HEAT OR CALORIC. n 



per part of which, is covered with muslin ; a little ether being drop- 

 ped upon this part of the ball, dew soon begins to be deposited on 

 the other, and the temperature at which this happens, is called the 

 dew point.* Mr. Pollock of Boston constructs this instrument with 

 two balls, one immediately below the other ; the upper one is cov- 

 ered with muslin, and moistened with ether and the dew is deposited 

 on the lower ball. 



EXPERIMENTAL ILLUSTRATIONS OF THE LAWS OF EVAPORATION. 



1 . Loss of weight. Water balanced in scales, loses a perceptible 

 weight in a short time ; with alcohol and ether the effect is still 

 more remarkable. 



2. Heat applied to the fluid gives a much quicker result. 



3. Camphor, carbonate of ammonia, and other very volatile solids, 

 in the same circumstances, lose weight, although more tardily. 



4. Dip a finger successively into water, alcohol, and ether, and 

 observe that the sensation of cold, is stronger and quicker, the more 

 evaporable the fluid. 



5. Production of cold. When the atmosphere is apparently still, 

 we discover which tvay the wind is, by wetting the finger in the mouth 

 and holding it up to the air, it will feel coldest on the windward 

 side, the evaporation being there the most rapid, and consequently, 

 heat being there most absorbed, from the finger, to form the vapor. 



6. Water is frozen by the evaporation of ether, j- in the air ; this '19 

 conveniently done, by placing the water in a glass tube, sealed at one 

 end ; it may be one third or one half of an inch in diameter, and the 

 water may occupy two or three inches in depth ; a coiled wire may 

 be pushed into the tube to lift the ice out, (and perhaps to aid by its 

 conducting power, in the extrication of the latent heat ;) if the water 

 be colored, the effect will be the more pleasing ; now let a capillary 

 stream of ether, from a dropping tube or otherwise fall upon the tube 

 containing the water, which may be either naked or may have a little 

 gauze wrapped around it ; in a few minutes the water will be frozen 

 solid, and a momentary pressure of the tube in the hand will thaw 

 the outside of the ice, so that it may be withdrawn by the wire. 



7. Cold produced by the Palm Glass. Dr. Hare, from 7 to 13. 



" Two bulbs are formed, at 

 each end of a tube, one having 

 a perforated projecting beak. 

 By warming the bulbs, and 

 plunging the orifice of the beak 



* Phil. Trans. 1826 Edin. Phil. Jour. No. XVII. pa. 155. 

 i This fact was mentioned on p. 105. 



