692 SCIENCE PROGRESS 



or a Wells to conceive a world in which these substances might 



form the materials of everyday life. Many of them are intensely 



poisonous, others are decomposed by traces of moisture, and 



others again are spontaneously inflammable or even explosive 



in air. 



Cacodyl Derivatives 



The first worker in this field was Bunsen, who during the 

 period 1837 — 1843 undertook the study of organic derivatives of 

 arsenic. It had long been known that by distilling a mixture 

 of white arsenic (arsenious oxide) and potassium acetate a fuming 

 oily liquid was obtained having very poisonous properties and a 

 most disagreeable odour. This uninviting product, known as 

 Cadet's liquid, was examined systematically by Bunsen, who 

 showed that the pungent constituents of the mixture were two 

 substances containing arsenic. 



The main constituent contained the metalloid associated 

 with carbon, hydrogen, and oxygen ; the compound present in 

 smaller amount consisted of the three elements, arsenic, carbon, 

 and hydrogen. Both compounds were extremely poisonous. 



Bunsen's analyses showed that the oxygenated compound 

 had a composition indicated by the formula As 2 C 4 H 12 0. The 

 non-oxygenated compound had the empirical formula AsC 2 H 6 , 

 but the vapour density gave its molecular formula as As 2 C 4 H 12 . 

 The former of these substances when distilled with hydrochloric 

 acid yielded a volatile oil with the molecular formula AsC 2 H 6 Cl, 

 and this compound, when heated with zinc in an inert atmo- 

 sphere, lost its chlorine and became converted into the compound 

 As 2 v^4.ri 12 . 



x / VsoC 4 H 12 < As 2 C 4 H 12 



(0 2 ) 



On examining the foregoing formulae it will be seen that 

 there is a group [AsC 2 H e ] common to all. Such a group is now 

 called a compound radical, and this particular group was among 

 the first compound radicals to be definitely recognised. 



At first the group was 'called by Bunsen alkarsin, but later, 

 at the suggestion of Berzelius, the name of cacodyl was adopted. 

 This discovery of a compound metal afforded at the time a 

 striking confirmation of the radical theory according to which 

 organic substances are composed of these groups or compound 



