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 wliicli 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 hia 

 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 possibihty 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 ^erm 

 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 be 

 the means of providing us with a scientific method of protection 

 against all infectious 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 ta 

 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 



