
PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES, 507 
the acuteness of the angle, or, in other words, the relative length of the un- 
equal axis, increases with the increase of the equivalent volume. 
Equivalent. 8.G. Equiv. Vol. Atomic Vol. Angle. 
CaC 100 272 36:8 7:36 105° 5’ 
2FeC + 3MgC + 5Cad 99°6 3-02 32:9 6:58 106° 12’ 
CaC +MgC 94 2°88 32-6 652 106° 15’ 
MnG 115 3°59 32:0 6-40 106° 51’ 
FeG 116 3°83 30:3 6-06 107° 
FeC + MgC 102 3-42 29°5 5:90 107° 14’ 
Me 88 3-04 28:9 5°78 107° 29’ 
InG 125 4-44 28:1 5-62 107° 40’ 
The above law requires that if any one of the minerals in this table * be 
displaced by—7.¢. pseudomorphosed into—any member standing below it, the 
resulting crystals must be vacuous, or have what has been called “ fallen in ” 
sides. 
Of about two hundred and twenty pseudomorphs in the author’s cabinet, 
there are only twenty-nine which throw any light on this particular point. 
From two localities there are specimens of bitter spar replacing calcite, and 
both are hollow; there is one specimen of calamine after calcite, and one of 
siderite replacing calcite, which are both also vacuous. These bear out the law 
enunciated.t 
There is in the author’s cabinet one other case (occurring, however, in 
numerous localities), which from its being a replacement in equal number of 
atoms is a direct case in point; this is hematite replacing calcite, the resulting 
crystals being always vacuous. The subjoined formulation shows the accord- 
ance with the law. 
Hematite. Calcite. 
Fe,0, CaO,CO, 
160 = 4:9 = 326 100 + 2°72 = 368. 
“ True chemical interchange,” however, does not always demand equality in 
the number of interchanging atoms. Professor Fucus long since noticed that 
* From which the calculated angles and specific gravities before given have been derived. 
+ Being desirous to test the law more fully, the writer bethought him of a collection procured by 
the late Professor Jameson, and which is thus referred to at page 42 of the English edition of BiscHor’s 
“Chemical Geology.” ‘‘ This petrifaction is in the Edinburgh Museum of Natural History. Professor 
JAMESON commissioned Dr Krantz of Bonn to collect the most important minerals, pseudomorphs, &c., 
which are mentioned in the German edition of this work. I have closely examined this collection, 
which consists of 664 specimens, and have found many which illustrate the phenomena described much 
more clearly than the minerals which I used. I shall, therefore, frequently take occasion to refer to 
especially characteristic specimens in this collection.” Upon application being made to the Director of 
the Industrial Museum, all that could at first be found of this collection was the catalogue; though a 
certain number of specimens supposed to belong to it were afterwards seen :—it would appear that one of 
the results of the transference of the University collection has been the incorporation of this specially 
geologic collection with the mineralogic suite; where it is practically useless, and positively an eyesore. 
It is so far satisfactory to know that an attempt may be made by the Director to separate it. 
