382 Prof. Bunsen on the Radical 



triim, compared with the luminous one, will likewise give the 

 difference of aberrations, it' any, by means of a residual part 

 which the refraction of the heat-rays considered as coming 

 from the luminous centre will not account for. For this pur- 

 pose the thermal experiments ought to be continued for some 

 time after the sun has passed the aperture in the window-shut- 

 ter, and the thermometers or galvanometers ought to be ar- 

 ranged parallel to the ecliptic. 



London, Jan. 2, 1842. S. M. D. 



LVII. On the Radical of the Cacodyl Series of Compounds. By 

 Professor Bunsen of Marburg*. 



1. Isolation of Cacodyl. 

 QOME of the cacodyl compounds have the remarkable pro- 

 perty of being decomposed by metals. When suiphuret 

 of cacodyl is heated in contact with mercury in a large vessel 

 to 200° or 300° C, the mercury becomes covered with a stra- 

 tum of suiphuret of mercury, without any apparent disen- 

 gagement of gas. The Huid which condenses in the vessel 

 gives off fumes and takes fire of itself in air, if the heat has 

 been continued long enough, and the temperature sufficiently 

 high. This process is, however, not available for the exhibi- 

 tion of cacodyl, as the mercury only acts upon the sulphur 

 compound of cacodyl at a temperature at which cacodyl 

 already begins to be decomposed. Bromide of cacodyl be- 

 haves in the same manner; under similar circumstances a 

 mixture of bromide of mercury, and a fluid which fumes in the 

 air, is produced : — 



Kd Br \ _ /Kd 

 Hg / 7 iHg Br. 

 When this mixture is boiled in water, the bromide of mercury 

 is reduced, and bromide of cacodyl is regenerated and given 

 off with the watery vapour : — 



Kd 1 TKd Br 



HgBrl = ^Hg 

 HO J Lh O. 



The last reduction also takes place at too high a temperature 

 for the exhibition of the radical. The isolation is most easil)^ 

 and perfectly effected by using a metal capable of decomposing 

 water and forming a chloride, particularly zinc, iron or tin. 

 When tin, or any of the foregoing metals, is added to anhy- 

 drous chloride of cacodyl, the metal is dissolved at a tempe- 

 rature of 90° to 100° C, without any evolution of gas. The 



• Communicated by the Chemical Society, having been read December 

 21,1841. 



