422 Experiments tending to prove that the Prism 



ble* or stone: and that frits, or unalterable metallic combina- 

 tions, have not been the only pigments employed by great artists ; 

 and that their varnishes have not been sought for amongst the 

 transparent 'combinations of the earths with water, or amongst 

 the crystalline transparent compounds unalterable in the atmo- 

 sphere f. 



Kome, January 14, 1815. 



LXXIII, Experiments tending to prove that the Prism has a 

 calorific Focus, and that Dr. Hekschel ivas viistake7i in sup- 

 posing he separated the Heat and Light of the Solar Raijs. 

 By Joseph Rjeade, M.D. 



To Mr. Tilloch. 



Sir, — rjAViNG fixed a piece of pasteboard in the frame of a 

 dressing-glass, in which I cut an opening or slit a little larger 

 than the bulb of a thermometer, and of a sufficient length to let 

 the whole extent of one of the prismatic colours pass through ; 

 and having placed a very sensible mercurial thermometer on an 

 inclined plane of wood covered with white paper, — I placed this 

 plane behind the dressing frame sustaining the pasteboard on a 

 table : I now set a prism moveable on its axis in the upper part 

 of an open window at right angles to the solar ray, and turned 

 it about till its refracted coloured spectrum became stationary on 

 the table placed at a proper distance from the window. The 

 experiment was so regulated as to let the rays of one colour pass 

 through the opening in the pasteboard and fall on the thermo- 

 meter, which when placed in the shade behind the screen stood 

 at 50'. 



Exp. 1. — Having arranged the thermometer, and allowed the 

 red, or more correctly speaking the orange rays to fall on the 

 bulb ; in five minutes it rose from 50 to 58, or eight degrees. 



Exp. 2. — I lowered my thermometer to 50", by plungeing it 

 in some dry sand at that temper|ture, and then replaced it on 

 the inclined plane : on bringing the green rays of the spectrum 

 through the opening, so as to immerse the entire thermometer 



* Copper, it is evident, from tlic specimens in the ruins of Pompeii, is a 

 very perishable material, and therefore, even enamels made on copper will 

 yield to time. Canviiss, by being impregnated "ith bitumen, is rendered 

 inucli more durable, as is evident from the duration o( the linen impreg- 

 nated with bitumen and aspiialtum used for infolding the Egyptian nuim- 

 inies. 



t The artificial hydr.it of alumina will probably be found to be a sub- 

 stance of this kind: possibly the solution of boracic acid in alcohol will 

 form a varnish. — 'I'lie solution of sulphur in alcohol is likewise worthy of 

 an experiment. Many other similar combinations might be named. 



as 



