330 
ON POISONING BY ARSENIOUS ACID. 
the same gas. He did not explain the nature of these spots, 
but affirmed that they could readily be distinguished from 
those of arsenic. 1. Because they are only partly volatile, 
and that with difficulty under a strong flame, at least when 
not exceedingly thin. 2. Because although cold nitric acid 
dissolves them rapidly, they never furnish on evaporation, 
a residue with which nitrate of silver will produce a brick-red 
color. Likewise, these spots cannot be obtained with a very 
small flame, although there may be much salt of zinc, lead, &c, 
in the apparatus, while the arsenical spots are produced 
readily with a feeble flame. MM. Thenard, Dumas, Regnault 
and Boussingault, have already obtained similar spots by 
sulphate of zinc. 
There are other spots which are readily produced even with 
a moderate flame, by allowing this to fall upon plates formed 
of certain kinds of pipe clay ; the oxides of lead and of tin 
which enter into the composition of the glazing applied to 
this kind of pottery, are reduced by the hydrogen, and there 
result spots either of a light or deep gray, and sometimes 
blackish, tarnished — slightly, or not at all volatile, and inso- 
luble in nitric acid, either hot or cold. These spots cannot be 
confounded with those from arsenic. 
In conclusion, these spots cannot be mistaken for arsenical 
spots, by any conversant with the subject. 
A chemist, accustomed to these researches will not be in 
the least degree embarrassed in establishing the distinction 
between these spots. 
A person of moderate ability, but attentive, will recognise, 
without difficulty, and solely by the character laid down, 
whether or not the spots are arsenical. 
M. Chevallier added to what M. Orfila had stated concerning 
the spots upon plate or pipe clay, that there were some spots 
which were due to carbon ; so that the plate used may afford 
spots not due to reduced lead, but resulting from the conver- 
sion into carbon, of substances which had penetrated the 
substance of the plate. He established that MM. Girardin 
and Morin had merited well of science by proving, in their 
