116 TEXT-BOOK OF BACTERIOLOWY. 



There are different kinds of incubators in use, and we will en- 

 deavor to explain their construction and arrangement by referring 

 to the two most usually employed. 



One is a large rectangular (" four-cornered ") sheet-iron case 

 covered outside with felt; it has water between its double walls, 

 and the depth of the water may at all times be seen by the gauge- 

 glass attached at the side. The water is brought up to a fixed 

 temperature, which it communicates to the space within. As this 

 warming proceeds equally and simultaneously from all sides, there 

 will seldom be an irregularity in the distribution of the heat, a 

 point which is of the first importance. For when the temperature 

 varies the cultures can only retain their vitality a short time : evap- 

 oration takes place here and consolidation there, the food media are 

 robbed of their moisture, they dry up and become useless. In order 

 to absolutelj' obviate these evils, it is desirable always to provide 

 the test-tubes in the incubator with small India-rubber caps. 



It is by no means easy to keep the temperature of the surround- 

 ing water constantly at the same point. This is managed by the 

 continual self-acting control of the amount of gas supplied to the 

 flame which heats the apparatus from below: if the water becomes 

 too warm, a smaller quantity of gas is admitted to the burner, and 

 vice versa. 



The flame is controlled by a thermo-regulator, of which there 

 are many different kinds in use. One of the commonest is the 

 quicksilver regulator invented by Bunsen and improved by V. 

 Meyer. A glass tube about 40 cm. long and closed at the bottom, 

 shaped like a large test-tube, is divided in the middle by a dia- 

 phragm of glass into an upper and a lower half. Yet a connec- 

 tion exists between them, inasmuch as the diaphragm sinks into a 

 funnel which narrows into an almost capillary tube and ends just 

 above the bottom of the vessel. The lower part is almost filled 

 with quicksilver; above this, however, and almost reaching the 

 diaphragm, is a mixture of alcohol and ether, which, as we know, 

 volatilizes when slightly heated. Indeed, it is only necessary to 

 hold in the hand that part of the glass which surrounds the liquid 

 and warm it a little in order to produce gas and drive away the 

 quicksilver. The latter can, however, only escape through the 

 capillary funnel tube, and the more it is warmed the more quick- 

 silver will gradually rise above the diaphragm. 



Into this latter space — through the India-rubber stopper which 

 closes the mouth of the whole vessel — runs a tolerably wide glass 

 tube cut off obliquely at the end. Above its oblique termination 

 the tube has a small lateral hole, not larger than a pin's head, the 



