1825.] Royal Society 225 



uhdertaken. The principal experiments were conducted in this 

 manner ; two thermometers, coated one with smooth black, and 

 the other with absorptive white, were exposed together under 

 exactly similar circumstances to the radiation from different lu- 

 minous hot bodies, such as iron raised to a considerable degree 

 of incandescence, and the flame of a lamp. This was done first 

 with, and then without the interposition of a glass screen. 

 After allowing for all the causes of error likely to have affected 

 the results, the object was to observe the ratio of the rates at 

 which the two thermometers rose when exposed ; and to com- 

 pare it with that similarly obtained when they were screened. 

 If the screen (according to the theory of de la Roche, &c.) only 

 intercepted a portion of one simple agent, the screened effects 

 would be merely diminished in absolute quantity, but would 

 remain unaltered in ratio. If this be not the case, it will follow, 

 that the transmitted portion of heating influence not only differs 

 from the rest in the property of transmissibility, but also affects 

 surfaces by a different law. 



In all the various experiments tried, one uniform result was 

 obtained ; viz., that the screened ratio was much greater than 

 the exposed. For example, in one instance, with the flame of 



a lamp the screened ratio was about -^p^ = - ; the exposed - ^ '^ 

 _ 1 



The general conclusion deduced is, that the radiant heating 

 influence is the united effect of two distinct agents ; one is 

 simple radiant heat, having the properties of relation to 

 texture and not darkness of colour, and stopped hy glass ; the 

 other, having relation to colour, and transmitted by glass, which 

 may be denominated " transmissible heat," or (from its close 

 association with light,) " the heating power of light." 



The distinct existence of two heating causes in the total 

 efi'ect from luminous hot bodies having been estabhshed in the 

 first part of this paper, the object of the second part is to ascer- 

 tain and compare the ratio subsisting between those two parts 

 of the heating effect in different instances. The instances tried 

 are, the flame of a lamp, when the combustion was in different 

 degrees of completeness ; the union of several flames compared 

 with one; the increase of density in a flame; and the different 

 degrees of incandescence in metals. 



In all these instances, a regillar increase of ratio was ob- 

 served, in correspondence with an increase in the completeness 

 of combustion, with the junction of different flames; with the 

 increase of density in a flame; and with the degree of incan- 

 descence in metal. 



Similar conclusions were also inferred from the experiments 

 of Mr. Brande, Count Rumford, &c. 



New Series, vol. ix. <? 



