284 SENARMONT ON THE OPTICAL CHARACTERS OF 



by the union of the sulphates of potash or ammonia with nearly 

 all the oxides of the magnesian family. 



The almost perfect identity of optical characters is especially 

 remarkable in these latter salts, referable to a symmetrical 

 oblique prism. For not only do the values of the three prin- 

 cipal elasticities maintain the same proportion, but likewise the 

 homologous axes of elasticity have almost the same direction in 

 the crystal, and still this direction is not in this case a forced 

 consequence of the geometrical symmetry. 



However, exceptions are not wanting, for instance in the laevo- 

 and dextro-tartrates. These salts possess an imperfect geome- 

 trical isomorphism, because they present an undisguised hemi- 

 hedrism ; they are no longer chemically isomorphous, because, 

 although possessing the same elementary composition, they are 

 capable of entering into combination in definite proportions, and 

 of developing heat in the act of union, giving rise to a substance 

 differing from both in its figure, composition and chemical cha- 

 racters ; but still these non-isomorphous Isevo- and dextro-tar- 

 trates present a perfectly absolute identity in their phaenomena 

 of double refraction. 



Lastly, there is found a third class of compounds, geometri- 

 cally isomorphous by reason of their perfect similarity of form, 

 chemically isomorphous by reason of their elementary composi- 

 tion, their chemical characters and the capability of crystallizing 

 together, but which nevertheless possess optical characters alto- 

 gether opposite. 



Thus we have seen, in the hyposulphates of lime, of strontia 

 and of oxide of lead, the single axis of double refraction situated 

 in the direction sometimes of the maximum and sometimes of 

 the minimum optical elasticity ; the axis of mean elasticity 

 situated in the same manner in the chromate and sulphate of 

 potash, while there is a reciprocal exchange in the directions of 

 the axes of minimum and maximum elasticity ; finally, in arra- 

 gonite and carbonate of lead, the same exchange takes place 

 between the axes of minimum and mean elasticity, whilst the 

 axis of maximum elasticity retains a constant direction. 



The double tartrates of soda and potash, of soda and am- 

 monia, present the same differences, which are rendered still 

 more striking by a very remarkable dispersion of the optical 



