BUNSEN AND KIRCHHOFF 335 



specifically lighter than water: the melting-point should fall 

 as the pressure rises. This was then tested and found to be 

 actually the case. 



Bunsen's work on cacodyl and its compounds was also im- 

 portant. He recognised in this substance, consisting of 

 carbon, hydrogen, and arsenic in fixed proportions, and 

 first prepared and examined by him, a 'radicle,' that is to 

 say a group of atoms always remaining together, which be- 

 haves like an element, and hence forms combinations of 

 various kinds. More than twenty years earlier, Gay- 

 Lussac had already discovered another radicle, cyanogen, 

 consisting of one atom of carbon and one of nitrogen, and 

 forming a first and particularly simple example of its class; 

 cacodyl was an example of a very complicated radicle, since 

 it contained altogether nine atoms, and led us again a step 

 further in our knowledge of the polyatomic substances of 

 organic chemistry. The investigation of the cacodyl series 

 may have particularly appealed to Bunsen's temperament; 

 it was a challenge to his skill to master substances which 

 were explosive, self-igniting, dangerous to life as poisons, 

 and of an unendurable odour even in traces. He had al- 

 ready previously found for the poison arsenic an absolutely 

 certain antidote - at that time a very important matter, since 

 arsenic was one of the commonest poisons. 



It is noteworthy that Bunsen, in his publications concern- 

 ing the cacodyl series, which belong to his youth (i 837-1 841), 

 makes use of the same atomic weights for representing the 

 composition of the compounds discovered by him, as are 

 known to be correct to-day (according to which, for ex- 

 ample, water is written HgO); but he later abandoned this 

 and took for example the atomic weight - or as he preferred 

 to say 'combining weight' - of oxygen as only eight times as 

 great as hydrogen (instead of sixteen times), as was the as- 

 sumption at a time when Avogadro's law was still unknown 

 or only an hypothesis; this leads to water being written HO, 



