50 VAPORIZATION. 



great advantage. When an extract is prepared in the ordinary 

 way, by boiling down an infusion or expressed juice in an open 

 vessel under atmospheric pressure, a considerable and variable 

 proportion of the active principle is always destroyed by the 

 high temperature and exposure to the air. But the extract is 

 not injured when the infusion or juice is evaporated at a low 

 temperature, and without access of air, and is generally found 

 to be a more active medicine. 



The temperatures at which different liquids are converted 

 into vapour are exceedingly various ; but other things remain- 

 ing the same, the boiling temperature is constant for any par- 

 ticular liquid. The following table exhibits the boiling points 

 of a few liquids, in which that point has been determined with 

 precision. 



Boiling point. 



Hydro-chloric ether 52 



Sulphuric ether . . . . .96 



Bisulphuret of carbon . . . .116 

 Ammonia, (sp. gr. 0.945) . . . .140 

 Alcohol, (sp. gr. 0.798) . . . . 1?2 



Water 212 



Nitric acid, (sp. gr. 1.42) . . . .248 

 Crystallized chloride of calcium . .302 



Oil of turpentine 314 



Naphtha 320 



Phosphorus 554 



Sulphuric acid, (sp. gr. 1.843) . . . 620 



Whale oil 630 



Mercury . ... . . .662 



The boiling point of water is uniformly elevated, by the so- 

 lution of salts in the fluid ; but much more so by some salts than 

 others. Tables have been constructed of the boiling points 

 of saline liquors, which are of useful application when we wish 

 to maintain a steady temperature somewhat above 2 12. Thus, 

 water saturated with common salt, (100 water to 30 salt,) boils 

 at 224; saturated with nitrate of potash, (100 water to 7 4 

 salt,) it boils at 238 ; saturated with chloride of calcium, at 

 264. 



When steam from water is confined, it increases in tem- 

 perature and acquires great force, and the experiment can only 

 be performed with safety in a boiler possessed of a safety valve. 



