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OF THE ELEMENTARY BODIES. 249 
But what holds good with respect to iridium, may be ima- 
gined to be the case from analogy with platinum, rhodium and 
palladium, although hitherto experimental proof is wanting. 
If the properties of the remaining metals are examined, we 
find here and there traces of analogous phenomena; but they 
have been so little followed out by experiment, that no positive 
conclusion regarding their existence can be drawn. I shall 
mention some of them. 
Copper, when reduced by hydrogen at a heat below that of 
redness, on exposure to air soon becomes converted throughout 
its mass into protoxide; and when it is triturated for some time 
with an equivalent quantity of sulphur, it combines with it, ac- 
cording to Bottcher’s experiment, producing flame and forming 
sulphuret of copper. If, however, the copper be reduced by hy- 
drogen at a red heat, still considerably below the temperature at 
which it softens and begins to melt, it remains for years un- 
changed by exposure to air, and cannot be made to combine with 
sulphur without the application of heat*. Iron, cobalt and 
nickel, when reduced by hydrogen below a red heat, inflame 
after they have cooled if exposed to the air, and if they are imme- 
diately placed in water to avoid their taking fire, they inflame 
when they are again removed and have become nearly dry, If we 
compare this behaviour with that of iron reduced by heat, and 
with iron in that state in which it forms the conductor of a gal- 
yanic current without becoming oxidized, it would certainly ap- 
pear that these peculiarities depended upon something more 
than a difference of mechanical condition. 
When phosphate of iron is reduced by charcoal in a testing 
crucible, a regulus of phosphuret of iron is obtained. Both of 
its constituents have great affinity for oxygen, and the products 
of the combination with oxygen have a remarkable tendency to 
combine anew. But nevertheless this compound (the phosphuret 
of iron) is oxidized with great difficulty by heat, and is not in the 
least affected by dilute acids, and in order to dissolve it by nitric 
acid this must be very concentrated, and the phosphuret of iron 
must be finely powdered and boiled with it for a considerable 
time. Neither the properties of the phosphorus nor of the iron 
* I take this opportunity of referring to the condition which copper as- 
sumes at a low red heat in a continuous stream of ammoniacal gas. On cool- 
ing it is pale gray, excessively brittle, and of a granular aspect. This certainly 
appears like a peculiar allotropic condition, 
