286 BUNSEN ON THE CACODYL SERIES. 
would be too high, for arseniate of copper would be formed 
along with the arsenious acid, as Dumas himself had conjec- 
tured. 
It appeared to me, therefore, that the greatest care was neces- 
sary in the preparation of the oxide of cacody] in order to deter- 
mine the question. 
Although it is impossibe to prevent the anhydrous substance 
from decomposing, with the formation of alcargen, by contact 
with the air, it may yet be accomplished by employing the appa- 
ratus formerly described as the distillation tube*. The method 
formerly given is however attended with some great inconyeni- 
ences, as we are compelled, in driving out all carbonic acid, to 
allow such a quantity of inflammable alcarsin vapour to pass 
out at the same time that everything near becomes covered 
with arsenious acid. It is most advisable, in order to avoid 
this inconvenience, to commence the distillation without dri- 
ving the carbonic acid out beforehand, and which can be per- 
formed without any danger, provided the leg destined to receive 
the condensed vapour be of sufficient length. The bursting of 
the apparatus from the tension of the vapour is prevented very 
easily, by keeping that portion of the tube which is destined for 
the reception of the same, cool in water at 8° to 10° C., and 
by hastening the distillation as much as possible. It is quite 
impossible to prevent an explosion if the distillation is carelessly 
conducted, or the heat so long continued that permanent gases 
are evolved, or allowing that portion of the bulb above the level 
of the liquid to become too hot. Should a drop of the liquid 
‘during the ebullition fall on that part, the whole apparatus is 
shattered to pieces, and an arsenious flame several feet high rises 
up, covering everything near with a black layer of offensive 
arsenic. It is therefore best to conduct this distillation behind 
some boards, in which a small glass pane is fixed, through 
which we can see how to regulate the operation. After it is 
finished the lamp can be removed by the operator by means of 
a string conveniently fixed in the board, and there is no more 
danger so soon as the apparatus has cooled. 
I have not thought it too great a digression to go into all 
these details, as nearly the whole of the compounds belonging 
to this series can be procured pure only in the moist way. 
The product of the distillation was divided into two portions. 
* Memoirs of the Chemical Society of London, p. 51. 
