CH. II] PREPARATION OF CULTURE MATERIAL 25 



end a fine hole. When the temperature of the water rises, the mercurial column 

 rises, and at a certain temperature rises above the lower open end of the small inner 

 glass tube just mentioned. If this point is reached, then the burner at C receives 

 only the amount of gas that passes through the fine lateral hole of that inner glass 

 tube. If the temperature of the water falls, the mercury falls, and the lower end of 

 the inner glass tube becomes again free, and now the burner at C receives a much 

 greater supply of gas. If so, the temperature of the water again rises, the mercury 

 rises, obstructs the lower end of the inner glass tube, the supply of gas is reduced to 

 what can pass through the fine lateral hole, and consequently the temperature again 

 falls, and so on. To adjust the regulator it is necessary when the thermometer in- 

 dicates the required degree of temperature to push the outer large glass tube, and 

 with it the inner tube, of the regulator so far down that the top of the mercurial 

 column just obstructs the free end of the inner glass. The temperature then regulates 

 itself for the reasons stated previously. These regulators are sufficient for all 

 practical purposes when it is not a question of small differences in temperature, since 

 they are tolerably constant within one or two centigrades. The trouble one ex- 

 periences in the working of these and other similar regulators arises from the incon- 

 stancy of the main gas supply, this, as is well known, varying within wide limits. 

 The stopcock, £^ obviates this to a limited extent ; when this is put at an angle of 

 45° only a limited amount of gas passes from the main supply tube to the regulator, 

 and therefore the variations in pressure of the gas are not felt to their full extent. 

 A Sugg's regulator interposed between E and the main supply tap is very useful. 



B. Thermometer to indicate the temperature in the chamber. 



C. Gas burner. 



D. Chamber of incubator. The front and back of the incubator is either a movable 

 tin plate or glass covered with black paper. 



E. Stopcock to regulate, when required, the supply of gas. 



F. Main supply. — The upper, lower, right and left walls of the incubator are 

 made of a double layer of tin ; between the two is water. The front and hack of 

 the chamber are closed by a movable plate. 



An excellent incubator for constant temperature is made by the Cambridge 

 Scientific Instrument Company. It has a double gas supply : one small permanent 

 flame, and a second one subject to the regulator. 



peratures varying between 20° and 38'^ C, are necessary in 

 order to study more accurately the life-history of the septic 

 as well as the pathogenic organisms. Moreover, large 

 numbers of them become available in a short time, and 

 their relation to disease can be tested more conveniently. 

 For if it should be found that, having carried on outside the 

 animal body successive cultivations of a particular organism, 

 the re-introduction of this cultivated organism into the 

 animal body is again productive of the same disorder as 

 before, then the conclusion becomes inevitable that this 

 organism is intimately related to the causation of the 

 disease. It must be conceded that after several successive 

 cultivations in fluids any hypothetical substance supposed 

 to be the materies morbi^ and introduced at first from the 

 blood or tissues, being in a very diluted condition in the 



