340 PROFESSOR JOSEPH LISTER. 
at length succeeded, both in the mouse and in the rabbit, in 
developing true Bacillus anthracis out of the progeny of the 
hay-bacillus. When large quantities were introduced, the 
animals died rapidly from the merely chemical toxic effects 
of the injected liquid; but, in some instances in which a 
smaller amount was injected, after the period for these pri- 
mary effects had passed, a fatal disease supervened—attended, 
as in anthrax, with great swelling of the spleen, the blood 
of which was found peopled, as in that affection, with newly 
furmed bacilli; and the spleens affected in this way were 
found to communicate anthrax to healthy animals, just 
like those of animals which have died of ordinary splenic 
fever.) 
Supposing these results to be trustworthy (and the record 
of them bears the stamp of authenticity) I need scarcely 
point out tou a meeting like the present their transcendant 
importance as bearing upon the origin of infective diseases, 
and their modifications as exhibited in epidemics. 
I trust that these examples may suffice to convey some 
idea of the work now going on with reference to the rela- 
tions of micro-organisms to disease. 
Since the address was delivered M. Pasteur has published 
the method by which he produces the “ attenuation ” of the 
virus, or in other words, the enfeeblement of the organism of 
fowl-cholera which fits it for securing immunity from the 
fatal form of the disease. This method consists in cultivat- 
ing the organism, pure and nnmixed,in chicken broth, to 
which access of air is permitted while dust is excluded, and 
simply allowing some months to elapse before it is employed. 
If the period does not amount to more than about two 
months, the organism retains its virulence little abated, but 
if the period is extended to three or four months it is found 
that animals inoculated with the organism take the disease, 
but have it in a milder form, and a considerable proportion 
recover ; and if the time is made still greater, as, for ex- 
emple, eight months, the organism has so far lost its potency 
that though chickens inoculated with it still go through an 
attack of the disease all recover. If the period is sufficiently 
prolonged, there comes a time when the organism is found 
to have lost its vitality altogether, so that it will no longer 
give rise to new development when introduced into fresh 
cultivating liquid. 
In considering by what agency this enfeeblement of the 
organism and ultimate extinction of its vitality was brought 
1 See ‘ Ueber die experimentelle Erzeugung des Milzbrandcontagiums 
aus den Heupilzen,’ von Hans Buchner, Miiuchen, 1880. 
