DIVISION II. 



PHYSICAL INFLUENCES. 



CHAPTER I. 

 MOISTURE. 



Moisture may be called the most important factor of life. Not only 

 bacteria, but every microscopic and macroscopic being requires a con- 

 siderable amount of moisture. Living organisms contain on the average 

 between 70 per cent and 90 per cent of water, and only 10 per cent to 30 

 per cent of solid matter. Microorganisms which live entirely submerged 

 in liquids need water not only within but without the cells. Bacteria, 

 yeasts, molds, and some protozoa obtain their food by diffusion through 

 the cell-membrane; their food-substances must be soluble and dissolved. 

 No other liquid can take the place of water. 



The amount of water required by microorganisms cannot be stated 

 briefly. Several factors have to be taken into consideration, as the 

 osmotic pressure, the insoluble and the colloidal substances, the species 

 of organisms, temperature, and perhaps others. 



OSMOTIC PRESSURE. In the organic world we find very commonly 

 membranes which will allow water to pass through but retain some 

 compounds dissolved in the water. Such so-called semi-permeable 

 membranes are found surrounding the protoplasm of cells. They are 

 not the cell wall, but separate the protoplasm from the cell wall. Similar 

 properties are found in parchment paper, pig's bladder, and other organic 

 membranes. 



If a salt solution is poured in water, the two liquids will mix in a short 

 time 'and soon every smallest portion of the mixture will have the same 

 concentration. If a salt solution and water are separated by a membrane 

 which does not allow the salt to pass, the water will go through the 

 membrane toward the salt with a certain amount of pressure. This 

 pressure depends upon the nature of the dissolved substance as well as 

 upon its concentration. 



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