CONDITIONS OF GROWTH. 119 



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 j.i, 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, ...... 100 



Cane sugar, ...... 10 



Tartrate of ammonia. . . ... . 1 



Ashes of one gramme of yeast, .... 0.075 



CORN'S SOLUTION. 



Distilled water, . . . . . . 100 



Tartrate of ammonia, ..... 1 



Ashes of yeast, ....... 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 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 



