184 SEVENTH REPORT—1837. 
30. In throwing out the suggestion Mitscherlich compares 
only the carbonates of lime and magnesia. I shall take a greater 
number of these carbonates in order to test it more closely*. 
Equivalent. Observed | Calculated | pig Angle of the Dif. 
specific grav. |specific grav. rhomboid. 
Cale spar o.seesssees 632-456 (2-721 | cesses | ceeees 105-4 Mit.| °..... 
Carb. of Magnesia...) 534:790 {2-884 2:30 =| 0584) 106°15 Mohs, 1-11 
Tron......... 71565 = |3°829 3:097 |0°75 | 107-0 1:56 
2 ; ; : 106-30 Phil. | 1-26 
ZANCssccseses 779663 + |3°379 3354 | 0-025 107-40 Woll. | 2:36 
Manganese | 722:337  |3:592 3:107 | 0°485) 106-51 Mohs.| 1-47 
A general agreement with the hypothesis is observable in 
these substances. The densities are all greater than they should 
be, compared with that of calc spar, and the acute angles of their 
crystals less, but no ratio is observable between the differences 
of density and of angle indicated by the 5th and 7th columns. 
The observed densities are those given by Mohs, as taken from 
crystallized specimens, but there is no evidence that the speci- 
mens measured were in any case those of which the density was 
also taken, so that in the absence of more correct data our test 
cannot be rigidly applied. Different crystals of the same sub- 
stance have not only different densities but also different an- 
gular dimensions. Breithaupt states that the crystals of horn- 
blende vary as much sometimes as 5°, those of pyroxene as 2°, 
and no doubt the density would vary in proportion. The same 
observer found the density of a calc spar of 105°°8’ to be 
2°741, and of another (tautokline) of 106°°10’ to be 2°968t, 
both of which cases are accordant with the notion that even in 
the same substance plesiomorphous differences may arise from 
condensation or expansion analogous to that produced by a di- 
minution or increase of temperature. All these examples show 
that our determinations of the angles and densities of crystal- 
lized bodies must be ranked among wncertain knowledge till 
accurate observations of both are made from one and the same 
specimen. Such results would enable us to try, it might be 
* Taking that of cale spar, in which the acute angle is greatest, as a stand- 
ard, the specific gravities of the other substances are compared with it and cal- 
culated from it. 
Sp. grav. of calc spar 
At. wt. of calc spar 
+ Karsten found in two specimens of pure cale spar that the one with the 
less angle had a density of 26978, that with the greater of 2°7064. 
X at. wt. of A= sp. grav. of A. 
