TiCHBORNE — On Dissociation hy Heat of Compounds. 183 
Eefore disposing of these experiments, I shonld wish to draw 
attention to some speculations upon this subject which have been pro- 
mulgated by Dr. Percy in his lecture on Chemical Geology — 
''It" (ferric oxide) '' is found pseudomorphous after iron pyrites ; 
it is a puzzle to know exactly how this pseudomorphism has taken 
place. If iron pyrites is exposed to the weathering action of the air, it 
is converted, and must be converted, first of all, into a common sul- 
phate of protoxide of iron. That becomes oxidized by exposure to 
the air passing to the state of a sulphate of sesquioxide of iron. That 
sulphate of sesquioxide of iron, by the action of water, decomposes 
into two salts — an acid salt, which will dissolve away, and a more 
basic salt which would remain. AVe can easily understand the trans- 
formation of iron pyrites under these conditions into a highly basic 
sulphate, and even iuto hydrated sesquioxide, because we have only 
to call into our aid bicarbonate of lime, which frequently occurs in 
mineral waters. The trace of sulphuric acid might be easily removed, 
and the gypsum formed washed away, leaving nothing but hydrated 
sesquioxide of iron in the manner described, and then exposed to a 
certain degree of heat, whereby the water of the hydrate may be 
expelled, and the red oxide left. But that theory will not suffice, 
because we find certain crystals of hydrated sesquioxide of iron which 
are partially converted on the outside into red oxide of iron. It is 
perfectly evident that the temperature sufficient to decompose the 
hydrate could never have existecl when such crystals were formed, be- 
cause, if it had, it is impossible to conceive that any nucleus of hy- 
drated sesquioxide of iron should remain. The formation of this 
anhydrous sesquioxide of iron in nature is yet unexplained satisfac- 
torily. We can produce it readily enough in our laboratories by means 
of heat, but there is no reason for supposing that in the condition in 
which it has been produced in nature, a high temperature has prevailed, 
or at least that remark applies to many cases of the formation of that 
mineral." 
As regards the ultimate removal of the sulphuric acid, it would 
seem, from the experiments detailed, that it is not absolutely necessary 
to call in the aid of a second base, such as lime, it being quite possible 
and probable that the same results may be obtained under the laws of 
dissociation by the aid of immense pressure and a long period of time, 
such as would be at work in geological changes. 
Again, as we see that we can have perfect dehydration even in th& 
presence of water or steam, the second difficulty which presents itself 
in the crystals of anhydrous oxide, containing the nucleus of hydrate^ 
is disposed of, and is explained by Ex. XYI. 
The chromic oxide behaves exactly in a similar manner, except that, 
ceteris 'paribus, the point of dissociation is higher, and therefore it re- 
quires more careful investigation to observe the changes produced by 
* " Chemical News," voh iv., p. 147. 
