22S Proceedings of Philosophical Societies. [Sept. 



rated solution of Glauber's salt, which preseives its liquidity when 

 cooled in a vacuum, crystallizes as soon as air is admitted. But M„ 

 Gay-Lussae lias ascertained that this is far from being the case with 

 other salts ; and that even with respect to sulphate of soda, the 

 phenomenon does not depend upon the presence or absence of {he 

 pressure of air. When the contact of air, for example, is inter- 

 cepted by a covering of oil, the liquid remains fluid just as well as 

 when it is in a Vacuum ; while, on the other hand, the pressure of 

 a column of mercury does not in the least accelerate the crystalliza- 

 tion. A solution v. Inch is made to pass through mercury freed 

 from air by boiling, does not crystallize ; but when passed through 

 mercury in its ordinary state, it crystallizes directly. Agitation, the 

 introduction of a small crystal, and many other causes, determine 

 the crystallization, whatever be the pressure. Therefore M. Gay- 

 Lussae concludes that it is not by its pressure that air diminishes the 

 dissolving power of water. He has ascertained, likewise, that it is 

 not by absorbing air that water loses this power; but he thinks it a 

 phenomenon more or less analogous to that of pure water, which, 

 as is known, remains liquid when cooled some degrees below its 

 freezing point, whenever we can prevent it from being agitated ; 

 and congeals the moment it receives the slightest shock. 



The most evident source of heat upon the globe is the rays of the 

 sun. But it has been long remarked that these rays when separated 

 by the prism do not produce equal degrees of heat : and Dr. 

 Herschel, the celebrated astronomer, ascertained some years ago 

 that the heating power goes on increasing from the violet to the 

 red. He even assures us that bevond the spectrum there are rays, 

 which, though not luminous, have the power of heating more 

 strongly than the red rays. Messrs. Hitter, Ba^ckman, and Wol- 

 laston, announced soon after that the power of the rays of light to 

 produce certain chemical changes goes in the inverse order, and is 

 strongest in the violet ray and beyond that ray. 



M. Berard, a young chemist of Montpellier, who has repeated 

 with much delicacy and precision these two kind of experiments, 

 has ascertained their accuracy in several respects. He has even 

 found that the chemical power of light goes on diminishing to the 

 middle of the spectrum, and is not sensible beyond that point. 

 According to him, it is at the extremity of the red ray that the 

 greatest heating power resides, and beyond the spectrum it dimi- 

 nishes. M. Berard has ascertained, likewise, that these properties 

 belong to light reflected by a mirror, and to light which lias passed 

 through Iceland crystal, as well as to direct light. 



Equally decisive results have not been obtained respecting the 

 property of magnetizing steel ascribed to the violet ray by M. 

 Morichini, a well-informed Roman chemist. Although needles 

 exposed to this ray appeared magnetized in certain experiments, they 

 underwent no such change in many other trials; and at present no 

 jeason can be assigned for this difference: for in both cases every 



