DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 283 



The double sulphate of potash and cobalt presents a remark- 

 able dichroism. A plate normal to the bisecting line, observed 

 with a NichoPs prism, appears of a wine-yellow colour [cou- 

 leur de vin paille) or violet-red colour, according as the plane 

 of polarization is parallel or normal to the plane of the optical 

 axes. A plate parallel to that plane likewise appears violet-red 

 or rose-coloured with a shade of orange, according as the plane 

 of polarization is successively parallel or normal to the bisecting 

 line. 



The double sulphate of ammonia and cobalt is very slightly 

 dichroic, the two salts of nickel still less so. 



In all these salts the bisecting line is the axis of minimum 

 optical elasticity. 



Three kinds of facts result from the preceding experiments, 

 and present in some sort a resume of them. 



In the first place, it is seen that in many groups of substances 

 chemically and geometrically isomorphous, there is a similarity 

 of optical characters altogether comparable with the similarity 

 of their crystalline forms ; isomorphism itself permits of slight 

 deviations of almost the same order in the geometrical elements, 

 and the agreement is such as should be expected to attend a 

 specific difference of substance with an almost perfect identity 

 of structure. 



Such, for example, are the binarseniates and biphosphates of 

 potash and ammonia ; the double chlorides of copper and potas- 

 sium, of copper and ammonium ; the sulphates of zinc and mag- 

 nesia, the chromate of magnesia; the sulphates of baryta, of 

 strontia, oxide of lead ; then the neutral phosphate and the 

 arseniate of soda ; and the numerous group of double salts formed 



x 2 



