-1859] WRITINGS OF JOSEPH HENRY. 221 



taining sulphuric acid, which will absorb the vapor as fast 

 as it is formed. 



We may convince ourselves immediately of the evapora- 

 tion of ice by exposing a given weight of it during a cold 

 day in the shade while the temperature is below freezing. 

 It will be found sensibly, though slowly, to diminish in quan- 

 tity. The same effect is exhibited in the process of drying 

 clothes in cold weather, which though they may be stiffened 

 by the frozen water with which they have been wetted, soon 

 become dry and pliable b}'- the evaporation of the ice. 



The apparatus of Daltou enables us to make the follow- 

 ing experiment, which has an important bearing on some 

 of the phenomena of meteorology. If, while the column of 

 mercury is at the temperature, for example, of 60°, and a 

 small quantity of water is resting on its upper end, the space 

 above being filled with vapor due to this temperature, we 

 place under the lower end of the tube beneath the surface 

 of the mercury a small crystal of common salt, it will rise 

 through the mercury by its specific levity, and be dissolved 

 in part or whole by the stratum of water at the top. Now 

 as soon as this solution begins to take place we shall see the 

 column of mercury ascend; a portion of the vapor will be 

 absorbed, and the tension of the remainder be diminished. 



In this case the attraction of the salt for the particles of 

 water neutralizes a part of their repulsive force and thus 

 diminishes the weight of mercury the vapor can support. 

 For the same reason salt water boils at a temperature several 

 degrees higher than 212°, though the vapor produced in 

 this case has only the elastic force of that due to pure water. 

 From the foregoing we conclude that the quantity of vapor 

 from the surface of the ocean is less and has less tension and 

 density than that from the surface of fresh-water lakes at 

 the same temperature. 



The table which was furnished by Dalton, and has since 

 been corrected by more refined experiments, is of great value 

 in various branches of science. The very simplicity of the 

 method employed is an evidence of scientific genius of the 

 highest character, and is well calculated to excite our admi- 



