DETERMINATION OF THE BOILING POINT. 767 



The mouth of the flask is lightly closed with a loose 

 piece of cotton-wool, which prevents the external air 

 from entering the flask and cooling the steam within. 

 As long as the interior of the neck of the flask appears 

 cloudy the steam has not obtained the requisite tem- 

 perature, for the cloudiness is due to condensed steam, 

 that is, to moisture. Steam itself is transparent and as 

 invisible as air ; when it comes in contact with cooler 

 air it is condensed to very small drops of water, and has 

 then the appearance of ' fog.' 



The boiling point of water, or rather the temperature 

 of steam produced by boiling water, is not so absolutely 

 constant as that of the freezing point, but depends on 

 the pressure of the atmosphere ; the less the atmospheric 

 pressure, the lower is the temperature of the boiling 

 point ; and the greater the atmospheric pressure, the 

 higher is the temperature of the boiling point. A 

 correct thermometer immersed in steam arising from 

 boiling water should indicate a temperature of 100 C., 

 when the pressure of the atmosphere at the time 

 and place where the experiment is made is equal to 

 that of a column of mercury 760 mm high. This tem- 

 perature of 100 C. is taken as the normal boiling 

 point of water. In localities which are situated several 

 hundred metres above the level of the sea, the atmo- 

 spheric pressure is never so great as that mentioned, and 

 in other places it is scarcely ever exactly equal to it 

 at the precise time when the boiling point is determined. 

 Hence it is necessary to know to what extent the boiling 

 point varies, when the pressure of the atmosphere differs 

 from the normal pressure. If the pressure indicated by 

 the barometer is not higher than 7SO rara nor lower than 



