CHAPTER IX COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

transformed into urea. The composition of both substances is identical, CN 2 H 4 O, but the structure, or disposition of, and connection between, the elements is different: in the ammonium cyanate one atom of nitrogen exists in the form of cyanogen, CN—that is, united with carbon—and the other as ammonium, NH 4 , but, as cyanic acid contains the hydroxyl radicle of carbonic acid, OH(CN), the ammonium in this salt is united with oxygen. The composition of this salt is best expressed by supposing one atom of the hydrogen in water to be replaced by ammonium and the other by cyanogen— i.e. that its composition is not symmetrical—whilst in urea both the nitrogen atoms are symmetrically and uniformly disposed as regards the radicle CO of carbonic acid: CO(NH 2 ) 2 . For this reason, urea is much more stable than ammonium cyanate, and therefore the latter, on being slightly heated in solution, is converted into urea. This remarkable isomeric transformation was discovered by Wöhler in 1828. Formamide, HCONH 2 , and hydrocyanic acid , HCN, as a nitrile, correspond with formic acid, HCOOH, and therefore ammonium formate, HCOONH 4 , and formamide, when acted on by heat and by substances which take up water (phosphoric anhydride) form hydrocyanic acid, HCN, whilst, under many conditions (for instance, on combining with hydrochloric acid in presence of water), this hydrocyanic acid forms formic acid and ammonia. Although containing hydrogen in the presence of two acid-forming elements—namely, carbon and nitrogen—hydrocyanic acid does not give an acid reaction with litmus (cyanic acid has very marked acid properties); but it forms salts , MCN , thus presenting the properties of a feeble acid, and for this reason is called an acid . The small amount of energy which it has is shown by the fact that the cyanides of the alkali metals—for instance, potassium cyanide (KHO + HCN = H 2 0 + KCN) in solution have a strongly alkaline reaction. If ammonia be passed over charcoal at a red heat, especially in the presence of an alkali, or if gaseous nitrogen be passed through a mixture of charcoal and an alkali (especially potash, KHO), and also if a mixture of nitrogenous organic substances and alkali be heated to a red heat, in all these cases the alkali metal