TEMPERATURE. 



15 



A 



8 



~l 



Benzolim 



and by observation of the barometer any deviation from 100 can be 

 accurately determined. But, as we know, the boiling-point varies in a 

 definite manner with the variation of pressure, 

 so that it is possible, by regulating the pressure, 

 to keep the steam at temperatures other than 

 100. This method is made use of in one class 

 of constant-temperature apparatus. Water, or 

 some other liquid suitably chosen, and contained 

 in a closed vessel, is supplied with so much heat 

 that it boils. The vapour passes into a cooling 

 arrangement, so that it is condensed back into 

 liquid as fast as it is formed. The pressure is 

 so regulated by varying the amount of air in 

 the vessel, that the boiling-point is the desired 

 fixed temperature. To adapt this apparatus to 

 secure a constant temperature, the vapour is made 

 to surround the chamber in which the constant- 

 temperature operations are being carried on. 



The sketch in Fig. 7 will illustrate an applica- 

 tion of the method. 



This will be seen to be merely an adaptation 

 of Regnault's apparatus for determining the 

 pressure of water vapour at high temperatures, 

 described in chapter x. 



Ramsay and Young have investigated the 

 change in vapour pressure of a number of liquids 

 in the neighbourhood of their boiling points, choos- 

 ing liquids which are suitable for use in such an 



apparatus as this, and tables embodying their results will be found in 

 the Journal of the Chemical Society, Sept. 1885, vol. xlvii. p. 640. The 

 liquids investigated were : 



Approximate 

 Liquid. Boiling-Point. 



Carbon bisulphide . . . 46 



Ethyl alcohol .... 78 



Chlorobenzene . . . . 132 



Bromobenzene .... 155 



Aniline 184 



Methyl salicylate .... 222 



Bromonaphthalene . . 280 



Mercury ..... 358 



In another class of constant-temperature apparatus, the constant 

 temperature enclosure is heated by gas, and the supply of gas to the 

 burner or burners is regulated by means of a " thermostat," so that if 

 the temperature tends to rise above that required, the gas supply is 

 checked, while if it tends to fall below it, the gas supply is increased. 

 There are many devices for effecting this. The following (Nicol, Phil. 

 Mag., 1883, xv. p. 340) will serve as an example. The thermostat 

 (Fig. 8) is placed with its bulb in the constant-temperature chamber. 

 The gas passes from A to B and thence to the burner, partly through 



FIG. 8. Thermostat. 



