10 MICRO-ORGANISMS A^D DISEASE. [CHAP. 



near the lower 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 indicates 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 suffi- 

 cient 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 experiences in the working of these and other similar regulators 

 arises from the inconstancy of the main gas supply, this, as is well known, 

 varying within wide limits. The stopcock, E, 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 be- 

 tween 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. 



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 

 back of the chamber are closed by a movable glass plate. 



An excellent incubator for constant temperature is mt.de by the Cambridge 

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

 permanent flame, and a second one subject to the regulator. 



varying between 30 and 40 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 hypo- 

 thetical substance supposed to be the materies morli, and in- 

 troduced at first from the blood or tissues, being in a very 

 diluted condition in the first cultivation, would after several 

 cultivations be practically lost. But if this last cultivation 

 should be found to act in the same manner pathogenically, i.e. 

 if every droplet of it, charged with the new brood of the 



