200 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
that we do know are very different things indeed, both physically and 
chemically. Without knowing whether they are present or not, or how much of 
them are present, we jumble them all together, and set them down as one thing, 
in formulating minerals. Is it rational to set down as one thing that which we 
know to contain two or more perfectly different things, and call our elaboration 
a rational formula ? 
- In the analyses which are to follow, the “ silicas” which have come out of 
different minerals,—nay, the “silicas” which have come out of the same mineral 
from different localities,—nay, the “ silicas ” which have come out of more than 
one portion of the same specimen, analysed by treatment purposely identical, 
have proved so different as to be perplexing beyond measure. 
Tabulating the results of the researches of RAMMELSBERG, JENZSCH, VOM 
Ratu, and Gustav Rose, we have— 
Crystalline Silicas. 
Spec. Gray. 
: 1. Silica, insoluble in hot potash solution, . ; . 2°65 
Hexagonal, hemihedral, { 9, Silica, alabla ‘ 2+ 65 
Hexagonal, holohedral, 3. Tridymite, insoluble in hot potash solution, : - 2°25 
Prismatic, 4, Asmanite, . : ~ ee) . : : Bee gra 
Pyramidal, 5. IniZireony ~ . : 7 F ; ’ : b (2) 
Amorphous Silicas. 
6. Opal Silica, insoluble in hot potash solution, : « 2°18 
7. Opal Silica, soluble 2. rR a a 
8. Jenzschite, soluble in hot potash Olabion, : : «2104: 
2 and 8 possibly are the same,—the interpenetration of a moderate amount of 
a colloid within a much greater amount of a crystalloid might not materially 
interfere with the optical properties of the latter ; and RAmMELsBERG found but 
a small quantity of 2 in the general mass of the crystals of 1. 
That some of these modifications, if the word may be used, are mutually 
convertible into each other is unquestionable ; and that the processes employed 
_ in our analyses must and do so convert them is also unquestionable ; but this 
only renders it the more difficult for us to ascertain in what state or states the 
silica was. originally present in the mineral ;—or, in other words, which one, or 
which two, or how many of these modifications, and how much of each was 
present, before our operations induced changes upon them. 
Seeing that under the adoption of a uniform process we obtain the silica 
from certain minerals only im one state—from certain other minerals in two 
states—and from others in three, is it irrational to suppose that in these latter 
cases it had existed in the mineral in more states than one 2 
If this be granted, is it, to go a step further, irrational to suppose that one — 
of the modifications was in union with one ingredient, or one isomorphous set of 
ingredients—another modification with another set—the third with a third; — 

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