28 Manual of Veterinary Microbiology. 



adverse to their multiplication ; other experimenters 

 have arrived at directly opposite results. 



2. Physical influences. — a) Humidity. — Water is in- 

 dispensable to microbes; a medium containing less 

 than sixty per cent of this liquid arrests- their multi- 

 plication. Desiccation causes all active life to cease 

 (latent life) and may in this way finally lead to their 

 death. 



Similarly, it is to their deficiency in water content 

 that we must attribute the preservation of vegetable 

 juices by sugar, of meat pastry by fat, etc. 



b) Temjperature. — A temperature too high or too 

 low is detrimental to microbes. They are generally 

 more sensitive to an elevation of temperature than to 

 a depression. The vegetative forms are killed by two 

 hours' exposure, to a temperature of 48° to 60° C. The 

 spores, however, are much more resistant and require 

 for their destruction a temperature of 140° when they 

 are in a dry medium, 100° when they are moist. 



The temperature of predilection of microbes is from 

 20° to 39° ; above the latter temperature disturbances 

 of microbic activity, either temporary or permanent, 

 are liable to ensue (principle of attenuation by heat). 



Cold arrests the multiplication of microbes (latent 

 life), but kills them only with difficulty. Some have 

 been exposed to a temperature of — 105° without their 

 vitality being completely destroyed. 



c) Light. — Light is a puissant cause of destruction 

 to microbes ; it excites oxidation of their constituent 

 principles and especially of the hydro-carbonaceous 

 substances ; its action is quite rapid and continues even 

 after the germs are again removed from the light. 



d) Electricity. — The action of continuous and of 



