1 2 Sketch of the Improvements in Science [Jan. 



second glass screen, of a similar nature, a much smaller diminution 

 of their intensity than they did in passing through the first screen. 



Fourth Proposition. — The rays emitted by a hot body differ from 

 each other in their faculty to pass through glass. 



Fifth Proposition. — A thick glass, though as much or more 

 permeable to light than a thin glass of a worse quality, allows a 

 much smaller quantity of radiant heat to pass. The difference is 

 so much the less as the temperature of the radiating source is more 

 elevated. 



Sixth Proposition. — The quantity of heat, which a hot body 

 yields in a given time by radiation to a cold body situated at a 

 distance, increases, Cffiteris paribus, in a greater ratio than the 

 excess of temperature of the first body above the second. 



5. These observations favour the notion that light and heat are 

 modifications of each other. The experiments of Berard render 

 this opinion still more probable. He confirmed the experiments of 

 Dr. Herschell, that the heating power of tlie solar ray increases 

 from the violet to the red end. He found its maximum at the 

 extremity of the red ray ; though it still continued perceptible at 

 some distance beyond the visible spectrum. He found that the 

 rays of heat were polarized by reflection, as well as the rays of 

 light. Tije chemical power was greatest at the violet end, or a 

 little beyond it, as had been previously observed by Dr. Wollaston. 

 There is reason, from his experiments, to conclude, that this che- . 

 mical power extends over the whole spectrum, though it is too 

 feeble at the red end to be perceptible. 



6. The cold produced by the evaporation of liquids has been long 

 known to chemists ; and the effect of ether, in particular, was 

 explained many years ago by Dr. Cullen. Dr. Marcet has lately 

 added two new facts to this interesting branch of chemistry. He 

 has found that if a glass tube be filled with mercury, surrounded 

 with a cotton cloth wet with ether, and inclosed in the receiver of 

 an air-pump along with sulphuric acid, in the manner of Mr. 

 Leslie, the mercury speedily freezes if the receiver be exhausted of 

 air. The freezing of mercury may be still more simply performed 

 by the sulphuret of carbon. It is only necessary to surround the 

 mercurial tube with lint moistened with alcohol of sulphur, and 

 Then to exhaust the receiver. The mercury immediately freezes. 



7. We owe another beautiful fact respecting freezing to Dr. 

 \A'olIaston. If two glass balls be joined together by a long glass 

 tube, one of them half filled with water, and the whole apparatus 

 be hermetically sealed when exhausted of air, the water in the ball 

 will speedily freeze if the other ball be introduced into a freezing 

 mixture. 



8. The freezing of alcohol, said to be performed by Mr. Hutton, 

 of Edinburgh, is conceived to be brought about by compressing air 

 over the alcohol, cooling it as much as possible by means of a 

 freezing mixture, ar\d then suddenly allowing the air to escape. 



