BACILLI IN CHRONIC INFECTIOUS DISEASES. 479 
Sawizky in 1891 made a series of experiments to determine 
the length of time during which dried tuberculous sputum retains 
its virulence. He arrived at the conclusion that virulence is not sud- 
denly but gradually lost, and that in an ordinary dwelling room 
dried sputum retains its specific infectious power for two and one- 
half months. 
Tizzoni and Cattani (1892) have presented some experimental evi- 
dence which indicates that injections of Koch’s tuberculin into 
guinea-pigs may produce in these animals a certain degree of im- 
munity against tuberculosis; and that this immunity depends upon 
the presence of an anti-tuberculin formed in the body of the partially 
immune animal. 
Numerous experiments made by veterinary surgeons upon tuber- 
culous cows show that the injection of Koch’s tuberculin in these 
animals, in doses of thirty to forty centigrammes, produces a rise of 
temperature of from 1° to 3° C. The febrile reaction usually occurs 
in from twelve to fifteen hours after the injection. Its duration and 
intensity do not depend upon the extent of the tuberculous lesions, 
but is even more marked when these are slight than in advanced 
cases. In non-tuberculous animals no reaction occurs, and the ex- 
periments made justify the suspicion that tuberculosis exists if an 
elevation in temperature of a degree or more occurs as a result of 
the subcutaneous injection of the dose mentioned. 
When the number of tubercle bacilli in sputum is comparatively 
small they may easily escape observation. Methods have therefore 
been suggested for finding them under these circumstances. Ribbert 
(1886) proposed the addition to the sputum of a two-per-cent solution 
of caustic potash, and boiling the mixture. The tenacious mucus is 
dissolved, and when the mixture is placed in a conical glass vessel 
the bacilli are deposited at the bottom and may easily be found in 
the sediment after removing the supernatant fluid. The same object 
is accomplished by Stroschein (1889) by the addition to sputum of 
three times its volume of a saturated solution of borax and boracic 
acid in water. 
A method of estimating the number of baciili in sputum has 
been proposed by Nuttall, which appears to give sufficiently ac- 
curate results and to be useful in judging of the progress of a 
case or of the results of treatment. For the details of this method 
we must refer to the author’s paper (Johns Hopkins Hospital Bulle- 
tin, vol. xi., No. 13, 1891). It consists essentially in first making 
the sputum fluid by the addition of a solution of caustic potash; in 
then shaking it thoroughly in a bottle containing sterilized gravel 
or pounded glass ; in carefully measuring the total quantity of fluid, 
and in dropping upon glass slides uniform drops by means of a grad- 
