TEXT-BOOK OF BACTERIOLOGY. 69 



danger the success of the whole operation. Thus the steam may 

 show a temperature of 130° C. while the fluid inclosed in it is, in 

 some portions, not more than 70° or 80° C. It is true that such 

 differences will not occur as a rule. In dealing with small articles — 

 for example, with thin silk threads — on which are dried spores and 

 which are usually exposed to the action of this apparatus, such 

 variations can scarcely occur. In such cases the operation of sat- 

 urated vapors under pressure are unexceptionable, and there is no 

 doubt that with increased pressure — that is, with increased temper- 

 ature — the results will be better. Thus Globig found that the spores 

 of a particular species of bacteria belonging to the class of potato 

 bacilli, and which are of specially tenacious vitality, were destroyed 

 by steam at 100° C. in 'five and a half to six hours; by steam at 

 about 110° C. in three-quarters of an hour; at 113° C. in twentj'-flve 

 minutes; at 120° C. in ten minutes; at 126° C. in three minutes; at 

 127° C. in two minutes; and at 130" C. in a moment. 



Nevertheless, if compressed steam be but seldom employed in 

 our laboratorj'^ and onlj' in exceptional cases, the reason is to be 

 sought partly in the disadvantages already mentioned, but chiefly 

 in the fact that we possess another, more convenient, less expensive 

 means, which is easier to manage, fulfils all requirements in the 

 great majority of cases, has been in use for years, has stood the 

 test of many thousand experiments, and rarely proves inefflcient. 



This process is sterilization by the freelj''-escaping steam of boil- 

 ing water. Koch and his collaborators, Gaffky and Loffler, found 

 that such steam, when properly confined and protected from the 

 cold outside air, keeps the boiling temperature permanently, which 

 it also speedily and perfectly communicates to liquids exposed to 

 its action, so that an exposure of from half an hour to an hour suf- 

 fices, as a rule, to render them germ-free. 



On this extremely important fact depends the construction of 

 the " Koch steam generator," which is almost exclusively used in 

 sterilizing our food solutions. 



A cylinder about 3 to 4 m. high and 30 cm. in diameter, of plain 

 tinned iron, or still better of sheet copper, is enveloped in a close 

 laj'er of felt, to protect it from loss of heat. At the top it has also 

 a felt-covered lid, called the " helmet," which fits loosely and must 

 not be air-tight. In this helmet a thermometer is usually fixed. 

 The cylinder itself has a grating in its interior, at the boundary of 

 the lower third, above the water which is made to boil in it. The 

 height of the water can always be seen by a gauge-pipe at the side. 



The grating, therefore, divides the cylinder into a lower water 

 space and an upper steam space. 



