APPARATUS FOR PHYSIOLOGICAL USE. 269 



get the ferments for yourselves. These ferments are con- 

 tained in the stomach, in the pancreas, and in the small 



FlG. 7 shows a common form of apparatus lor experiments on artificial diges- 

 tion, furnished with one of Geissler's thermostats, a is a tin vessel nearly full 

 of water, supported on a tripod and heated by a gas lamp placed underneath. 

 b, c, d, e is the thermostat, h is a beaker, into which the substances to be 

 digested are put. The thermostat consists of a wide tube, b, partially filled 

 with mercury, furnished with a perforated cork, and a glass T-tube, c, f, 

 inside which is a smaller and shorter tube, d, the upper ends of d and c being 

 luted together, c is now pushed through the cork into the mercury in b, and 

 connected by india-rubber tubing with the gas main and lamp. The gas 

 passes from the main through the india-rubber tube, g, down the gas tube, d, 

 out at its lower end into c, f, and thence to the lamp. As the mercury in t 

 becomes warm it expands, and rising in c closes the lower end of d. The 

 supply of gas to the lamp would thus be entirely cut off, were it not for a 

 small hole, e, in d, which just allows sufficient gas to pass through to keep the 

 lamp alight. The instrument is adjusted by putting c just so far through the 

 cork in 6 that the mercury does not touch the lower end of d until the liquid 

 in a is at the exact temperature desired. The thermostat would act per- 

 fectly well if b were filled with mercury only, but in order to render it still 

 more delicate 6 is provided with a horizontal diaphragm in its middle, 

 from the centre of which a tube passes nearly to the lower end of 6. When 

 mercury is poured into i some runs down to the bottom and shuts up the air 

 in the space t, under the diaphragm. When the air in i gets warm and ex- 

 pands, it pushes up the mercury from the lower end of b, and closes the orifice 

 of d. As it expands more quickly than the same bulk of mercury it renders 

 the instrument more sensitive. It is sometimes difficult to get the hole, e, in 

 the tube, d, sufficiently small. The temperature consequently rises above 

 the required point, even when the end of d is closed by the mercury. To 

 obviate this another form has been invented by Page, in which the gas re- 

 quired to keep the lamp alight passes through another tube, regulated by a 

 stopcock, as shown in Fig. 8. 



