62 
DR. A. E. H. TUTTON ON THE MONOCLINIC DOUBLE SULPHATES. 
temperature. There can be no doubt, however, that each salt has its own particular 
crystallographic angular values, and that there are real although very minute 
angular differences between the crystals of the various salts of the series, in accordance 
with the law of Hauy, which the author (Tutton) considers he has fully substantiated. 
This law provides that, except when the symmetry imposes identity of crystal angles 
(as in the cubic system and the 90°, 45°, 60°, and 30° angles of the tetragonal and 
hexagonal systems), every substance is characterised by its own specific crystalline 
form, the cases of isomorphism being only close resemblances within the limits 
of identity of symmetry. 
