96 THE DISEASES OF DOMESTIC ANIMALS. 



The excretions of purely contagious diseases conform more to 

 this condition than external media ; but when these are exposed to 

 the action of an excess of moisture, decomposition of the media and 

 germinal changes soon follow, which are opposed to further con- 

 tagion. 



We are justified in asserting that contagions which are sup- 

 ported in moisture not their natural media soon lose their activity. 

 Such disturbance takes place more rapidly in a warm than in a 

 cool temperature. 



The more water a given medium contains, the poorer its nutri- 

 tious qualities, and the quicker destruction or change takes place 

 upon germs suspended in it. 



In a frozen condition media as well as germs suffer but little 

 change. Distribution of infectious elements in a desiccated or dried 

 condition takes place either by means of the atmosphere, or upon 

 the surface of or within desiccated vehicles. 



The elements of infection retain their vitality longer in a desic- 

 cated condition than when in or upon media of a moist nature, or 

 when the desiccation takes place with sufficient rapidity to prevent 

 decomposition or changes in the media, or the germs in or upon it. 



If the processes of desiccation take place with such a rapidity 

 and to such a degree as to remove all moisture, the germs perish. 

 Some degree of moisture is necessary to their vitality. Germs re- 

 tain their activity longer in a cool, moist atmosphere than in a 

 warm and dry, and less long when suspended in a dry atmosphere 

 than when contained in a dry substance, where they are protected 

 in some measure from further desiccation. We know that the 

 transportation of infectious elements to any great distance by means 

 of the atmosphere does not take place, but that they may be trans- 

 ported a long distance by means of diseased individuals or infected 

 vehicles. From what has been said you must infer that infectious 

 elements are largely transported by means of the air or desiccated 

 objects, and that these are the chief sources whence to gain entrance 

 to the animal organism. 



Infectious elements must be generated in media containing 

 moisture. They are not volatile — gaseous — and must first become 

 desiccated and reduced to powder before they can be taken up and 

 transported by the atmosphere. 



Some authorities have mistakenly asserted that the germs are, 

 as it were, torn away from their moist media with the molecules of 

 water which pass off with evaporation. That this is false is proved 

 by the evaporation of salt water. The salts remain ; the volume is 



