398 APPENDIX. 



thin "preserve" or "type-writing" paper; other objects, such as watch 

 glasses, slides, cover glasses, cultivation plates, &c., may be enclosed in 

 small iron or copper boxes, or they may be carefully wrapped in cotton 

 wadding, or in a couple of thicknesses of the above paper, before they are put 

 into the oven to be sterilized. One must always be careful that the_ paper 

 or wadding does not come into actual contact with the bottom or sides of 

 the oven, the hot metal of which may cause singeing. Even hot air at I50°C. 

 will " brown" cotton wadding or paper slightly, so that such atemperature 

 in the chamber may be at once recognized, even when no thermometer can be 

 obtained, by the appearance of a slight brown coloration of these materials. 



Steam Sterilizer. 



For sterilizing by means of steam, an ordinary fish kettle or potato 

 steamer may be used ; either of these placed on a fire will, for private work, 

 usually serve the purposes of the more complicated Koch steam sterilizer. 

 In this steaming apparatus the heat penetrates rapidly and does its work 

 thoroughly, moist heat being much more effective as a sterilizing agent than 

 dry heat. If it is thought necessary to obtain a piece of special apparatus 

 for the purpose, the best is a tin or zinc cylinder about eighteen inches high 

 and six inches in diameter, to which a small water gauge is added. The 

 bottom of this should be made of block tin ; the top is closed by a conical 

 lid in which is a hole for a thermometer. The lid and sides are usually 

 covered with felt, though this is not essential, unless economy of heat is 

 advisable. The felt should not in any case extend quite to the bottom, or 

 it is readily singed by the heating flame. Inside the cylinder are a couple 

 of shelves, one about one inch, and the other about ten inches, above the 

 level of the water ; on each rests a perforated tin plate on which tin vessels 

 about four or five inches in diameter and with perforated bottoms are sup- 

 ported. The objects to be sterilized are placed in these vessels, the cylinder 

 is filled to a depth of three or four inches with water, the pails are put into 

 their places, and the lid placed in position. After the water has com- 

 menced to boil briskly, the steaming is continued for about twenty minutes, 

 at the end of which time most of the pieces of apparatus are thoroughly 

 sterilized. This sterilizer may be lengthened by the addition of a tin 

 cylinder with a ring or collar near the base, which fits into the top of the 

 sterilizing cylinder, the lid being placed at the top of the additional cylin- 

 der. This lengthening portion is especially useful for sterilizing Hesse's air 

 analysis tubes. 



Incubating Apparatus. 



Most of the ordinary micro-organisms may be cultivated at the temperature 

 of the room, but many of the pathogenic organisms, such as the tubercle bacil- 

 lus, will only grow at a temperature approximating that of the human body, 

 and for the cultivation of these some sort of incubating apparatus is necessary. 

 The simplest apparatus, however, if supplied with a good body of water 

 or a wrapping of felt and cotton wadding, will in most cases serve our pur- 

 poses. Any one with a little ingenuity will be able to devise an incubator 

 for his own use, if he has a greenhouse or any warm room at his disposal, 

 an ordinary oil lamp being sufficient, if properly trimmed and regulated, to 

 keep a double-walled chamber covered with felt at a fairly constant 

 temperature, varying only three or four degrees for months together. Where 

 systematic investigation is to be carried on, however, one of the ordinary 



