304 RADIANT HEAT. 



4.) The most recent and curious researches on this part of the sub- 

 ject (and extending, as we shall see, to other parts also) are those of 

 MM. Nobili and Melloni. — (Annates de Chimie, Oct., 1831; Recherches 

 sur plusieurs Phenomenes Calorifiques, &c.) 



The authors commence by describing their ther mo -multiplier, by the 

 aid of which their researches were carried on. This consists in a 

 thermo-electric combination, susceptible of excitation from the feeblest 

 conceivable application of heat, and connected with a delicate gal- 

 vanometer, which gives a measure of the effect produced, and conse- 

 quently of the heat. 



The pile is in a case coated with the smoke of a flame when used 



for radiant heat, but left naked when for heat of temperature, on 



account (as they observe) of the bad conducting quality of this coating. 



They applied this instrument to the examination of the different 



reflecting, absorbing, and radiating powers of surfaces. 



They confirmed in general the results of Leslie and others already 

 mentioned. They found that polish augments the reflecting power 

 much less than usually supposed. Non-metallic substances possess 

 scarcely any reflecting power, whatever be the state of their surfaces. 

 They examined the absorptive power of different substances, taking 

 lamina? of equal thickness and similarly fixed, &c; these having been 

 heated for a few minutes in the rays of the sun, were placed in pairs 

 on apertures at the opposite sides of the thermo-multiplier, and in 

 this way the order of their absorptive powers was considered to be 

 obtained by the degree of heat they respectively radiated; and the 

 results were, that the effect increased by blackness of color and with 

 roughness of surfaces. Also the following surfaces were in this order: 

 silk, wool, cotton, flax, hemp, (all white,) which is the inverse of their 

 conducting powers. In like manner, with metals of nearly the same 

 color and polish, the order was — copper, silver, gold, steel, iron, tin, 

 lead, exactly in the inverse order of the conducting powers; the same 

 with several woods and minerals. 



On these experiments I must remark, that the heat acquired from 

 the sun's rays is so obviously dependent on color that it is astonishing 

 that any experimenter should adopt this as affording any ground for 

 making conclusions respecting the comparative absorbing or radiating 

 powers for heat in general, The later results, wdien the surfaces were 

 all of the same color, are extremely important. Supposing they all 

 acquired the same degree of solar heat which was thus converted into 

 heat of temperature, and then radiated from the surfaces as simple 

 heat, the real conclusion established is, that the kadiating poivers of 

 surfaces for simple heat are in the inverse order of their conducting 

 jonvers. 



e.) Effect of screens on heat from non-luminous hot bodies. 



1. Pictet found a difference in the interceptive effect, according 

 as the plain or the silvered side of a glass screen was towards the 

 source of heat. 



