370 MISCELLANEOUS PRODUCTS USED IN AGRICULTURE. XVI. 



4. Keducing agents, e.g., sulphites, ferrous salts, &c., remove 



oxygen and so destroy aerobic organisms. 



5. Oxidising agents, e.g., chlorine, ozone, hydrogen peroxide, 



permanganates, &c., destroy by oxidation both the 

 bacteria and their food. These are the most perfect 

 disinfectants. 



6. Some easily reducible metallic salts are assimilated by the 



bacteria with the deposition of the elements within 

 their tissues. This deposition when it becomes 

 great enough kills the organisms. This happens with 

 salts of gold and silver. In some eases very minute 

 quantities of these poisons promote the growth of the 

 same organisms, which larger doses quickly destroy. 



7. Some substances which are germicides act in a manner 



which can only be described as physiological and not 

 chemical. To this class belong boric acid and the 

 borates and many of the aromatic compounds. 



The number of substances which have been used as dis- 

 infectants is very great and is constantly being increased. 



Among many others, the following may be mentioned : 



Chlorine, both as the free element, as hypochlorous acid, 

 and as hydrochloric acid. 



Bromine and iodine. The former has some advantages 

 because of its being liquid and is now sufficiently cheap to 

 permit of its use ; the latter is less convenient and too costly 

 for general purposes. They act, like chlorine, best in the 

 presence of moisture. Iodine trichloride, IC1 3 , has also been 

 highly recommended. 



Hydrofluoric acid, HF, and especially hydrofluosilicic acid, 

 H 2 SiF G , and the silicofluorides, are highly antiseptic. 



" Salufer" is a patented disinfectant, the basis of which is 

 silicofluoride of sodium, Na 2 SiF ti . The use of hydrofluosilicic 

 acid as a preventative of the decay of farm-yard manure has 

 already been alluded to. Fluorides are also employed to pre- 

 vent undesirable fermentations in breweries. 



Oxygen is the best natural disinfectant, and free admission of 

 oxygen destroys many micro-organisms. It is by absorption 

 of atmospheric oxygen that river water polluted by putrefying 

 ergamc matter purifies itself. Unfortunately the process is slow. 



