202 VETERINARY HYGIENE 



higher temperature than pure water, 225 F. as against 212 F. 

 Steel instruments do not rust if boiled in a solution of sodium 

 carbonate. 



Steam. Steam is used as ordinary steam, superheated steam, 

 under pressure greater than atmospheric pressure and under 

 reduced pressure. 



The temperature of steam at ordinary atmospheric pressure, i.e., 

 15 Ibs. per square inch, is 212 F. (100 C). Most spores, if kept in 

 direct contact with steam at this temperature for ten minutes, are 

 destroyed. 



Hot dry air has very little penetrating power, and so it is 

 that organisms contained within folds of rugs and other materials 

 that are bad conductors of heat are not affected, unless the tem- 

 perature is such as will damage the more superficial layers. Hot 

 dry air on coming into contact with cooler substances has its tem- 

 perature reduced, as a certain amount of the heat is used up in 

 the evaporation of the moisture that all " air-dried " substances 

 contain. 



With steam there is no reduction of heat, as evaporation of the 

 moisture obviously cannot take place. Instead of there being a 

 reduction of heat there is, on the contrary, an increase owing to the 

 amount of latent heat contained in steam. Steam on coming in 

 contact with a cooler substance undergoes condensation, and the 

 latent heat set free as condensation takes place is available for 

 heating the cooler body. 



Thus, in fact, steam on condensation has more available heat 

 than it had in its original form. The latent heat of steam is the 

 heat or energy expended in converting water at the boiling point 

 into steam. At ordinary atmospheric pressure both the steam and 

 the boiling water have the same temperature, 212 F., and yet the 

 steam contains, in a latent form, extra heat, which it parts with on 

 condensation. 



The penetrating power of steam is very great, and it is brought 

 about in this manner. Ordinary or " saturated " steam condenses 

 into a comparatively small volume with a very slight degree of cool- 

 ing. On coming in contact with, say, a bundle of rugs, it warms and 

 penetrates the outer layer, undergoes condensation and reduction in 

 volume with the formation of a partial vacuum as a natural 

 consequence, more steam rushes in to fill the vacuum, which, in turn, 

 passes on into the interior to fill the vacuum caused by the steam 

 that has gone on before. 



Steam heats irrespective of whether the material to be dis- 

 infected is a good or bad conductor of heat, which is not the case 



