258 
Thoughts on Miner alogical Systems. 
[Sept, 
these axes, we should he able to refer it to its proper system, andconsequently, by 
combining other characters, to discriminate it and fix its name. Now the pheno- 
mena of polarisation, it appears, afford a means of effecting this ; and many happy 
applications of the principle have been made by Dr. Brewster, who in this manner 
has detected the crystalline forms of several substances till then doubtful or misun- 
derstood, which determinations have been afterwards confirmed by the crystallo- 
grapher, on obtaining more perfect specimens or a fuller series. But I believe 
that to examine the phenomena of polarisation in this way requires that the miner- 
al should be more or less translucent ; and that in the case of opaque minerals the 
optical mineralogist will be at a loss. Dr. Brewster is, however, we understand, pre- 
paring a system founded chiefly on these characters ; and till the work appears and 
the full practical application of the method is explained, it would be rash to decide on 
its value. That the method will afford valuable aids to the analytical method, how- 
ever, cannot admit of a doubt. 
11. There are other characters which, though not susceptible of numerical deter- 
mination, may yet be found of some use at least in the formation of the larger divi- 
sions. Colour is vague, and has a great range even in the same species ; but it is 
found, that some minerals which possess colour retain it when reduced to powder- 
while others in the state of powder are white, however bright their colours in the 
solid state. This property is sufficiently remarkable and definite to be applied in 
separating minerals into at least two grand classes. The same may be said of lustre, 
which in most of its modifications is sufficiently vague, and also variable in the same 
species : under one aspect alone it appears to be an essential character, and therefore 
such as may be relied on, and in this relation it is also definite : this is the kind of 
lustre called metallic, a term which requires no explanation. It is susceptible of one 
modification, which enables ns to proceed one. subdivision lower: it maybe retained 
or lost in the powder. 
12. There still remain a few characters which are in like manner applicable to 
the separation of minerals into two classes ; such as solubility in water, nearly synony- 
mous with taste ; solubility iu acids, in which it is to be distinguished whether they 
effervesce, gelatinise, or are quietly dissolved ; fusibility' before the blow pipe, or the 
contrary, each of which is susceptible of many sub-divisions. In general these cha- 
racters though they arc sufficiently definite and certain, yet belonging as they do 
to many minerals, they are better fitted for the establishment of the larger divisions ; 
while the specific gravity, hardness, and crystalline form, where it occurs, are the 
true tests of the species. The particulars relating to these characters, and the modi- 
fications of which they are susceptible, will be considered further on. At present 
it will be necessary to take a view of the remaining characters. 
13. These as they have been enumerated by mineralogical writers are ; fracture ; 
electric phenomena by friction or heat ; phosphorescence ; magDetism. The first 
character is utterly worthless *, and has long occupied a place iu our systems to the 
exclusion of more valuable matter. Writers have rung their wearisome changes 
on their conclroidal, even, and uneven fractures ; frequently making one mineral sus- 
ceptible ol every fracture, while hundreds are described in the very same terms. 
In reality the substance of many of these miscalled systems has consisted of little 
beyond a “ darkening of counsel by much words." And the disappointed student 
finds himself as far from the knowledge or discrimination of minerals, or farther 
even, than when he first began. I may speak thus of them without imputation of 
* presumption or unbecoming harshness ; for, I have had occasion to lament my 
dependence on them, and to pronounce them by sad experience “the persecutors of 
students and the thieves of time.” 
14. Electric phenomena are doubtless very useful in distinguishing species where 
other methods fail, but the application of this test is more nhvim.e in the smaller 
