158 On Steam and Brines 



condensation of this steam in the mixture of salt and brine.' 

 It is, therefore, of equal importance to observe accurately the 

 temperature of pure saturated steam as to observe that of the 

 boiling mixture. The same apparatus suffices for both pur- 

 poses. Before charging, and being clean and dry, the receiver 

 is connected with the boiler, and steam blown through it. 

 Some of it condenses and collects at the bottom of the re- 

 ceiver, forming a pool of distilled water boiled by steam, the 

 steam produced by which is perfect for the purpose. As the 

 division marking 100 C. is usually some way down the stem, 

 the thermometer is preferably pulled up, so that the bulb is 

 entirely in the steam. The steam so produced in an apparatus 

 of the proportions described, must be truly saturated steam 

 of the tension equal to the actual barometric pressure, and 

 must, therefore, have exactly the temperature which corre- 

 sponds to this pressure. This is the temperature of steam 

 condensing on pure water. 



The supply of steam is abundant, and the latent heat of 

 steam is very great ; the thermometer, therefore, must take, 

 in a very short time, exactly the temperature of the steam 

 with which it is completely surrounded. The thermometer 

 may be constructed so that a millimetre on the stem corre- 

 sponds to a hundredth or a thousandth of a degree. Its indi- 

 cation will be perfectly steady, provided that the conditions 

 remain unchanged. The slightest change in the barometric 

 pressure makes itself at once apparent, and at all times, 

 especially during unsettled weather, the temperature of satu- 

 rated steam must be observed at frequent intervals. 



It is convenient to have a separate apparatus for this 

 purpose. If we imagine the receiver, D, with the entry tube 

 at the bottom straight instead of bent, so that it can take the 

 place of the T-tube in the boiler, we have a perfect apparatus 

 for determining the temperature of saturated steam and con- 

 sequently also for fixing the point corresponding to 100 C. 

 on the scale of the thermometer. It is assumed that the 

 steam tube is large enough to take the whole working part of 

 the thermometer. It is unsuitable in itself or to the steam 

 generator used if the steam makes its exit with an audible 



