Some Points in the Life-history of Bacteria. 127 



oxygen from the dissolved carbonic acid, little bubbles will form 

 upon it, and round these the bacteria will collect. Engelmann 

 has shown that the coloured bacteria are attracted by the red 

 rays of the spectrum. Janowski, on the other hand, found that 

 the typhoid bacillus was killed by a few hours' exposure to direct 

 sunlight, and that the violet or actinic portion had this destructive 

 power. Phosphorescence in waters has been found to be largely 

 due to several species of bacteria. 



Temperature Effects. — As might be expected, this has a 

 great effect upon the growth of bacteria. Different species have 

 a temperature at which their growth is most luxuriant, as well 

 as temperatures above and below this point at which vegetation 

 ceases. These several points are called respectively, optimum, 

 maximum, and minimum. The optimum is commonly from 80° 

 to 100° F. The maximum is often not much above the optimum. 

 There is a higher temperature still, which quickly kills the 

 bacteria. In liquids this is from 120° to 140° F., and destruction 

 is certainly effected in boiling water. Spores are far more 

 resistant to heat than the developed cells. In producing arti- 

 ficial immunity to infectious diseases it is well known that a 

 profound effect upon the powers of certain bacteria is exercised 

 by prolonged exposure to certain temperatures. 



Interaction between Bacteria and their Nutrient Media. — 

 This is a very important and extensive subject, and can only be 

 somewhat sketchily dealt with in this paper. These plants 

 having, with the exception of the coloured forms, nothing like 

 chlorophyll in their protoplasm, are, like the fungi, entirely de- 

 pendent upon organic compounds for their carbon and nitrogen. 

 Experiment has shown which combinations go to make the most 

 suitable media for individual species, wherein they may reach 

 their fullest development, but all, unless we except the case of 

 the water bacteria, must contain carbon and nitrogen in organic 

 combination, and mineral elements as well. Now the effect 

 upon the substratum is often very marked. Sacchoromyces cere- 

 visicE, the yeast plant, does not belong to this group, but its effects 

 are so well known that it may be taken as a type of other fer- 

 mentations. There must be present, besides sugar, nitrogenous 

 organic compounds and ash, and at least three-fourths of water. 

 The products are carbonic acid, succinic and lactic acids, glycerine, 

 and alcohol. Yet the yeast consumes for itself but 1-25 per cent, 

 of the sugar, 4-5 per cent, goes to make the acids and glycerine, 

 and 94-95 per cent, becomes broken up into alcohol and carbonic 

 acid. Chemically, the rearrangement of the sugar molecule 

 forms two molecules of alcohol, two of carbonic acid, and one of 

 water. When the alcohol amounts to one-fifth of the weight of 

 the liquid it arrests the fermentation. Acetous fermentation is 

 set up when an acid nutrient fluid containing a small percentage 



