DISEASES OF CATTLE 37 



generally known under the name of formalin. The solution is sup- 

 posed to contain 40 per cent of formaldehyde, though in reality the 

 amount of formaldehyde present rarely exceeds 37 to 38 per cent. 

 Formaldehyde may be used for disinfection in either a liquid or 

 gaseous form. 



Liquid Formaldehyde. Solutions of formaldehyde are best 

 prepared by making a 5 per cent solution of formalin in water. 

 This is applied directly to substances that require disinfection, and 

 in the case of refuse, excreta, and similar substances should be thor- 

 oughly mixed with them. A 5 per cent solution of formalin is gen- 

 erally regarded as superior to carbolic acid of the same strength 

 as a general disinfectant. 



Gaseous Formaldehyde. In disinfecting with formaldehyde 

 gas it is essential that the compartments to be disinfected be tightly 

 closed so that a sufficient concentration of the gas may be held in 

 contact with the infected substances a sufficient length of time. The 

 temperature of the air is an important factor in securing efficient 

 action, the formaldehyde being much more energetic in a warm 

 atmosphere than in a cold. The best authorities state that gaseous 

 formaldehyde disinfection should not be attempted if the temper- 

 ature of the air is below 50 F. The gas is most conveniently se- 

 cured by liberating it from the concentrated aqueous 40 per cent 

 solution or from the solid paraform. A number of methods for ac- 

 complishing this in practice have been devised, the more important 

 of these 'being as follows : 



GAS PROM FORMALIN. 



Heating Under Pressure. Portable autoclaves specially designed 

 for the purpose are charged with a sufficient amount of for- 

 malin, the quantity depending upon the cubic air space to be disin- 

 fected. The autoclave is closed and heat is applied until the re- 

 quired pressure within the autoclave is attained. The gas which 

 has been liberated from the solution by the heat is allowed to flow 

 through an outlet tube into the closed chamber which is to be dis- 

 infected. The room is then kept closed for from two to twelve 

 hours ; the shorter time if only smooth surfaces are to be acted upon ; 

 the longer if penetration into fabrics is desired. Ten ounces of 

 formalin should be used for each 1,000 cubic feet of air space. 



Heating Without Pressure. This method is similar to that 

 just described. Formalin is placed in a specially designated retort 

 and heated with a lamp. The gas is conducted into the compart- 

 ment to be disinfected by means of a small tube which passes 

 through the keyhole, or other small aperture. Ten ounces of for- 

 malin is required for each 1,000 cubic feet of space. The evolution 

 of gas by this method is less rapid than when generated under 

 pressure and a longer time is required for disinfection. The com- 

 partment should remain closed for at least six hours, and for twelve 

 hours if penetration into the interior of fabrics, hay, etc., is re- 

 quired. 



Spraying. In this method the formalin is sprayed upon the 

 surface of objects which require disinfection or upon sheets which 



