PREVENTIVE MEDICINE 203 



with hot dry air whose penetrating power is limited by the conduct- 

 ivity of the article. 



Superheated steam is steam that has a temperature higher than 

 that of ordinary or saturated steam under equal atmospheric 

 pressure. It may be made by increasing the temperature of the 

 steam, without altering its pressure, by surrounding the chamber in 

 which it is contained with a second chamber or jacket into which is 

 passed steam under an increased atmospheric pressure, and thus 

 having a temperature higher than the steam in the inside chamber. 

 The inner steam is therefore heated by borrowed heat from the 

 outer steam ; this increased heating also drys it. 



The temperature of steam can also be raised by generating it 

 from a saline water the boiling point of which is 225 F., as com- 

 pared with 212 F. of ordinary water. This fact is put to good 

 account in practical disinfection as in Thresh's disinfector. 



Saturated steam condenses at the slightest reduction of heat. 

 Superheated steam, on the contrary, cannot condense until it has 

 lost by conduction its " superheat." 



Steam that has been superheated by passing it over hot pipes or 

 surrounding it with a jacket containing steam under increased 

 atmospheric pressure, is, as has been said above, much drier than 

 ordinary or saturated steam, and is rather a hot gas than steam. 

 Owing to its dryness its germicidal power is much less than 

 saturated steam. 



Steam under Pressure. Variation in the pressure under which 

 steam is generated can only be effected in an air-tight container or 

 boiler, since the altered pressure would soon revert to the normal, if 

 not in fact impossible to obtain, if there were any continuity with 

 the air at general atmospheric pressure. Alteration in pressure 

 may be either an increase or a decrease. When water is boiled in a 

 sealed vessel, which is necessarily fitted with a safety valve and a 

 pressure gauge, the steam accumulates and, as more steam is 

 formed, undergoes compression by the ever-increasing volume. 

 The pressure within the boiler is therefore greater than that of out- 

 side air and, as has been said, water boils at a higher temperature 

 as the atmospheric pressure increases. The temperature of the 

 steam produced under pressure is therefore greater than that pro- 

 duced at ordinary temperature, as the temperature at which the 

 water boils is greater. 



Unlike superheated steam, " pressure steam " is still saturated 

 with moisture, it is not dried, and possesses therefore the great 

 advantage of increased heat over ordinary steam, and is free from 

 the disadvantage of dryness inseparable from superheated steam. 



