498 THE LIFE-WORK OF A CHEMIST. 



to communicate to the Academy of Sciences, results of observations 

 and experiment which, striking at the root of the evil, pointed the way 

 to the means of securing immunity from the dreaded plague. This pa- 

 per was freely criticised. Here, it was said, was a chemist who, quitting 

 his proper sphere, had the hardihood to lay down rules for the guid- 

 ance of the physician and biologist in fields specially their own. Why 

 should his proposals be more successful than all the other nostrums 

 which had already so egregiously failed"? 



In order to appreciate the difficulties which met Pasteur in this in- 

 quiry, and to understand how wonderfully he overcame them, I must 

 very shortly describe the nature of this disease, which is termed pebrine, 

 from the black spots which cover the silk-worm. It declares itself by 

 the stunted and unequal growth of the worms, by their torpidity, and 

 by their fastidiousness as to food, and by their premature death. 



Before Pasteur went to Alais the presence of certain microscopic cor- 

 puscles had been noticed in the blood and in all the tissues of the dis- 

 eased caterpillar, and even in the eggs from which such worms were 

 hatched. These micro-organisms often fill the whole of the silk organs 

 of the insect, which in a healthy condition contain the clear viscous 

 liquid from which the silk is made. Such worms are of course value- 

 less. Still this knowledge did not suffice, for eggs apparently healthy 

 gave rise to stricken worms incapable of producing silk, whilst again 

 other worms distinctly diseased yielded normal cocoons. These diffi- 

 culties, which had proved too much for previous observers, were fully 

 explained by Pasteur. " The germs of these organisms," said he, " which 

 are so minute, may be present in the egg and even in the young worms, 

 and yet baffle the most careful search. They develop with the growth 

 of the worm, and in the chrysalis they are more easily seen. The moth 

 derived from a diseased worm invariably contains these corpuscles, and 

 is incapable of breeding healthy progeny." 



This moth-test is the one adoptell by Past(gtir, and it is an infallible 

 one. If the female moth is stricken, then her eggs, even though they 

 show no visible sign of diseas^e, will produce sick worms. If in the moth 

 no micrococci are s^een, then hel" immediate proge^ny at any rate will be 

 sound and free from inheritied faint, and will always prbilucB the ncfrmal 

 quantity of silk. But this is not all. Pasteur found that healthy worms 

 can be readily infected by contact with dis\3aseil ones, or through germs 

 contained in the dust of the ro'oms in which the worms are fed. Wofms 

 thus infected, but free from inherited taint, can however (as stated) 

 spin normal coicicrons, but — and this is the important point — the moths 

 which such chrysalids yield invariably produce diseased eggs. This 

 explains the anomalies previousl^^ noticed. The silk- worms which die 

 without spinning are those in which the disease is hereditary, viz, those 

 born from a diseased mother. Worms from sound eggs which contract 

 the disease during their life-time always spin their silk, but they give 

 rise to a stricken moth, the worms from which do not reach maturity 

 and furnish no silk. 



