other substance, was melted on this ring by the ordinary
blowpipe, or even in some cases by the oxy-hydrogen blowpipe, and when the
drop, retained in its place by the ring, was thoroughly hot and fluid, a
platina wire from the opposite pole of the battery was made to touch it,
and the effects observed.

402. The following are various substances, taken from very different
classes chemically considered, which are subject to this law. The list
might, no doubt, be enormously extended; but I have not had time to do more
than confirm the law by a sufficient number of instances.

First, _water_.

Amongst _oxides_;--potassa, protoxide of lead, glass of antimony, protoxide
of antimony, oxide of bismuth.

_Chlorides_ of potassium, sodium, barium, strontium, calcium, magnesium,
manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver.

_Iodides_ of potassium, zinc and lead, protiodide of tin, periodide of
mercury; _fluoride_ of potassium; _cyanide_ of potassium; _sulpho-cyanide_
of potassium.

_Salts._ Chlorate of potassa; nitrates of potassa, soda, baryta, strontia,
lead, copper, and silver; sulphates of soda and lead, proto-sulphate of
mercury; phosphates of potassa, soda, lead, copper, phosphoric glass or
acid phosphate of lime; carbonates of potassa and soda, mingled and
separate; borax, borate of lead, per-borate of tin; chromate of potassa,
bi-chromate of potassa, chromate of lead; acetate of potassa.

_Sulphurets._ Sulphuret of antimony, sulphuret of potassium made by
reducing sulphate of potassa by hydrogen; ordinary sulphuret of potassa.

Silicated potassa; chameleon mineral.

403. It is highly interesting in the instances of those substances which
soften before they liquefy, to observe at what period the conducting power
is acquired, and to what degree it is exalted by perfect fluidity. Thus,
with the borate of lead, when heated by the lamp upon glass, it becomes as
soft as treacle, but it did not conduct, and it was only when urged by the
blowpipe and brought to a fair red heat, that it conducted. When rendered
quite liquid, it conducted with extreme facility.

404. I do not mean to deny that part of the increased conducting power in
these cases of softening was probably due to the elevation of temperature
(432. 445.); but I have no doubt that by far the greater part was due to
the influence of the general law already demonstrated, and which in these
instances came gradually, instead of suddenly,