222 Notices respecting New Books. 



condensed vapour, whose quantity will depend partly upon the 

 moisture existing in the atmosphere, and partly upon the differ- 

 ence in sensible heat between the air and the liquid in the ves- 

 sel. There are certain saline substances that lower the tempe- 

 rature of the water in which they are dissolved; if one of these, 

 in fine powder, be added gradually to a portion of water con- 

 tained in a vessel, the temperature may be lowered by slow 

 degrees, until it reach that point at which deposition will just 

 begin to take place. A thermometer placed in the liquid will 

 show this temperature, and mark the degree of heat at which 

 saturation would occur, with the quantity of moisture then con- 

 tained in a given bulk of atmospheric air. This degree of the 

 thermometer is called the dew point, and will, by mere refer- 

 ence to tables deduced from the experiments of Dalton, give 

 the absolute quantity of moisture that is present. 



This experiment would be attended with some difficulty in 

 practice. Mr. Daniell has therefore adopted another method 

 of performing it. If water be placed in one of two balls, con- 

 nected together by a tube bent twice at right angles and ex- 

 hausted of ail', the immersion of the empty ball in a freezing- 

 mixture will cause the congelation of the water contained in 

 the other; for the aqueous vapour that rises rapidly in vacuo 

 will be as rapidly condensed by the cold application ; its place 

 will be supplied by a fresh evaporation from the surface of the 

 water, and the formation of this new vapour will carry off so 

 much heat from the mass, as rapidly to reduce its temperature 

 to the freezing point. If ether be substituted for water in the 

 balls, and if the ball that contains no liquid be coated with a 

 bibulous substance, moistened also with ether, the evaporation 

 of this last will produce a great degree of cold ; and this will not 

 be manifested in the loss of heat by the ball to which the ether 

 is applied, but by the rapid passage of the inclosed ether in the 

 state of vapour from the other bulb, the temperature of which 

 is lowered in consequence. The loss of temperature in the 

 naked bulb may be rendered evident by inclosing within it, 

 and the contiguous stem, a very delicate thermometer ; as soon 

 as the surface of this bulb is cooled down to the point at which 

 the aqueous matter contained in the atmosphere would lie pre- 

 cipitated, it becomes clouded with a thin film of moisture ; a 

 practised eye will readily seize the precise instant at which 

 this takes place, and will at the same moment read the tempe- 

 rature shown by the included thermometer. In the whole cir- 

 cle of physical science there is no instrument more simple and 

 beautiful in principle than this hygrometer. Its use is not at- 

 tended with any difficulties; and it fully satisfies all the condi- 

 tions laid down by Saussure, as essential to the perfection of 



hygrometric 



