BACILLUS OF TUBERCULOSIS. 267 
The tubercle bacillus when exposed to direct sunlight 
is killed in from a few minutes to several hours, accord- 
ing to the thickness of the layer and the season of the 
year; it is also usually destroyed by diffuse daylight in 
from five to seven days when placed near a window. 
This fact is worthy of note, as it has an important 
hygienic bearing. Thus, tuberculous sputum expector- 
ated upon sidewalks, etc., being exposed to the action — 
of direct sunlight, will in many cases, especially in 
summer, be disinfected by the time it is in a condi- 
tion to be carried into the air as dust. For the same 
reason, consumptive patients should occupy light, sunny 
rooms and live as much as possible in the open air and 
exposed to the action of direct sunlight. 
The tubercle bacillus is a strict parasite—that is to 
say, its biological characters are such that it could 
searcely find natural conditions outside of the bodies 
of living animals favorable for its multiplication. But 
it has been noted that when it is cultivated for a time 
in artificial media containing glycerin it may grow on 
the surface of plain veal or chicken bouillon, in which 
media it fails to develop when introduced directly from 
a culture originating from the body of an infected ani- 
mal. This would indicate the possibility of its acquir- 
ing the ability to grow as a saprophyte. The experi- 
ments of Nutall also show that the bacillus may multi- 
ply, under favorable conditions, in tuberculous sputum | 
outside of the body. Notwithstanding these facts, 
it is probable that the growth of tubercle bacillus out- 
side of the living bodies of man and animals is so 
slight as to have no practical importance in causing 
infection. 
On account of their slow growth and the special con- 
