CLXIII.—10-Chloro-5 : 10-dihydrophenarsazine and its derivatives. Part IX. The synthesis of nitromethyldiphenylamine-6′-arsinic acids and their conversion into nitromethyl derivatives of 10-chloro-5 : 10-dihydrophenarsazine. Constitution of 10-chloro-5 : 10-dihydrophenarsazine