366 APPLIED BACTERIOLOGY 



practice thick blankets and similar objects oppose resistance 

 to the penetration of steam, and this resistance is liable to 

 vary with circumstances. It would be desirable in any 

 case to have a margin of temperature ; and the least, 

 where it is possible to procure it, is an exposure for fifteen 

 minutes to saturated steam at 115° C. Disinfectors giving 

 a less temperature may, if properly designed to give 

 saturated and not superheated steam, be useful in com- 

 munities where the cost of a disinfector under the pressure 

 which is desirable is utterly beyond the funds available. 

 But it can give no certainty except for the organisms of 

 diseases such as those named above, which have been 

 shown to perish in all circumstances at temperatures well 

 below 100° C. Disinfectors without pressure should, for 

 the reasons named above, not be used at all. 



The difference in efficiency between various steam-disin- 

 fectors depends not only on the general method of their 

 construction, but also on the proportions and details of 

 their design. 



The Constraction of Steam Disinfectors. — The chief principles 

 involved have been stated above. To these must be added 

 one of great importance — that the steam must be free from 

 air. Koch in 1880 omitted this precaution, and found in 

 consequence that steam at 212° acted more powerfully than 

 at a higher temperature in a digester. Heydenreich and 

 others showed what has been confirmed by all subsequent 

 experiments, that this result was due solely to the presence 

 of air in the vessel, and that when the air was allowed 

 to escape, the steam acted more rapidly and effectively 

 when under pressure than when used as a current without 

 pressure. 



The disinfectors principally known in this country are 

 described below. 



Lyon's Disinfector. — This consists of a horizontal chamber, 



