546 Intelligence and Miscellaneous Articles. 



ON THE ALLOTROPY OP SELENIUM. BY M. HITTORF. 



It is well known that selenium is softened by heat, becomes semi- 

 fluid at 212°, and melts at a few degrees higher. In cooling, it be- 

 comes viscous, thickens more and more, like wax, and then solidifies 

 into a reddish mass, with a shining surface and a conchoidal and 

 vitreous fracture. Berzelius had already observed, that when the 

 cooling takes place very slowly, the selenium acquires a reddish 

 colour, a rough surface, and a dull granular fracture, but that it loses 

 this appearance when it is melted again and cooled rapidly. 



The author, having attentively studied these phaenomena, has 

 found that they are due to the existence of an allotropic modifica- 

 tion of selenium analogous to that presented by sulphur, and evi- 

 denced by the fact that the crystallized selenium melts without any 

 previous softening, but at a temperature of 422 0- 6 F. 



When the substance is melted and the temperature raised, for 

 instance to 428°, and left to cool with a thermometer immersed in 

 it, the temperature is seen to descend gradually without the ther- 

 mometer becoming stationary, without its even being possible to ob- 

 serve a single point where its cooling appears to slacken until it has 

 attained the temperature of the medium. At the same time the 

 selenium passes through all degrees of viscosity until at about 122° 

 it entirely solidifies into a resinous mass. In these circumstances 

 the substance has therefore solidified in the amorphous state without 

 having lost its latent heat of fusion ; and it may retain it indefinitely, 

 for it persists in this amorphous state at the ordinary temperature. 

 But it passes into the crystalline state when kept for some time at 

 a temperature between 176°-422°, and it then parts with its latent 

 heat. 



Between 176° and 212° the transformation requires several hours ; 

 and in this case the disengagement of heat which accompanies it is 

 not appreciable. Between 257° and 356° it is very rapid ; and if 

 we operate upon 20 grms. of selenium, which are heated in an oil- 

 bath, a thermometer immersed in the interior of the substance, after 

 having attained the temperature of the bath, will rapidly exceed it, 

 and will rise from 70° to 90° above it for some minutes. This 

 phenomenon is still more striking when a hot air-bath is substituted 

 for an oil-bath. In one experiment, in which the bath was heated 

 to 266°, the author observed the thermometer, after having risen 

 slowly to 257°, ascend suddenly to between 410° and 419°. 



When the selenium is employed in the state of powder, its meta- 

 morphosis is more rapid. In this case, even in a bath heated merely 

 to 212°, the crystalline state may be developed so rapidly that the 

 heat rises from 45° to 52° above that of the interior. 



These phenomena are exactly similar to those which are pre- 

 sented by sulphur. When this substance is strongly heated, and it is 

 then suddenly cooled, it is converted into an amorphous, soft and 

 elastic mass. At the ordinary temperature it passes gradually, but 

 very slowly, from this amorphous state into the hard and crystalline 

 state in which it is ordinarily met with. At a temperature ap- 

 proaching 212°, it passes in a few minutes into this state; and it is 

 well known that M. Regnault noticed in this case the temperature 

 of the soft sulphur rise spontaneously to 232° in a chamber heated 



