216 VETERINARY HYGIENE 



convenient than the crude sulphur, as combustion is more certain 

 and more complete. Sulphurous acid gas is also liquefied and 

 stored under pressure in cylinders. 



SO 2 is a heavy gas and does not diffuse well ; though it is the 

 usual custom to place the sulphur on the floor for combustion, 

 better results are possibly obtained when the holder containing the 

 sulphur is suspended near the ceiling or roof. 



Foodstuffs such as hay, oats, &c., very readily take on the dis- 

 agreeable odour of the gas. 



CHLORINE GAS. Chlorine gas is generated from bleaching 

 powder by causing a mineral acid, such as sulphuric acid, to slowly 

 drip on to it. The reaction is as follows : 



CaOCl 2 + H 2 SO 4 = CaSO 4 + C1 2 + H 2 O 



The chlorine gas unites with the hydrogen of water and liberates 

 nascent oxygen. 



As with sulphur dioxide, chlorine gas is practically useless in a 

 dry atmosphere. The same quantity of bleaching powder is required 

 as in the case of sulphur, namely, 3 pounds per 1000 cubic feet, but 

 from 1 to 2 per cent, of the gas in the air is considered sufficiently 

 powerful to destroy non-sporulating bacteria. The exposure should 

 not be less than three hours, and a longer time is desirable. 

 To acidify 2 Ibs. of bleaching powder approximately 1 Ib. 

 of acid is required. Chlorine is little used now for aerial 

 disinfection. 



FORMALDEHYDE GAS. The chief use of formaldehyde is in the 

 vapour form. When formalin is evaporated in the air, the gas 

 formic aldehyde is liberated but in a dry atmosphere polymerisation 

 to paraformaldehyde soon takes place as before stated. Therefore 

 for the gas to have any potency the atmosphere must be moist, and 

 the best results are obtained when the gas is generated along with 

 vaporisation of water from a special lamp. The gas formed com- 

 bines with the vapour so that disinfection by formaldehyde gas is 

 rather a vapour action than a gaseous one. The object is to pro- 

 duce a fine mist that will fall evenly throughout the room. It is 

 thus seen that with vapour disinfection the surface of things exposed 

 to the film are the only parts affected. It is an air disinfectant of 

 some value and may be used in the disinfection of the air in kennels 

 and catteries following an outbreak of distemper and influenza. 

 There are several forms of vaporisers on the market, directions for 

 use being given with each. 



THE POTASSIUM PERMANGANATE METHOD. With this method 

 the 40 per cent, commercial formaldehyde is poured on to crystals 

 of potassium permanganate; powdered permanganate is said to be 



