216 
HERMANN M. BIGGS. 
as in the first instance, the period of incubation becomes gradually 
shorter, and, after the passage through a series of twenty-five rab¬ 
bits, the time is finally reduced to eight days. In the transmission of 
this virus through'a second series of twenty-five rabbits, the period 
of incubation is further reduced to seven days, but at this point 
apparently becomes stationary, as this time is not materially dimin¬ 
ished in the passage through a third series of forty animals. 
Commencing these experiments in November, 1882, at the end of 
three years Pasteur had inoculated in this manner successfully a 
series of more than ninety rabbits, without having had this series 
once interrupted or broken. He says of this: “Nothing is easier, 
therefore, than to have at one’s disposal, after considerable inter¬ 
vals of time, a virus of perfect purity always identical, or nearly 
so. This constitutes the practical point of the method.” 
If, now, the cord is removed (with all possible care to prevent 
contamination with foreign matters) from a rabbit just dead from 
rabies, and is suspended in a jar the air of which is artificially 
dried and kept at a constant temperature of 20°C., it gradually 
loses its virulence until, at the end of about fifteen days, this has 
become quite extinct. The time required for the extinction of 
the virulence is somewhat dependent upon the thickness of the 
spinal marrow and upon the external temperature. The spinal 
cords which have been subjected to this process of desiccation for 
a shorter period than fifteen da}-s possess a gradually increasing 
virulence inversely proportionate to this time. The conditions 
remaining the same under which the cords have been preserved, 
the degree of virulence is always constant, or nearly so. If the 
infectious cords, while in a moist condition, are protected from 
the atmospheric air and submerged in carbonic-acid gas, the viru¬ 
lence may be preserved unaltered for several months, providing 
only that no foreign micro-organisms gain access to them. Ac¬ 
cording to Pasteur, “ these observations constitute the scientific 
points of the method.” 
To render a dog refractory to rabies the following method is 
pursued: 
On the first day the animal is inoculated with a portion of 
the cord (removed from a rabbit dead of rabies appearing after 
