COMPOUNDS CONTAINING NITROGEN. 547 



phate, CO(NH,) 2 .H 3 PO 4 . It also unites with metallic oxides, thus : CO(NH 3 ) a .- 

 HgO, and also with salts, of which HgCl 2 .CO(NH 2 ) 2 and HgO.CO(NH 2 ) 2 .HNO, 

 are examples. 



Ureids. Urea contains ammonia residues and, therefore, acts in many 

 respects like ammonia. Thus, one or more hydrogen atoms of the NH 3 groups 

 can be replaced by acid radicals, forming compounds analogous to the acid 

 amides. These are known as ureids. The following ureids are of interest. 



CO NH, 



Oxalyl urea, Parabanic acid, | \CO, is formed when uric 



CO NH X 



acid is boiled with concentrated nitric acid, and also when a mixture of urea 

 and oxalic acid is treated with phosphorus oxychloride,POC! 3 , which abstracts 

 the elements of water. It acts as an acid, since the hydrogen of the NH group 

 can be replaced by metals. 



Malonyl urea, Barbituric acid, CH 2 <JQ~*J|*>CO, like the previous 



compound, can also be obtained from uric acid, and synthetically by treating 

 a mixture of urea and malonic acid with phosphorus oxychloride. It breaks 

 up into urea and malonic acid when treated with an alkali. Closely related 

 to it is 



Veronal, Diethyl-malonyl urea, c 2 H 5>C< CO NH >CO ' which oc ~ 

 curs as a white, faintly bitter, crystalline powder, melting at 191 C., and sub- 

 lirnable without residue. It is soluble in about 145 parts of water at 20 C., in 

 about 12 parts of boiling water, and readily in warm alcohol. Veronal is one 

 of the most valuable hypnotics, being prompt and relatively innocuous in small 

 doses (8 grains), and dangerous only in large doses. 



Uric acid, C 5 H 4 N 4 O 3 , xanthine, C 5 H 4 N 4 O 2 , theobromine (dimethyl-xanthine), 

 C 5 H 3 (CH 3 ) 2 N 4 O 2 , and caffeine (theine, trimethyl-xanthine), C 5 H(CH 3 ) 3 N 4 O 2 + 

 H 2 O, are all interesting and important compounds, but rather complex, for a 

 discussion of which see Index. 



Cyanogen compounds. 



Cyanogen itself does not occur in nature, but compounds contain- 

 ing it are found in a few plants (amygdalin), and also in some animal 

 fluids (saliva contains sodium sulphocyanate). Gases issuing from 

 volcanoes (or from iron furnaces) sometimes contain cyanogen com- 

 pounds. Their formation from inorganic matter can be shown by the 

 action of ammonia on red-hot charcoal, when ammonium cyanide and 

 methane are generated : 



4NJEL, -f 3C = 2(NH 4 CN) -j- CH 4 . 



The univalent radical cyanogen, C = N, or CN, was the first 

 compound radical distinctly proved to exist by Gay-Lussac in 1814. 



