. By 
the use of a temperature of 108 F. he obtained cultures that were 
so weak as to be unable to produce a fatal disease, even in suscep- 
tible animals. 
After reaching this result Pasteur, by an ever memorable 
experiment, demonstrated to the world the possibility of combating 
infectious diseases by the use of what are now known as weakened 
cultures. He inoculated half of a lot of fifty susceptible animals, 
including cattle and sheep, with his weakened virus. The animals 
were slightly indisposed, but suffered no evil consequences. In a 
few days he inoculated them with a second, stronger culture, with 
a like harmless result. Having thus prepared these test animals, 
he summoned to a public experiment an assemblage of noted men 
in Paris, and, in the presence of them all, inoculated the entire 
fifty animals with the strong infectious material taken from an 
animal dead from the disease. Two days later the company 
assembled again to find all of the unprotected animals either dead or 
dying from a violent case of anthrax, while of the protected animals 
not a single one showed the slightest evil result from the inoculation. 
A more beneficial discovery has hardly ever been made. From 
the date of Pasteur's experiment a constant succession of bacteri- 
ologists has been trying to apply the same principle elsewhere. 
We cannot here attempt to follow the development of the work, 
but can only state that practical results of the utmost value have 
been obtained. It has not been found possible to use just the same 
method in other diseases that Pasteur used, but by a modification 
of it, or by others that have come from it, it has been found possible 
to withdraw the terrors from some of the most dreaded diseases. 
