126 CONDITIONS OF GROWTH, 
tilled water was pure, it was distilled a second time in a clean glass 
retort, but the result was the same. Bolton remarks, with reference 
to these results: ‘‘If we seek to explain this remarkable fact we 
must remember, in the first place, what an extremely small abso- 
lute mass is represented by an enormous number of bacteria, and 
what a minute amount of material is required for the formation of 
this mass. In ten cubic centimetres of distilled water, in the experi- 
ment last referred to, there were about twenty million bacteria (two 
million per cubic centimetre). If we estimate the diameter of each 
at one ym, with a specific weight of 1, the absolute weight would 
be for the entire number one-one-hundredth of a milligramme— 
that is to say, a quantity which cannot be determined by any of our 
methods of weighing.” 
Bolton supposes that the small amount of organic pabulum re- 
quired fell into the water in the shape of dust, or was attached to the 
walls of the test tube in spite of all the precautions taken. 
Nitrogen is chiefly obtained from albuminoid substances, but 
Pasteur has shown that it may also be obtained from ammonia. 
This is shown by cultivating bacteria in a medium containing an 
ammonia salt, as in the following : 
PASTEUR’S SOLUTION. 
Distilled water, : ‘ ‘ A : 100 
Cane sugar, ‘ ‘ 5 ; 10 
Tartrate of ammonia, ; i a Z 
Ashes of one gramme of yeast, i ‘ F : 0.075 
COHN’S SOLUTION. 
Distilled water, 3 : . . : 100 
Tartrate of ammonia, : : ‘ : ? 1 
Ashes of yeast, . ‘ ‘ ‘ 5 ¥ é 1 
Many bacteria multiply abundantly in these solutions. 
Carbon is obtained from the various organic substances contain- 
ing it; among others, from starch, sugars, glycerin, organic acids 
and their salts, etc. 
Temperature.—There are certain limits of temperature within 
which development may take place, but these differ greatly with 
different species. As a rule, growth is arrested when the tempera- 
ture falls below 10° C. (50° F.), but some species multiply at a still 
lower temperature. Thus Bolton observed a very decided increase 
in certain water bacteria kept in an ice chest at 6° C., and other ob- 
servers have witnessed development at the freezing temperature. 
Most saprophytic bacteria grow within rather wide temperature 
limits, but the rapidity of development is greatest at a certain favor- 
able temperature, which is usually between 25° and 30° C. The 
