TRANSACTIONS OP THE SECTIONS. 575 



would induce those who are in possession of specimens to ex- 

 amine them with attention, and especially in reference to empty 

 or filled cavities, and to veins or portions of foreign matter 

 which may exist in the mass. 



Remarks on the value of Optical Characters in the discrimina- 

 tion of Mineral SjJecies. By Sir David Brewster. 



If minerals were all formed from solutions containing the 

 same ingredients, having the same temperature, and crystallizing 

 in perfect tranquillity, the differences recognised by the chemist, 

 the crystallographer, and the optical observer would have no 

 existence ; but as this hypothetical state of the mineral, when 

 in a state of fluidity or solution, is inadmissible, we must consider 

 minerals as having been formed under the influence of many 

 disturbing causes. In order to illustrate this remark, the author 

 takes the case of chabasie, which he regards as a congeries of 

 several substances, formed in succession round a central rhomb 

 of the same mineral in a perfect state. The central rhomb has 

 a certain degree of double refraction, which is equal in all 

 parallel directions ; but there is another rhomb formed round 

 it which has a less double refraction, and each successive 

 I'homb has its double refraction successively diminishing till it 

 disappears altogether, at which period the form of the crystal 

 would be a cube. Beyond this neutral line an opposite kind 

 of double refraction appears, corresponding to a new series of 

 rhombs, deviating more and more from the cubical structure. 



Now it is very obvious that these changes may have, or rather 

 must have, taken place, either from some agitation in the fluid 

 which prevented its particles from assuming the perfect type 

 of the mineral, or from the addition or abstraction of some of 

 the ingredients of which the central rhomb was composed. 



If a crystallographer, therefore, were to examine such a 

 mineral, he would report to us only the condition of the outer 

 rhomb, while the chemist would detail to us the elements which 

 form the whole compound mass. The optical observer, how- 

 ever, is alone admitted into the secret, and his results are in- 

 fallible. The changes which take place in the optical characters 

 of minerals by heat, do not in the least affect their value, any 

 more than similar changes affect the ordinary characters which 

 are employed by mineralogists. The specific gravity of bodies 

 varies also with heat, and probably the hardness also of the 

 softer minerals ; and it is well known that changes of tempera- 

 ture not very great may drive off the more valuable ingredients 

 of minerals, and thus prevent the chemist from obtaining their 

 actual composition. 



