KNOBLAUCH ON RADIANT HEAT. 



Table V. {continued). 



9OB 



The deflection of the galvanometer-needle, produced by direct 

 radiation, upon the constancy of which the accuracy of all the 

 comparisons detailed depends, had been previously tested before 

 the insertion of each diathermanous substance. 



Three only of the latter were used in succession in the in- 

 vestigation with regard to the various sources of heat ; so that 

 the observations relating to them, each of which required a 

 minute and a half or two minutes, never extended beyond a time 

 during which all the conditions of the experiment could be con- 

 sidered as sufficiently constant*. 



To preserve this uniformity as much as possible, the posi- 

 tion of the thermal pile remained unchanged, whilst the source 

 of heat was more or less approximated to it, until the constant 

 direct deflection used for comparison was produced. The dia- 

 thermanous bodies were always inserted at the same spot behind 

 a diaphragm, and at a constant distance from the thermoscope. 



II. On the Heating of Bodies by Radiant Heat. 

 It is a fact, which has been long known and proved by an 

 extended series of observations — - 



1. That different substances are heated to a different extent 

 by the radiation of heat from one and the same source. 



2. That in every instance the extent of the increase of heat 

 depends upon the structure of the surface. 



More recent experiments by Baden Powell and Melloni have 

 shown — 



• For this reason the numbers in these, as well as in all the subsecpient in- 

 stances, should only be compared so far as I have brought them into relation 

 with one another. 



