108 PRINCIPAL FORBES ON AN EXPERIMENTAL INQUIRY INTO 



lish, for a few points of a metallic bar, the difference between the superficial 

 and the internal temperature of the bar in any transverse section. This might 

 be done by thermo-electric methods, such as, I think, were used by the late M. 

 Langberg of Christiania in his experiments on the conduction of heat in bars. I 

 made some attempts (which were not unpromising) in a different way, by applying 

 to the surface of the bar small portions of fusible alloys or other substances, lique- 

 fying at definite temperatures. There did not appear to be much difficulty by 

 gently sliding these proof-pieces along the bar from the cooler towards the hotter 

 part, of ascertaining with considerable precision the co-ordinate of the superficial 

 point, corresponding to the fusing temperature of the alloy or other substance used. 

 The five following substances, in a descending scale, were found to have tolerably 

 definite fusing-points, and to be sufficiently suitable for the experiment :— Tin ; 

 solder (tin 9 parts, lead 5 by weight) ; fusible metal (consisting of bismuth 2 

 parts, lead 1 part, and tin 1 part by weight) ; napthalic acid ; and bees-wax. The 

 fusing temperatures of the three first were carefully ascertained by direct experi- 

 ment to be — 



Tin,* . 229°0 Cent. = 444°-2 Falir. 

 Solder, . 181°-6 „ =358° 9 .. 

 Fusible metal, 94M5 „ =20l°4 



The fusing points of the others were not ascertained by me. 



127. The experiments which I made in this manner were entirely tentative 

 and preliminary. The following is a specimen: — Statical experiment; 1851, 

 March 14. l|-inch bar, naked [see Table I., page 78 of this paper.] " At l h 40 m 

 I tried the following experiment to test the difference of temperature of interior 

 and exterior of bar. Taking small sharp-pointed pieces of tin [and] fusible 

 metal (prepared on purpose, bismuth 2, lead 1, tin 1 by weight), I rubbed them 

 gently on the surface of the iron bar till I found the melting point, keeping 

 them gently in motion so as not to allow the surface to heat beneath them. I 

 fixed these points with very considerable exactness, in the case of the fusible 

 metal (the best observation), to perhaps within ^th inch. I did not find the 

 position sensibly [to] vary on the centre of the top, and on the centre of the side 

 of the bar, nor even towards the angle of the bar (with the fusible metal). These 

 experiments deserve repetition. 



" l h 40 ra Tin melted when rubbed on ) 



the centre of one side of the bar, j> from origin at the edge of the crucible, 

 at . . ft. 065 in. ) 



Fusible metal, . . 10-45 „ ,, „ „ „ 



Bees- wax, . . 1 4 7 „ „ „ 



* The melting point of tin seems to be 'one of the best determined of the higher temperatures. 

 According to Crichton, Senior (of Glasgow), it is 442° Fahr. [T. Thomson]; Kupffer, 446°; 

 Daniell, 441°. On the melting point of lead, see Art. 70. 



