182 Prof. Bunsen on Kakodylic Acid, 
and consequently the experiment agrees exceedingly well with 
the theory. 
Kakodylic acid forms large, glassy, perfectly transparent 
crystals, which are oblique quadrilateral prisms with oblique- 
angled unequal terminal faces; they belong to the trimetric 
system. ‘The substance is not altered by exposure to dry air, 
but is decomposed by moisture. It is less soluble in absolute 
alcohol than in water, but not at all soluble in zther. Among 
all the kakodyle compounds it is the only one which does not 
possess the least smell. In a toxicological point of view this 
substance is very remarkable; for although it contains more 
than 72 per cent of arsenic, and oxygen in the same propor- 
tion as arsenious acid, it does not exhibit the least poisonous 
properties: 8 grains dissolved in water were injected into the 
jugular vein of a rabbit, but produced neither death, nor 
indeed any symptom of a poisonous action. This unexpected 
fact is in perfect concordance with one which has not been as 
yet regarded, but which is evident in the pharmaco-dynamical 
properties of organic bodies, and on which is founded one 
of the characteristic distinctions between the inorganic bodies 
and those produced by the interference of vitality. Ifnamely 
certain matters are added to inorganic bodies, their phar- 
maco-dynamical effects are thereby somewhat altered, but 
not destroyed. If, on the contrary, they combine together to 
form organic substances, these properties are completely lost. 
Copper, mercury, lead and barium do not lose their poisonous 
qualities, whatever the soluble compound may be in which they 
are present. Carbon, hydrogen, nitrogen, and oxygen, which 
in strychnine, atropine and coniine form the most violent poi- 
sons, are altogether harmless in thecompoundsof proteine. ‘This 
fact receives a beautiful confirmation from the kakodylic acid ; 
arsenic as if combined by organic affinity has become a harm- 
less body. Kakodylic acid contains to 1 atom of radical 
4 atoms of oxygen, of which 1 atomis combined with hy- 
drogen to form basic water, which cannot be drawn out by 
heat, but only by stronger bases. _ Its formulais C* H® As? O® 
+ H?O, which has been deduced from new analyses made 
with chromate of lead: the formula which I had previously 
assumed was C* H® As? O* + H?O. The analyses were made 
with oxide of copper, and were therefore incorrect, for kako- 
dylic acid is one of the most difficultly combustible substances, 
and cannot be completely burnt with oxide of copper. 
The acid can support a very high temperature without 
undergoing decomposition: this commences at 200° C. The 
radical in this compound seems to possess a much greater 
stability than in the oxide ; I have observed that the acid in this 
