130 M. A. J. Angstrom on a Neiv Method of 



We conclude, then, from the foregoing — 



1. That Professor Cima's experiment is only another instance 

 showing how easily we can mistake one thing for another, and 

 induce others to do the same. 



2. That intuitive perception of relief may be indefinitely 

 increased in degree by exercise — showing that this sense follows 

 the same law under which we employ our other faculties. 



XVII. New Method of determining Jhe Thermal Conductibility of 

 Bodies. By A. J. Angstrom*. 



TO the properties of matter which have been the subject of 

 continuous investigation, the thermal conductibility of the 

 metals undoubtedly belongs ; but our knowledge of this import- 

 ant element is by no means so accurate or complete as we are 

 entitled to expect, and the following contribution may therefore 

 not be uninteresting. 



The methods hitherto used for determining the conducting 

 power are especially two. It has either been attempted, starting 

 from the formula 



*^r-* « 



to determine the heat which traverses a metal screen of the 

 thickness Ax, when its two surfaces have the temperatures u 

 and v! ; or the distribution of heat has been observed in a 

 metal bar of constant temperature, the differential formula 



d 2 u hp . . 



first proposed by Biot, being taken as a basis ; in which case, as 

 well as in what follows, u indicates the temperature of a given 

 point of the bar, h the radiating power of the surface, k the 

 conducting power, p the perimeter of the bar, and w its section. 

 The first method appears to promise no great accuracy, and, 

 from a theoretical point of view, is not unobjectionable. For if 

 both surfaces are kept at a given temperature by contact with 

 steam or water, the conducting power of the metal screen, or, 

 more correctly, the value of Q, is modified to such an extent that, 

 as Peclet has found, the difference between various metals quite 

 disappears in comparison with the small conductibility which 

 water possesses. Peclet has endeavoured to obviate this error 

 by renewing, by means of a special apparatus, the layer of water 

 in contact with both surfaces as often as 1600 times in a minute. 

 In this way this source of error must doubtless have been less- 

 ened, although it cannot be said to have been entirely destroyed. 



* Translated from Poggendorff's Annalen, vol. cxiv. p. 513. 



