LABORATORY STERILIZATION 249 



typhosus is the organism selected for the purpose, and the strength 

 of solution of both the unknown and known solutions are carefully 

 measured. The time and temperature of exposure of the organism to 

 the disinfectant solutions and the nature of the medium in which the 

 exposure is made are carefully controlled. Even with the most rigor- 

 ous attention to details, the carbolic coefficient of the same disinfectant 

 determined by this method varies nearly 50 per cent, in the hands of 

 different observers; 1 for the present, the standards of the Public 

 Health and Marine Hospital Service 2 are regarded as official for the 

 United States. 



Gaseous Disinfectants. Pathogenic bacteria which are known or 

 suspected to be present upon fabrics or furnishings injured by chemical 

 disinfectant solutions, as well as bacteria promiscuously distributed 

 in rooms through droplet infection and by dust may be killed by 

 gaseous disinfectants, of which several are available. 



1 . Formaldehyde. Formaldehyde is the most efficient of the gaseous 

 disinfectants for superficial disinfection, but its limited power of 

 penetration must be borne in mind. Formaldehyde is dispensed com- 

 mercially under the name "formalin," which signifies a 40 per cent, 

 volume solution of the gas (formaldehyde) in water. Commercial 

 formalin rarely contains more than 36 per cent, of formaldehyde by 

 volume, however, and in practice it is well to estimate 35 per cent, as 

 a working basis. Commercial solutions, it must be remembered, are 

 always acid, and the gas itself in small amounts is irritating to mucous 

 membranes. Prolonged exposure to concentrations of the gas suffi- 

 cient to kill bacteria may be fatal to animals. The gas has practically 

 no insecticidal value. In sufficient concentration the gas is inflam- 

 mable and may be ignitecrby any free flame. 



In the past formaldehyde was liberated from its aqueous solution 

 in the gaseous state in complicated retorts, autoclaves or lamps of 

 special design. Much simpler methods have been evolved, which are 

 now used almost exclusively in practical gaseous disinfection. Of 

 these the permanganate method and the " sheet- volatilization method" 

 are the most widely used; the former possesses the dual advantage 

 of a quick liberation of the entire available amount of disinfectant, 

 and very simple apparatus ; the latter is advantageous when a gradual 

 evolution of gas and a prolonged exposure to its action are desired. 



The Permanganate Method. When formalin is poured upon crystals' 

 of potassium permanganate, an energetic reaction with the evolution 



1 Hamilton and Ohno, Jour. Pub. Health, 1913; ibid., 1914, iv, 163. 



2 Bull. Hyg. Lab., Washington, D. C., April, 1912, No. 82. 



