436 M. BECQUEREL ON CHEMICAL DECOMPOSITION AND 
lower part, which is contiguous to the clay. We then perceive a great 
number of crystals, derived from the regular octahedron, and exhibiting 
that golden hue and brilliancy of aspect which belong to the iodide of 
lead. 
Copper, subjected to the same: sort of action, gives at first a double 
iodide in white crystallized needles; and then, after the decomposition, 
we obtain some octahedral crystals. The iodide of silver also is ob- 
tained without difficulty. 
It is probable that the other metals whose iodides are insoluble would, 
with due precaution, afford similar results. The insoluble bromides 
and selenurets can unquestionably be obtained by the same process. We 
shall, in this place, confine ourselves to the bare mention of the fact, be- 
cause their formation flows from a general principle which has been suffi- 
ciently developed in this chapter to render new details unnecessary. 
ACCOUNT OF A GENERAL METHOD OF OBTAINING IN A CRYSTAL- 
LIZED STATE, SULPHUR, THE SULPHATE, AND THE CARBONATE 
OF BARYTES. 
The Principle employed in the Formation of those Compounds. 
We have already shown that when a body transferred by a current 
meets another body with which it forms an insoluble compound, the 
combination takes place, and the precipitate is produced instantaneously. 
But we have not yet adverted to that which happens when an acid en- 
counters a salt, the base of which has a greater affinity towards that acid 
than towards the acid with which it is actually combined. The follow- 
ing experiment will show what it is that happens in such a case. 
A glass tube several millimetres in diameter, open at each end, and 
containing in its lower part some very fine potter’s clay moistened with 
a solution of nitrate of potash, and in its upper part some common 
alcohol, is placed in another tube filled with a solution of sulphate of 
copper. The communication between the two liquids is then esta- 
blished externally by means of an arc, consisting of two plates of cop- 
per and lead soldered end to end, the copper side being immersed in 
the sulphate and the leaden side in the alcohol. The sulphate of 
copper is quickly decomposed, in consequence of the electric effects re- 
sulting from the reaction of the two liquids on each other, and that of 
the alcohol on the lead. The copper of the sulphate is reduced on the 
plate of copper, which is the negative pole ; the oxygen and the sulphu- 
ric acid are transferred to the side where the plate of lead is; but, in- 
stead of obtaining sulphate of lead, we see, after an interval of a few 
days, that a great number of octahedral crystals of nitrate of lead have 
been formed. This fact proves, beyond all doubt, that the sulphuric 
acid, in traversing the clay impregnated with nitrate of potash, decom- 
posed this salt and combined itself with the potash, because of its hay- 
