
310 THE EVOLUTION OF LIFE, 
ment that the expectorated matter dried slowly, and 
that its conversion into dust was neither simple nor 
easy. Spread thickly upona glass plate and exposed 
to ordinary daylight in the laboratory, the sputum 
adhered in drying as a bright varnish, and was not 
easily powdered into dust till the tenth or twelfth 
day. While as regards (2) the activity of this dried 
sputum, he found that even on the sixth day a con- 
siderable quantity of the powder was required to 
cause a discrete peritoneal tuberculosis, as a result 
of inoculation. Again, the sputum spread upon a 
marble plate, and kept above a stove for fourteen 
days, was found to have lost its virulence for the 
guinea-pig. Spread upon a porous plate and ex- 
posed to sunlight it was found to be innocuous for 
inoculation after forty-eight hours. Cadéac gives 
other evidence of like kind, and comes to the 
conclusion that dust which will cause tuberculosis 
with so much difficulty by direct inoculation into the 
peritoneal cavity of a guinea-pig will have much more 
difficulty in infecting man’s respiratory passages. 
Behring, however, advocates the doctrine of 
contagion in regard to tuberculosis in a different 
manner. He believes that infection takes place in 
the main in infancy through the intestinal canal (the 
contagion being derived from milk), and that there- 
after the infecting Bacilli, lodging in different parts 
of the body, commonly remain latent for years, per- 
haps for a long series of them. All this, however, 
is pure hypothesis. As I have said elsewhere:! 
“He postulates long periods of latency after 
1 “The Nature and Origin of Living Matter,” 1905, p. 327. 
