148 LOUIS PASTEUK. 



distance, through the medium of the air and the dust it 

 carries, is a fact equally well established. It is suffi- 

 cient, by sweeping the breeding-houses, or by shaking 

 the hurdles, to stir up the dust of corpusculous excre- 

 tions and the dried remains of dead worms, and to 

 allow them to be spread over the hurdles of the 

 healthy worms, to cause, after a certain time, con- 

 tagion to appear among these worms. When very 

 healthy worms were placed in a breeding nursery at a 

 considerable distance from unhealthy worms, they, in 

 their turn, became infected. 



After so many decisive experiments it was no 

 longer possible not to see in pebrine an essentially 

 contagious disease. Nevertheless, among facts in- 

 voked in favour of non-contagion, there was one which 

 it was difficult to explain. There existed several 

 examples of successful cultivations conducted in 

 nurseries which had totally failed from the effects of 

 pebrine the year before. The explanation is, as shown 

 by Pasteur, that the dust can only act as a con- 

 tagion when it is fresh. Corpusculous matter, when 

 thoroughly dried, loses its virulence. A few weeks 

 suffice to render such matter inoffensive : hence the 

 dust of one year is not injurious to the cultivations of 

 the next year. The corpuscles contained in the eggs 

 intended for future cultivation alone cause the trans- 

 mission of the disease to future generations. 



And what can be more easilv understood than the 





