of the radiant heating lEffedsfrom terrestrial Sources. 4-39 



" absorptive texture," to signify that peculiar state of" division 

 in the particles of the surface, which has been shown by Pro- 

 fessor Leslie and others to be most susceptible of the influence 

 of simple radiant heat, and always to give a proportionally 

 greater radiating power. 



The question then is entirely one of facts, and involves no 

 hypothesis as to the nature either of light or of heat. The 

 object is simply to ascertain by experiment, whether, of the 

 total heating effect radiated from a luminous hot body, the 

 portion intercepted by a transparent screen is of the same 

 nature as, or different from, the part transmitted in its rela- 

 tions to the surfaces on which it acts. 



(6.) In conformity with this view of the object proposed, 

 the general principle of the following experiments is this : 

 taking different luminous hot bodies, to expose to their influ- 

 ence two thermometers, presenting one, a smooth black sur- 

 face, the other an absorptive white one ; thus obtaining the 

 ratio of the total direct effect on the two, we may compare it 

 with the ratio similarly observed when a transparent screen is 

 interposed. 



(7). ThLs principle of experimenting was applied with one 

 or two variations ; and though in the abstract sufficiently sim- 

 ple, it will in practice require an attention to several consider- 

 ations. I shall therefore proceed in the first instance to the 

 detail of the different particulars ; then give the results of the 

 experiments in a tabular form ; and lastly, recapitulate the 

 conclusions and make a Cew general remarks. 



I. (8.) In the following set of experiments two common 

 thermometers were employed. The diameters of their bulbs 

 were, thermometer A, O'fi inch. ; B 0*55. A was coated with 

 a wash of chalk and water, and B with Indian ink. 



In order to compare the effects to be observed with those 

 of simple radiant heat, I ascertained the ratio of the effects of 

 the latter on the two bulbs thus coated, by a few preliminary 

 trials, and found it to be very nearly one of ecjuality, or per- 

 haps, the effect of the white rather greater than that of the 

 black. 



'i'he two thermometers were graduated to quarters of centi- 

 grade degrees; and were both fixed on one mounting, with 

 their bulbs detached about one inch from its lowest pait, and at 

 the distance of about tiiree quarters of an inch from each other. 



(9.) In the 2d set of experiments they weie fixed into the 

 top of a box, the front of which was open, so that the glass 

 screen could be applied to it or not, as required. When the 

 screen was not used the box would accjuire more heat, and 

 radiate it to the bulbs in a small degree; which afli'cting (hem 



in 



