.'v and on the Isomeric Modifications of Selenium, 507 



at which it solidifies 150 degrees. Nevertheless as this last can- 

 not be fixed with precision, because selenium remains along time 

 pasty, we must believe that the point of solidification of selenium 

 is at 200 degrees, like its melting-point, and that it is repre- 

 sented by the moment at which melted selenium ceases to adhere 

 to the bulb of the thermometer which is plunged in it. 



The characters which these two chemists assign to selenium 

 are not very precise. Experiments which I have made on a mass 

 of selenium weighing 250 grms. have allowed me to fix them 

 better*. 



Selenium exists, in fact, in two isomeric modifications; the 

 vitreous modification and the metallic modification. In these two 

 states selenium presents very distinct physical properties ; and 

 the passage from the one to the other is attended by very curious 

 phsenomena, the production of which is not difficult. 



When selenium melts to a state of perfect liquidity, and is cast 

 in a small trench-shaped mould formed of thin sheet brass, or in 

 a glass tube, it solidifies in the form of a black mass with a bril- 

 liant surface, the fracture of which is exactly like that of black 

 glass or of obsidian, and which has by no means a metallic aspect. 

 This vitreous mass presents, when transparent, the colour of 

 ruby-red in very fine splinters. Its dust remains gray under the 

 pestle, and the red colour is not developeS. even by rubbing the 

 pestle on a sheet of paper. I have always obtained selenium in 

 this vitreous state, when, after having been melted, I have even 

 allowed it to cool slowly. 



■ But if selenium be so heated that its temperature rises very 

 slowly, at the moment at which the thermometer indicates 96 to 

 97 degrees, the temperature rises suddenly with great rapidity, 

 and in a few minutes it exceeds 200 to 230 degrees. If the sub- 

 stance is then examined, it is seen that its physical condition has 

 been completely changed. Its surface presents a bluish-gray 

 colour, and it has a fresh metallic lustre. Its fracture, instead 

 of being vitreous, is in very fine metallic grains, quite resembling 

 that of gray cast iron. Under the hammer the mass flattens 

 perceptibly before breaking, and the fracture has then very de- 

 cided metallic lustre. The file likewise produces surfaces having 



* At the time of publishing this memoir, I was acquainted with no other 

 researches than those I have named. Since then I have become acquainted 

 with a memoir, read by M. Mitscherhch at the Academy of Sciences in 

 Berlin in June 1855 ; that is at the time at which I was engaged on these 

 experiments. A translation of this memoir is printed in the Annates de 

 Chimie et de Physique for March 1856. It will be seen there, that M. Rit- 

 torf had observed before me, that selenium disengages heat at the moment 

 of its molecular transformation ; but the quantity of heat which he has 

 given is far less than that which I have found in my experiments. Not- 

 withstanding this, I have thought better to preserve the original text, which 

 gives my experiments in the order in which they were made. 



2 L2 



