694 SCIENCE PROGRESS 



years afterwards, when in 191 3 Ehrlich unfolded to the Seven- 

 teenth International Congress of Medicine assembled in London 

 his wonderful story of the therapeutic application of the organo- 

 arsenic compound " salvarsan " or " 606." 



Cacodylic acid, in the form of its sodium salt, has been 

 suggested for medicinal use, but at present it is largely super- 

 seded by arsenical preparations based on atoxyl. 



It should be pointed out that at the time of Bunsen's 



researches the hydrocarbon radicals themselves had not been 



recognised. Subsequent researches by Kolbe, Frankland, 



Cahours, v. Baeyer and others elucidated the inner constitution 



of cacodyl, and it is now known that this compound radical 



consists of tervalent arsenic associated with two methyl radicals: 



CH 3x /CH3 CH 3x 



>As - As< >As - CI 



ch/ N:h 3 CH/ 



Cacodyl b.p. 170 . Cacodyl chloride b.p. 109 . 



CH 3x /CH 3 CH 3x ,.0 



>As - O - As< >As/ 



ch/ x:h 3 ch/ x oh 



Cacodyl oxide b.p. 120° Cacodylic acid. 



In all these compounds but the last the arsenic is tervalent ; in 

 cacodylic acid it is quinquevalent. 



Cahours found that an alloy of arsenic and sodium when 

 heated with methyl iodide yielded cacodyl and another arsenical 

 compound, trimethylarsine (I) containing arsenic associated with 

 three methyl groups. This substance gives rise to a large 

 number of derivatives, of which I shall only mention two. 



The direct addition of methyl iodide yields a salt-like 

 compound, tetramethylarsonium iodide (II.), and from this product 

 by the action of moist silver oxide one obtains the base 

 tetramethylarsonium hydroxide (III.), which is a strong caustic 

 alkali having properties resembling those of potassium 

 hydroxide : 



CH 3X CH 3 v y CH 3 CH 3 \ y CH 3 



CHa >-CH, -* CH3)As/ - CH :7 A< qh 



I. II. HI. 



Zinc and Mercury Alky Is 

 A few years after Bunsen's investigations E. Frankland found 

 that on heating the alkyl iodides with zinc, especially when the 

 metal is rendered more active by the addition of a small amount 



