Crystallography of the Nitrosoterpenes of Dr. Tilden. 129 



sical. Chemical catalyses are those which consist of a true 

 chemical action; and are distinct from ordinary chemical 

 actions only in this, that one of the bodies remains in the 

 same state after the action as it was before it. 



According to this view of catalysis and of the constitution 

 of oxides, we retain the old terms protoxide, oxide, and per- 

 oxide (distinct from acid anhydride) with somewhat different 

 meanings. Oxide of copper (CuO) is no longer the protoxide 

 but the oxide ; the unknown yellow body is the hydrated per- 

 oxide, while Cu 2 is the protoxide. In like manner the oxygen 

 atoms which in peroxides are held with the least affinity may 

 be called the peroxygen atoms, and those which are the last 

 to leave the compound may be called the protoxygen atoms. 

 This nomenclature is distinct from the ordinary one founded 

 upon the use of the suffixes "ic" and " ous," and of nume- 

 rical prefixes, and need in no way interfere with it. Being 

 founded strictly upon the chemical behaviour of the compounds 

 and not upon the constitution of their molecules, it pos- 

 sesses evident advantages ; but the other terms have important 

 uses, and might with advantage be retained in the majority of 

 instances. 



XIX. Crystallography of the Nitrosoterpenes of Dr. Tilden. 



By N. S. Maskelyne, F.R.S.* 



[Plate VII. figs. 1-6.] 



THE varieties of nitrosoterpenes obtained in crystals by 

 Dr. Tilden belong to two crystalline types. The first 

 includes the substances formed (a) from ordinary turpentine, 

 (b) from French turpentine, (c) from juniper turpentine. 

 To the second type belong the substances obtained from the 

 oils of orange, of bergamot, and of caraway. 



I. First group. — The crystals of nitrosoterpene produced in 

 different ways from the American oil of turpentine have 

 already been described in connexion with Dr. Tilden's notice 

 of the substances in the Journal of the Chemical Society, 

 June 1875. They were of two kinds, differing in habit — the 

 one being twinned on the plane 01, and the other not evin- 

 cing any twin habit. The crystals obtained from French oil of 

 turpentine and from juniper oil are very dissimilar in appear- 

 ance to those made from the American oil ; but a goniome- 

 trical study of them proves that they belong to the same crys- 

 talline type with those previously investigated. The crystals 

 of the latter kinds furnished me by Dr. Tilden presented con- 

 siderable difficulty under measurement, since certain of the 

 faces are rounded ; and from their being very minute and in- 

 * Communicated by the Oystallological Society. 



