THE INCUBATOR 41 



gas passes from A to C, through an aperture inside the 

 valve, the size of which aperture can be adjusted by the 

 screw-needle S, hence the gas-flame which burns in a little 

 lantern below the incubator is never extinguished. 



The reader will no doubt have grasped the fact that the 

 expansion of the capsule, owing to the boiling of its contents, 

 provides the motive force for acting upon the lever BD, and 

 as this expansion only takes place at a predetermined 

 temperature, the lever will only be acted upon when the 

 critical temperature is reached, no sensible effect being pro- 

 duced at even one degree below that at which the capsule 

 is desired to act. We have thus a thermostat acting at a 

 nearly fixed and predetermined temperature, and without 

 any further additions to the apparatus already described, we 

 should have (were it not for slight barometric variations) an 

 absolutely fixed temperature regulator. 



Changes in atmospheric pressure, however, tend to make 

 the temperature fluctuate about a degree (P.) on either 

 side of the normal, if observations be taken extending over 

 considerable intervals of time. 



To compensate for these variations, if it be desired to do 

 so, a sliding weight runs on the lever-rod D. But this 

 weight serves a yet more important function. 



It gives us the opportunity of retarding within certain 

 limits the boiling-point, of the capsule, and of thus adjusting 

 the temperature at which the capsule shall expand several 

 degrees above that at which (with the weight quite to the 

 left) it first commenced to act. 



By this means the operator is enabled to obtain a range 

 of about 8° with any particular capsule, and as these can be 

 made to act at any temperature from 60° to about 300° P., 

 we are enabled to maintain any desired temperature in in- 

 cubators, sterilisers, water-baths, etc. 



In actual practice it is found that the temperature can be 



