410 THE SCIENTIFIC PAPERS OF 



became latent, a circumstance which would account for the greater 

 bulk assumed at that point. He might mention a circumstance, 

 analogous to that already given with regard to selenium, apper- 

 taining to iron itself. If an iron wire were heated to whiteness, 

 allowed gradually to cool, and the variations in its length and 

 colour observed, they would find that when it was almost becoming 

 black it suddenly lighted up again and expanded, showing that in 

 this wire of iron which was exposed to no sort of external heating 

 agency a sudden evolution of heat was produced. This evolution 

 could only be accounted for by the sudden departure, out of the 

 mass of iron, of latent heat. It was probable that the phenomena 

 brought forward in Mr. Wrightson's able paper might be found to 

 be the result also of such sudden changes of specific heat. 



In the discussion of the Paper 

 <" ON THE PHOTOPHONE," by PROFESSOR A. G. BELL, 



THE CHAIRMAN (F. J. Bramwell, F.R.S.) having invited Dr. 

 Siemens to explain his " Selenium Eye," 



DR. SIEMENS, F.R.S.,* said he had listened with intense interest 

 to the discourse which Professor Graham Bell had given. The 

 world had been astonished before with his invention of the tele- 

 phone, and now he came forward with an instrument equally marvel- 

 lous in its results. The property of selenium to alter its electrical 

 resistance under the influence of light, was, as had been stated, first 

 brought before the world by Mr. Willoughby Smith, and so remark- 

 able was this discovery that many physicists turned their attention 

 to the subject. His brother, Dr. Werner Siemens, took up the 

 inquiry with a view of determining the cause of this extraordinary 

 variation in resistance caused by light, and the conclusion to which 



* Excerpt Journal of the Society of Arts, Vol. XXIX. 1880-81, pp. 43, 44. 



