KNOBLAUCH ON RADIANT HEAT. 235 



This result is of some interest as regards the determination of 

 the specific heat of bodies. Thus, if ice in the calorimeter 

 absorbs the heat radiated by different substances, even within the 

 limits of temperature stated, unequally, i. e. a greater or less 

 portion of a constant amount of heat, according as it emanates 

 from one substance or the other, the quantity of ice melted 

 would not constitute a pure quantity for the amount of heat 

 radiated by ditFerent substances, upon the calculation of which 

 the whole determination rests. 



The results communicated moreover lead to a neio method of 

 ascertaining whether any substance transmits rays of heat or not. 



In the first investigations on this point the diathermancy of 

 certain substances was considered to occur when, on inserting 

 them before a source of heat, effects upon the thermoscope were 

 obtained which could not arise from the introduced media, either 

 because these had been made imperceptible to the instrument, 

 or because these effects were diminished by the means which 

 increased the absorption (see p. 191 and pp. 203, 204). 



This method however presupposes a certain intensity of the 

 transmission, and would not have been applicable, e. g. in cases, 

 as those previously considered (p. 228 to 234), in which small 

 quantities only of radiant heat were concerned. For these I 

 therefore made use of the method already described (p. 231 to 

 234), i. e. I examined first the effects on the inserted substances 

 of absorption alone with different sources of heat, by impeding 

 the direct transmission of the heat by a coat of lamp-black, then 

 produced the transmission, and then concluded from the effects 

 observed whether in fact it arose from absorption or not. 



The new method, which yields nothing to the former in delicacy, 

 has the advantage of not requiring the substance to be coated 

 w ith lamp-black. It will be most easily illustrated by an example. 



Suppose it was required to be ascertained if ivory is diather- 

 manous or not. 



To decide this, a plate of any substance known to be adia- 

 thermanous, as wood, pasteboard or charcoal, is heated by the 

 rays of an Argand lamp in such a manner that the radiation 

 upon the pile through a diaphragm produces a certain deflection, 

 e.g. of 35°, in the multiplier. A diathermanous substance is 

 then inserted before the thermoscope on this side of the perfo- 

 rated screen. The needle then recedes, e.g. with red glass 1'5 

 millim. in thickness to 10'^*25. The ivory plate is subjected to 



