268 LECTURES TO SCIENCE TEACHERS. 



during the assimilation of foods may be said generally to 

 consist in their taking up water and then breaking up into two 

 or more simpler bodies. The heat which the chemist applies 

 may be said to drive the water in and tear them apart, 

 while the ferment which accomplishes the same thing as 

 the heat may be supposed to insinuate itself into them and 

 let the water in so that they fall apart. You must remem- 

 ber that I use this illustration only in a general way, and 

 that I do not mean to say it is in every way exact. 



The ferments by which digestion is carried on act only at 

 a certain temperature, namely, that of the body. When the 

 temperature is low they hardly act at all, and above a 

 certain temperature they are destroyed ; so that it is only 

 within certain limits that we can carry on digestion. 

 The temperature of the body is constant, and if we wish to 

 imitate the digestion which goes on in the body we must 

 have a constant temperature. In order to do this we 

 employ a thermostat or gas regulator (3950 d and e in 

 Catalogue). 



This is a sort of self -regulating valve through which the 

 gas passes to the burner placed under the vessel we want 

 to keep warm. The thermostat is put in the vessel, so that 

 the temperature of both rise and fall together. The in- 

 strument is so adjusted that when its temperature falls it 

 lets more gas pass through it, so that the flame becomes 

 bigger and the vessel and thermostat consequently warmer. 

 As the temperature rises the thermostat allows less and less 

 gas to pass through it, the flame consequently becomes 

 less and the temperature again falls. In this way the 

 temperature can be kept oscillating within such narrow 

 limits as to be for most purposes practically constant. 

 There are various forms of thermostat, but in all of them 

 the supply of gas is regulated by mercury, which expands 

 when warm so as partially to block up the tube through 

 which the gas passes, and on cooling contracts so as to 

 leave it open. 



In this way we are able to keep fluids at any given 

 temperature for a long time together, and are thus able to 

 imitate out of the body the process of digestion which goes 

 on in it. 



Now, as some of you may wish to repeat this experiment 

 cf artificial digestion, I may as well tell you how you can 



