504 LECTURE LII. 



their operations, but produce precisely the same effects ; thus, an artificial 

 magnet, the force of which is quickly destroyed by heat, is affected more 

 slowly in a similar manner, when made to ring for a considerable time ; 

 and an electrical jar may be discharged, either by heating it, or by causing 

 it to sound by the friction of the finger. 



All these analogies are certainly favourable to the opinion of the 

 vibratory nature of heat, which has been sufficiently sanctioned by the 

 authority of the greatest philosophers of past times, and of the most sober 

 reasoners of the present. Those, however, who look up with unqualified 

 reverence to the dogmas of the modern schools of chemistry, will probably 

 long retain a partiality for the convenient, but superficial and inaccurate, 

 modes of reasoning, which have been founded on the favourite hypothesis 

 of the existence of caloric as a separate substance ; but it may be presumed 

 that in the end a careful and repeated examination of the facts, which have 

 been adduced in confutation of that system, will make a sufficient impres- 

 sion on the minds of the cultivators of chemistry, to induce them to listen 

 to a less objectionable theory. 



[Considerable advances have been made in our knowledge of the pro- 

 perties of heat since the first publication of these Lectures. They have 

 owed their existence, for the most part, to the discovery of a very delicate 

 measure of variation of temperature by means of its galvanic influence. 

 It is well known that when a current passes along a wire, a tangential 

 force is put in play which tends to cause the deflection of a magnetic 

 needle placed in the neighbourhood of the wire. (See additions to Lect. LV.) 

 Moreover it has been discovered that heat is capable of producing a gal- 

 vanic current, the intensity of which is proportional to that of the produ- 

 cing agent. This is exhibited in the following manner. A number of bars 

 of antimony and bismuth are arranged so as to lie compactly, whilst they 

 alternate with each other. They are then soldered together in pairs, so 

 that each bar of the one metal is connected at both ends with a bar of the 

 other. The extreme bars are united by means of a wire, and it is along 

 this that a galvanic current travels, on the application of heat to the sol- 

 dered ends of the bars. The instrument based on these principles, as sug- 

 gested by Becquerel and improved by Nobili, Melloni, and others, is called 

 a thermomultiplier. This name arises from the important fact, that by 

 crossing the wire so as to cause it to pass several times parallel to the mag- 

 netic needle, the simple effect may be increased to almost any extent, and 

 thus the instrument may be made to measure the minutest indications of 

 heat.* Amongst the earliest results of the use of this instrument was the 

 discovery of Melloni, that rock salt suffers heat of every kind to pass freely 

 through it, thus forming for heat a substance analogous to that which 

 glass constitutes for light.1* The field of discovery of the analogous pro- 

 perties of heat and light was now thrown open. That the former, as well 

 as the latter, suffers polarization under certain circumstances had been 

 conjectured by Berard and others. But the experiment on which the 



* For a description of the instrument see the Bibliotheque Universelle (new ser.) 

 ii. 225. Ann. de Ch. xlviii. 198. Ed. Tr. vol. xiii. 



f Ann. de Ch. liii., or Taylor's Scientific Memoirs, i. 32. 



