262 Prof. E. Wartmann's third Memoir on Induction. 



rays transmitted by various media, when these are the seat of 

 a sufficient electro-magnetic induction*. I have shown that 

 polarized calorific radiations are affected in a manner entirely 

 similarf. Are these effects limited to the cases in which they 

 have been detected ? Would not the action of the magnet 

 upon fluids, or rather on diaphanous and diathermanous bodies, 

 be capable of impressing on light and caloric other modifi- 

 cations hitherto unperceived ? The mere statement of these 

 questions is enough to indicate an unlimited field of research. 

 I shall content myself with describing the experiments which 

 I have made upon the rays of the spectrum, which may be 

 studied by processes capable of accurate admeasurement, and 

 whose theory is intimately connected with that of light. 



81. I long ago observed that the production of the rays, 

 their direction, number and distribution, are not affected by 

 the presence of a magnet in contact with the prism. I re- 

 peated these experiments, inducting magnetism in various 

 media through which the luminous bundle had to pass. 



82. My prism, of the finest flint glass, was made by Fraun- 

 hofer ; its angle of refraction is 45° 4' 20". 



83. The diaphanous bodies submitted to induction were 

 the following : — Among the gases, air and nitrous acid, dry 

 and moist: these were inclosed in a glass tube O^'ISO long 

 and O^'OOS in diameter. Lastly, among solids, a specimen of 

 very pure flint glass was selected, in the form of a square 

 prism 0'"-170 long and 0™-0195 the side. 



84. The rays were observed in a dark chamber with an 

 opening in the shutter of six metres, with an excellent comet- 

 seeker by Cauchoir. This instrument has a reduced opening 

 of 0'""069 and 0™"66 focal distance : it was used with a mag- 

 nifying power of seven times. 



85. The light sent horizontally by a heliostate with a sil- 

 vered or black mirror, or from a lamp, was polarized more or 

 less completely by reflexion or by its passage through a Nicol's 

 prism. A second similar prism served as analyser. 



80. The electro-magnet is formed of a horseshoe of very 

 soft iron, weighing 12'^'^*5, around which is wound a well- 

 annealed copper wire of 0™'003 diameter and 70™"8 long. 



* Philosophical Transactions, 1845, and Phil. Mag. vol.xxviii.p.2y4, &c. 



t L'lnstUut of May 6, 1846, No. 644. M. Riihmkorff has recently 

 repeated my experiments. He polarizes and analyses heat with Nicol's 

 prisms of large size. The induction is obtained by means of one of those 

 double helices, destined to reproduce the phacnomena discovered by I\Ir. 

 Faraday, and on which M. Biot has read a very favourable report to the 

 Institute. Acccrdina^ as the magnetism is engendered in this jjowerful 

 apparatus, or the voltaic circuit is broken, the deviation of the rheoinetric 

 needle varies several degrees. (October 1.) 



