Chemical Science. 229 



is removed, when it immediately crystallizes, and its tem- 

 pcvatuie becomes elevated. Dr. Thomson has lately expe- 

 rimented on this phenomenon ; not with a view of ascertaining 

 the cause of the crystallization, but on the source of the heat 

 evolved. Sulphate of soda and carbonate of soda were the 

 salts used; 51 parts of crystallized sulphate of soda to 49 

 parts of water, make a good solution for the experiment. If 

 carbonate of soda be used, 1 part should be dissolved in 4.22 

 parts of water, and the solution suffered to cool below 50° F. 

 When the sulphate of soda is used, the crystallization of the 

 mass is certain, at a temperature under 50°, on removing the 

 stopper, and the rise of temperature equals 24° F. If the 

 carbonate be used immediately the cork is drawn, an abundant 

 precipitation of small crystals in stars takes place, and the 

 temperature rises 14° F. From estimation made of the rise of 

 temperature ; the whole quantity of matter heated ; the quan- 

 tity of salt solidified; of wafer of crystallization combined with 

 it, Sfc.Sfc., Dr. Thomson concludes, that the latent heat of the 

 water of crystallization was, in both cases, the sole source of 

 the increase of temperature observed. — Annal. Phil. iii. 169. 



13. Heat in the moon's rmjs. — Mr. Howard, by exposing- a 

 concave mirror to the moon's rays, appears to have observed a 

 slight elevation of temperature in the focus, the thermoscope 

 used being particularly delicate. The experiment, on being re- 

 peated by M. Pictet, gave exactly opposite results ; for in place 

 of an elevation of temperature, there was depression, and the 

 thermoscope sank, although the same instrument that was 

 used by Mr. Howard. M. Prevost, in remarking on these re- 

 sults, points out some very important precautions to be taken 

 in this and similar experiments. As in a clear night, a 

 concave mirror exposed to the heavens will almost always in- 

 dicate a depression of temperature in its focus, in consequence 

 of its intercepting the warmer rays from the earth, and sub- 

 stituting those which are less heating from the heavens; the 

 quantity of effect of this kind becomes an important correction 

 in estimating the heat caused by the lunar rays, when con- 

 centrated by a concave reflector. The effect is variable, being 

 very much diminished by opaque vapour, and by reflection 

 from such terrestrial bodies as may enter into the field of the 

 mirror : in winter also the difference between the temperature of 

 the earth and the sky is diminished, and consequently this 

 effect with it. 



Again, M. Prevost remarks, that sometimes an elevation of 

 temperature may be obtained, not only because the air may 

 radiate much heat, but also because, on fine summer nights, 

 the upper air is warmer from having been less cooled (by radia- 

 tion) than the lower air or the soil. 



