SPN RR — <AIPRNRIRIINE 
Correspondence. 111 
pound bodies. 
These results call to mind a relation brought out by M. Hankel,* 
between the crystalline form of metallic iron (tesseral system) and 
oxyd of antimony (tesseral) on one side, and peroxyd of iron (rhombo- 
hedral) and metallic antimony (rhombohedral) on the other—an alternate 
relation like the preceding, having a resemblance to the above in this 
forded a generalization of a fact observed long since by Biot, namely, 
that substances not possessing double refraction may acquire it from 
tions which matter may undergo under the influence of torsion, trac- 
m 
Untempered steel, twisted in every direction, which retained perfectly 
the form which the torsion had given it, contrary to the opinion of 
some authors (Lagerhjelm and others) with regard to the elasticity of 
this metal, 
Lightning. —Communications to the Academy on cases of light- 
e globulaire,” “ globe fulminant,”) and according to 
es a 1 often accompanies strokes of ordinary lightning. 
'S desirable that the kind of Lightning conductor suggested ‘ 
* Poggendorfi’s Annalen. 
