504 PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 
specific gravity is increased as an effect, as will be afterwards shown; but 
certainly a porous pseudomorph can only result from the replacing substance 
being deposited in a bulk which is smaller than that displaced. 
There follows—“ The twenty-eight minerals which occur in forms of carbonate 
of lime, are all less soluble than it is; the less soluble mineral displaces there- 
fore the more soluble. Jt appears that the displacing mineral is always less 
soluble than that which is displaced, consequently a mineral which never appears 
in the form of another, would have a greater affinity for water than those 
minerals which occur in its form. The greater solubility of the displaced in 
comparison with the replacing mineral will cause the porosity of the former 
to increase during the pseudomorphic change.” 
Here then we have it clearly enunciated that relative solubility is the deter- 
mining agent both of the interchange and of the alteration in bulk—that 
adhesion, in fact, is the acting force. 
At the very outset, however, exceptions to the facts above stated are near 
at hand. On the very opposite page Bischor writes—“ We see in some belem- 
nites not only the whole sheath, but also the alveole filled by sulphate of 
baryta.” The latter has twice the specific gravity of the fibrous calcite of 
belemnites, but, instead of producing porosity, it is said to have jil/ed the whole 
thing. 
Again, according to Kremers, the solubility of carbonate of lime in water is 
one part in 12,860;* that of carbonate of magnesia, one in 5070,+ or twice and 
a half as great; yet in our Kinkell pseudomorph the very soluble substance is 
that which has replaced the less soluble—has been deposited in the solid form, 
while the less soluble has been swept away; but this is in direct opposition to 
BiscuHor’s statement; according to him, it should “ never appear.” 
Turning, however, to page 175 of his third volume, it will be seen that it 
has appeared even to him ; and appeared, moreover, so very important that he 
has devoted nearly fifty pages, directly or indirectly, to its consideration. 
We read-—“ Carbonate of magnesia is decidedly more soluble in carbonic acid 
water than carbonate of lime ; consequently it might be expected that it would 
be displaced not only by the minerals that displace carbonate of lime, but also 
by carbonate of lime itself;—but no pseudomorphs after magnesite are known 
as yet. The conversion of calc spar into bitter spar, which is actually proved 
by the occurrence of pseudomorphs, seems to be an exception to the generally 
greater solubility of the displaced mineral than that by which it is displaced. 
The pseudomorphs are more or less hollow, which shows that the displaced 
mineral was more copiously dissolved than that by which it was replaced.” 
“Seems to be an exception.” ‘This is nothing but skimming over a fact 
* BINEAU says it is soluble in 62,500 parts of water;—Pxticor in 50,000 parts. 
+ Fouroray says in 2504 parts. 
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