4 THE FARM DOCTOR. 



water and air must be carefully seen to, since in close places 

 they are usually charged with invisible particles of organic 

 matter in a state of decay, the most suitable field for the growth 

 of contagious principles. These, too, tend to purify them- 

 selves in a free circulation of air, and ventilation may be largely 

 relied upon for this purpose, unless the deleterious supplies are 

 too abundant from some adjacent putrid accumulation, as dung- 

 heaps, cess-pools, leaky drains, or soil saturated with filth. 

 Purity of the surroundings kills many contagious elements on 

 the principles of starvation. 



Of agents reputed to be disinfectants, some act merely by 

 changing the physical condition of organic matter, without any 

 abstraction from, or addition to its constituents. Thus, heating 

 to the boiling point (212° F.), coagulates albuminous matters 

 and destroys any infectious properties generally. But it must 

 be prolonged for a variable time according to the size of the 

 object to allow of the heat penetrating to all parts alike. 

 Clothing may be heated in an oven to 300° F., or safer, 

 boiled, and even the prolonged application of hot transparent 

 steam directed from a hose, upon wood-work, etc., previously 

 well cleaned, is found very effectual. Some poisons, like 

 that of Texas-fever, are destroyed by freezing, while others 

 are unaffected. • 



Other disinfectants act by changing the chemical relations of 

 organic matter, and hence of contagious principles, by uniting 

 with them to form new compounds, by abstracting some ot their 

 constituent elements or by adding a new one. Thus the alo- 

 tropic state of oxygen called ozone, produced abundantly during 

 thunder-storms, is supposed to be one of nature's most potent 

 disinfectants, acting by hastening the oxidation of organic 

 matter. Yet, at times, its excess seems to be without effect, as 

 in the influenza of horses in 1877. Camphor and many of the 

 odorous essential oils are supposed to be of some slight use by 

 reason of their developing ozone. 



