1864.] On the Colourmg -matters derived from CoaUtar. '34l 



without losing its solubility or passing into the pectous state, but at a tem- 

 perature near redness it undergoes a molecular change, losing at the same 

 time 2°42 per cent, of water. When water is added to unchanged tungstic 

 acid, it becomes pasty and adhesive hke gum ; and it forms a liquid with 

 about one-fourth its weight of water, which is so dense as to float glass. 

 The solution effervesces with carbonate of soda, and tungstic acid is evi- 

 dently associated with silicic and molybdic acids. The taste of tungstic acid 

 dissolved in water is not metallic or acid, but rather bitter and astringent. 

 Solutions of tungstic acid containing 5, 20, 50, 66*5, and 79'8 per cent, of 

 dry acid, possess the following densities at 19°, 1*0475, 1*2168, 1*8001, 

 2*396, and 3*243. Evaporated in vacuo liquid tungstic acid is colourless, 

 but becomes green in air from the deoxidating action of organic matter. 

 Liquid silicic acid is protected from pectizing when mixed with tungstic 

 acid, a circumstance probably connected with the formation of the double 

 compounds of these acids which M. Marignac has lately described. 



Molyhdic Acid has hitherto been known (like tungstic acid) only in the 

 insoluble form. Crystallized molybdate of soda dissolved in water is de- 

 composed by the gradual addition of hydrochloric acid in excess without 

 any immediate precipitation. The acid liquid thrown upon a dialyzer may 

 gelatinize after a few hours, but again liquefies spontaneously, when the 

 salts diffuse away. After a diffusion of three days, about 60 per cent, of 

 the molybdic acid remains behind in a pure condition. The solution of 

 pure molybdic acid is yellow, astringent to the taste, acid to test-paper, 

 and possesses much stability. The acid may be dried at 100°, and then 

 heated to 200° without losing its solubility. Soluble molybdic acid has the 

 same gummy aspect as soluble tungstic acid, and deliquesces slightly when 

 exposed to damp air. Both acids lose their colloidality when digested with 

 soda for a short time, and give a variety of crystallizable salts. 



XV. " Researches on the Colouring-matters derived from Coal-tar. 

 —III. Diphenylamine.'^ By A. W. Hofmann, LL.D., F.R.S., 

 Received June 16, 1864. 



In the course of last year * I published an account of some experiments 

 upon the composition of the blue colouring-matter discovered by MM. 

 Girard and De Laire when studying the action of aniline upon rosaniline. 



These experiments had established a simple relation between aniline-red 

 and aniline-blue, the latter exhibiting the composition of triphenylated 

 rosaniline, 



C,,H,N3, H,0 + 3^^ } N=3H3N + C,o^(.^]^- N3, H,0. 



The composition of aniline-blue has since been also investigated by M. 

 Schifff, who, in a paper published shortly after my first communication 



* Proceedings of the Royal Society, vol. xii. p. 578 ; and vol. xiii. p. 9. 

 t Comptes Rendus, Ivi. 1234. 



2c3 



