RADIANT IIEAT. 309 



effect, as the greater proximity of the second screen to the ther- 

 mometer; also more heat might be lost in communicating an equable 

 temperature to the first screen from its central and more heated part; 

 whilst the heat would be thus more equally radiated to allparts of the 

 second without such loss. 



Thus it appears that the fact stated by M. De la Roche is fully sub- 

 stantiated, while, on the other hand, it is satisfactorily accounted for 

 without supposing any new property of heat or any direct radiation 

 through glass. 



In some unpublished experiments of my own, I found, upon observing 

 the temperature acquired by a screen exposed to iron below luminosity, 

 first plain, and then coated with Indian ink towards the source of 

 heat, the thermometer being in contact at the central part on the 

 outside, that it rose rather more on the 2^0 in than on the coated screen. 



8.) MM. Nobili and Melloni, in the memoir before quoted, applied 

 their instrument to estimate the effects of transparent screens. Over 

 the thermo-multiplier were placed successively transparent screens of 

 glass, sulphate of lime, mica, and of water, oil, alcohol, and nitric acid 

 (enclosed between plates of glass?) and also of ice. 



The source of heat was a ball of iron, heated to a point below lumi- 

 nosity, suspended, or rather passed rapidly, at a certain distance 

 above the screen. 



The index indicated an instantaneous effect, greater or less in all 

 cases except those of water and ice, in which none was produced, even 

 when the iron was kept a longer time over the instrument, or even 

 heated to redness, and the screen reduced in thickness. 



9.) A set of experiments presenting some important results with 

 respect to the absorbing and radiating properties of surfaces, as well 

 as the action of screens in air and in vacuo, are given by Mr. W. R. 

 Fox, in the Phil. Mag. and Annals, New Series, No. G5, p. 245. A 

 brief statement of the results is as follows : 



A cylindrical tin vessel of hot oil with its surface polished, and 

 another similar, painted black, had their times of cooling a certain 

 number of degrees observed under a receiver first highly exhausted, 

 and then full of air; the cylinders being respectively first exposed, 

 and secondly enclosed in one and sometimes more tin cases with in- 

 tervals; the outer and inner surfaces being one or both polished or 

 blaekened. From all the different combinations of these results, of 

 which he states in detail, I collect the following general inferences: 



I. In vacuo: (1) the 'polished vessel had its cooling always accelerated 

 by the cases, and in this order — 



Case. 

 1 , 



Inside. Outside. 



Most accelerated • • bright black. 



black black. 



bright (3 cases) black. 



bright (3 cases) bright. 



bright (1 case) bright. 

 Least accelerated black bright. 



