146 Domestic Science 



Further boiling at once takes place, and this phenomenon 

 may be caused to occur several times. 



It will be well to consider rather closely what 

 happens in this experiment, in order to account for 

 the strange fact that cooling the flask makes the water 

 start boiling again. At the instant of corking the flask 

 the upper portion of the latter is filled almost entirely 

 with steam, the small remaining portion being air which 

 has not been completely expelled by the steam. When 

 cold water is poured over the bottom of the flask, some of 

 this steam, which is in the vaporous condition, is rapidly 

 condensed to water, and this water only occupies an 

 extremely small volume as compared with that of the 

 steam from which it was made. The pressure of the 

 remaining steam above the water is much less than that 

 previously exerted by the whole quantity of vapour 

 originally present. To restore in some part the original 

 conditions, some of the water is vaporised, with the 

 consequent appearance of bubbles of steam in the liquid. 

 Further disturbance of equilibrium by repetition of the 

 cooling is followed by a further outbreak of ebullition. 



The important observations made in Experiments 49 

 and 50 may be summed up thus : 



The temperature at which a liquid boils is dependent 

 upon the pressure to which its free surface is subjected, 

 increase of pressure raising the boiling-point, while 

 diminution of pressure has the opposite effect. 



93. The two fixed points the melting-point of ice 

 and the boiling-point of water having been marked 

 upon the scale of a mercury thermometer, the space 

 between these two points is divided into a number of 

 equal parts, called degrees. Two scales are in use in 

 this country. In that found on most thermometers in 

 domestic use, the number of degrees between freezing 



