Notices respecting New Books. 77 



absolute units, or (since 1° C. is equal to 216 x 10 5 ) 2780° C. 

 This would thus be the greatest increase of temperature which 

 the element could produce with a constant current in a circuit if 

 the element itself were kept at a constant temperature. This 

 increase of temperature would in fact only take place if the ele- 

 ment or its poles could be cooled down to absolute zero; if, on 

 the other hand, we assume for this the temperature of 20° C. 

 above the freezing-point of water, we shall obtain from equa- 

 tion (17) 2502° C. as the greatest increase of temperature. 



In a thermoelectric element of copper-pyrites and copper inves- 

 tigated by Bunsen*, the electromotive force was about equal to 

 one tenth of a DanielPs element; the positive tension therefore 

 was about equal to 278° C. when one junction was heated to the 

 melting-point of tin, and the other to about 60° C. above the 

 freezing-point of water. In fact the greatest increase of tempe- 

 rature, if it were calculated as above, would be about 111° C. 



Yet it cannot be concluded from this that, for instance, the 

 last-named element could not produce a spark (and consequently 

 a far greater heating) through the interruption of the circuit ; I 

 am convinced, on the contrary, that this is possible. To show 

 with what extraordinary facility the electrical spark can result 

 from a break in the circuit, I will adduce the following experi- 

 ment. An electrical current, whose intensity in absolute mea- 

 sure was 20, was passed through a copper ware 1 millim. in 

 thickness. This was connected with a sharp knife-edge, with 

 which another part of the wire was scraped with rapid strokes. 

 It was then seen that in complete darkness there was still a 

 bright space between the steel knife-edge and the copper wire 

 when the distance between the two points of the copper wire 

 which were placed by the knife-edge in conducting communica- 

 tion was only 400 millim s. This facility with which a spark 

 results from the interruption of the current, however, shows 

 only that induction here plays an important part. 



VII. Notices respecting New Books. 



Light Science for Leisure Hours. Second Series. 

 By Richard A. Proctor, B.A. London : Longmans and Co. 



MR. PROCTOR tells us in his titlepage, quoting from Tenny- 

 son, " the truths of science [are] waiting to be caught;" and 

 numerous at the present day are the fishers for these truths. In the 

 work before us the author performs the office of urging scientific 

 facts combined with his own and others' opinions into the nets pre- 

 pared and spread for their reception, these nets being the minds of 

 his readers. In the short notice which we propose to give of this 

 * Pogg. Ann. vol. cxxiii. p. 505, 



