VAPORIZATION. 325 



When the mercury in the manometer tube is 760 mm. 

 above the level of the mercury in the boiler, the steam 

 has a tension of two atmospheres, and the thermometer 

 will record a temperature of about 121 C. 



508. Concerning Steam. A given mass of 

 water in the aeriform condition occupies nearly 

 1700 times as much space under a pressure of 

 one atmosphere as it does in the liquid condition. 

 In other words, a cubic inch of water will yield nearly a 

 cubic foot of steam. Steam is invisible. What is 

 commonly called steam is not true steam, but little globules 

 of water condensed by the cold air and suspended in it. 

 By carefully noticing the steam issuing from the spout of 

 a tea-kettle, it will be observed that for about an inch from 

 the spout there is nothing visible. The steam there has 

 not had opportunity for condensation. The water particles 

 visible beyond this space passed through it as invisible 

 steam. The steam in the flask of Fig. 247 and in F of 

 Fig. 248 is invisible 



509. Reference Tables. Boiling Points under a pressure 

 of one atmosphere : 



Alcohol.. . 78 C. 



Ammonia 40 C. 



Sulphurous anhydride. .. 8 



Ether 35 



Carbon bisulphide 48 



Water (pure) 100 



Mercury 350 



Sulphur 447 



Some solids, as iodine, arsenic and camphor vaporize without 

 visible intermediate liquefaction. The process is called sublimation. 



Boiling Points of water at different pressures : 



Atmospheres. 

 1 

 2 

 3 

 6 



10 

 20 



