52 BACTERIOLOGY. 
yield of residue from five different varieties of cholera 
spirilla: Albumin 65 per cent. and ash 31 per cent. 
when grown in soda bouillon, while in Uschinsky’s 
solution there was only 45 per cent. albumin and 11 
per cent. ash. The five varieties of spirilla which in 
soda bouillon yielded almost exactly the same quanti- 
ties of albumin and ash, in the other medium free from 
albumin exhibited a very variable composition. This 
shows how little dependence can be placed upon any 
single chemical or cultural reaction for the differentia- 
tion of two species of bacteria. Judging from the per- 
centage composition of the cholera spirilla when grown 
in the Uschinsky medium, these five varieties, taken 
from Paris, Hamburg, Shanghai, etc., might be con- 
sidered to be different species, whereas they were prob- 
ably merely varieties of the same species of bacteria. 
CONDITIONS OF GROWTH. 
Culture Media. Although there are among the bac- 
teria related to disease a number which are met with 
only in the bodies of living animals or plants, and, 
therefore, so far as we know, strictly parasites, yet 
most pathogenic bacteria can be cultivated more or less 
readily in artificial culture media under suitable con- 
ditions, as, for example, the tubercle bacillus and the 
gonococcus. The majority of bacteria which occur 
usually as saprophytes are easily cultivated artificially ; 
but there are some, such as various micro-organisms 
found in the saliva and in water, which, with our 
present knowledge, are either difficult or impossible to 
cultivate. 
All bacterial culture media must contain an abun- 
