458 Recent Improvements in Science. 



sphere, and, consequently, less than it would have at its 

 maximum density, at a temperature much above 212°. It 

 appears natural to suppose, that each bubble of vapour which 

 is elevated, immediately acquires the temperature of the li- 

 quid, which surrounds it on all sides, and continues at the 

 same time, to undergo a certain expansion, until its elasticity 

 is equal to the pressure of the atmosphere. Rudberg, 

 however, has drawn a different conclusion from his expe- 

 riments. (Poggenclorff's Ann. 1835, No. 2.) He has found 

 that the temperature of the vapour produced from a boil- 

 ing saline solution is independent of the nature and qua- 

 lity of the salt dissolved ; it is under the same barometric 

 pressure, absolutely the same as the vapour raised from 

 pure water. He also concludes that the temperature of the 

 vapour which rises from a boiling saline solution, is, at 

 whatever pressure the ebullition takes place, always the 

 same, in the same circumstances, as that of the vapour of 

 boiling water. 



These results are quite different from those obtained by 

 Dalton, Gay Lussac, and Princep, with regard to the 

 vapour formed from saline solutions, by evaporation. They 

 consider that the vapour of a saline solution has much less 

 elasticity than that of pure water, if the two liquids are at 

 the same temperature. Inversely, then, it follows, that 

 for the same elasticity, the vapour of a saline solution is 

 hotter than that of pure water. They find, also, that this 

 difference of temperature increases both the quantity of 

 salt dissolved, and that it varies much, according to the 

 nature of the salt. Hence, it appears, that between the 

 temperature of vapour and its elasticity, the relation is 

 quite different, if this vapour is produced by the ebullition 

 of a saline solution, or by evaporation from the surface. 

 The cause of this difference requires further investigation. 



Chemical action of the solar spectrum. Professor Hessler, 

 of Gr'atz, has observed, that the action of the solar spec- 

 trum, upon a piece of paper, covered with gum water and 

 chloride of silver, varies according to the nature of the 

 prism employed. The time required with water and spirit 

 of wine, was very short ; with turpentine and oil of cassia, 

 two to thirteen minutes : with flint glass, 2 3 ; with crown 

 glass, 1'5. The maximum lay in the spectrum of the spi- 



