26 
with the disease, that, when developed into moths, they invariably 
laid diseased eggs, which in their turn would yield no cocoons. 
Here then was the solution of the problem—worms in which the 
disease is inherited spin no cocoons, and can, as moths, lay only 
diseased eggs; worms in which the disease is contracted and not 
inherited, can spin cocoons, but only yield diseased eggs. Here, as. 
in all M. Pasteur’s discoveries, the aid of the microscope is indis- 
pensable, and by careful examination of the worms, moths, and 
eggs, it is easy for the diseased specimens to be identified and 
destroyed, and the disease may thus be practically stamped out.. 
So successful indeed has been the application of M. Pasteur’s tests, 
that the silk industry is once more flourishing, and Pebrine is, 
comparatively speaking, a disease of the past. 
But a still greater task awaited M. Pasteur in elaborating his 
experiments as to the possibility of weakening the virus of specific 
microbes, and thus placing preventive inoculation on a scientific 
basis. In the course of his experiments with ferments, M. 
Pasteur had first discovered the possibility of separating and. 
cultivating any special germ in some suitable broth or liquid, 
much as a gardener raises some special seed in a_par- 
ticular soil. His attention was first directed towards the 
subject of infectious diseases and preventive inoculation, by 
the prevalence of a disease called chicken cholera, caused by 
poisonous microbes in the blood and tissues of the diseased fowls. 
M. Pasteur succeeded in isolating and cultivating this specific germ 
without weakening its poisonous properties, but he also found that 
by exposing it for some time to the air, and subjecting it to a 
certain degree of heat, it was possible so to weaken its virus that a 
chicken inoculated with this attenuated lymph was made only 
slightly ill. This mild attack, however, caused by the weakened. 
virus, renders the fowl proof against the more deadly disease. 
This brilliant discovery that the virus of these micro-organisms is. 
alterable at will, and can be cultivated up or down to any degree 
of virulence, is of incalculable value, and affords the clue to the 
theory of preventative inoculation, which may possibly in time he 
the means of providing us with a scientific method of protection 
against all iifectious diseases. It is an interesting question 
whether the varying strength of the virus of infectious complaints 
may not possibly account for the variable strength of the attacks, 
a variety of circumstances conspiring either to preserve or to 
weaken the poison. 
M. Pasteur’s next subject of investigation was that fatal disease 
called anthrax, charbon, or splenic fever, which in severe cases 
often kills cattle and sheep in twenty-four hours. In ore dis- 
trict of Russia alone, between the years 1867 and 1870, no less 
