272 



THE CHEMISTRY OF CREATION. 



which it would be deposited in the liquid form, 

 as dew, would be greater than if the temperature 

 were 70. Now, when any substance or sur- 

 face becomes cooled down, whether naturally 

 or artificially, to a little below the point of 

 maximum density of watery vapour, the latter 

 immediately loses the form of vapour, and 

 becomes condensed in minute drops upon the 

 cold surface ; the point at which this takes 

 place is called the " Dew-point." The late 

 Professor Daniell invented an interesting little 



instrument, of con- 

 siderable importance 

 in hygrometric pur- 

 suits, entitled the 

 '' Dew - point hy- 

 grometer." It con- 

 sists of a bent glass 

 tube, terminating in 

 two bulbs, one of 

 which is half filled 

 with ether, the whole 

 being vacuous as re- 

 spects atmospheric air. In the ether bulb is a 

 delicate thermometer ; a piece of muslin wetted 

 with ether covers the empty bulb outside. The 

 cold produced by the evaporation of this ether 

 condenses the ether vapour inside the tube, and 

 rapidly lowers the temperature of the ether bulb 



DANIBLL'S HYGROMETER. 



