METALS AND METALLOIDS 693 



radicals combined with elementary radicals. Berzelius wrote 

 of Bunsen's work, " The research is a foundation stone of the 

 theory of compound radicals of which cacodyl is the only one 

 the properties of which in every particular correspond with 

 those of the simple radicals." 



The analogy between the compound metal, cacodyl, and 

 two of the metallic elements, sodium and thallium, may be 

 illustrated as follows : 



Cacodyl. Cacodyl oxide. Cacodyl chloride. 



As 2 C 4 H 12 = [AsC 2 H 6 ] 2 (AsC 2 H 6 ) 2 AsC«>H 6 Cl 



Kd 2 Kd 2 KdCl 



Metal. Metallic oxide. Metallic chloride. 



2Na . Na 2 NaCl 



Tl 2 T1 2 T1C1 . 



From the large series described by Bunsen, two other 

 cacodyl derivatives may be selected for special mention. On 

 distilling cacodyl oxide with mercuric cyanide a well-defined 

 crystalline substance, cacodyl cyanide, was obtained. This 

 product is of interest, as consisting of a combination of two 

 of the first compound radicals (cacodyl and cyanogen, CN) to be 

 definitely recognised. 



Kd 2 + Hg(CN) 2 = HgO + 2 Kd . CN. 



Cacodyl cyanide is a terribly poisonous substance, a few 

 grains left to evaporate in a large room speedily attack the 

 occupants, producing tingling and numbness of hands and 

 feet, giddiness and finally unconsciousness. In addition to 

 this disagreeable property, the vapour of the compound is 

 explosive, and in attempting to determine the vapour density 

 Bunsen lost the sight of one eye. Nevertheless, he persisted 

 in the investigation and left on record a complete description of 

 this deadly substance. 



Cacodyl itself is spontaneously inflammable in air, but if 

 allowed only a moderate amount of free oxygen, or preferably if 

 oxidised with moist mercuric oxide, it changes successively into 

 cacodyl oxide Kd 2 and then to an extremely soluble compound 

 Kd O . OH which, having acidic properties, is termed cacodylic 

 acid. When compared with the cacodyl derivatives already 

 mentioned, it may seem extraordinary that this oxidised com- 

 pound, although containing 54 per cent, of soluble arsenic, is 

 nevertheless non-poisonous. Bunsen first observed this differ- 

 ence in 1843, and his observation remained fallow until seventy 



