562 Prof. Johnston on ike Composition of certain Mineral 



in which we observe an interesting approximation to the 

 number of atoms of carbon in the retinic acid. In fact if 

 gum elemi were so changed that an atom of carbonic acid 

 should replace two of hydrogen, or colophony so that one of 

 carbonic oxide should replace two of hydrogen, retinic acid 

 would be formed, since 





C,iHi4 03or7(C3H,) + 30 



That the resinous matter formed upon or exuding from the 

 trees deposited in the Bovey coal field was ever identical in 

 constitution with recent pine resins, or that if so it has during 

 its long burial undergone a change so simple as that indicated 

 by the formula, it is impossible for us to determine ; neverthe- 

 less researches of this kind are, I think, likely to throw an 

 additional light on the nature and products of the vegetation 

 of remote epochs, correcting or confirming the deductions of 

 the fossil botanist, and it may be suggesting to him new in- 

 quiries. 



Salts of Retinic Acid, 



Retinate of Silver, — Alcoholic solutions of nitrate of silver 

 and of retinic acid give a slight precipitate when mixed, 

 which is determined however more fully by the addition of a 

 small quantity of ammonia. It is of a brown colour, but 

 speedily blackens by the action of the light. It is soluble to 

 a considerable extent in alcohol, giving a dark brown solution. 

 It is washed therefore with difficulty, and is in great part car- 

 ried through the filter before the purity of the remainder 

 can be depended upon. The filtered solution on standing 

 gradually deposits a black precipitate containing more silver, 

 due in all probability to a decomposition of the acid and re- 

 duction of the silver, or to the presence of some foreign re- 

 ducing substance. The small quantity which falls on the ad- 

 mixture of the two alcoholic solutions, previous to the addi- 

 tion of ammonia, contains also an excess of silver, which may 

 be due to a similar cause. 



Heated to 300° Fahr., this and all the other retinates give 

 out the peculiar resinous odour of the acid, and at a higher 

 temperature the metallic salts melt, give off combustible pro- 

 ducts, and leave a bulky charcoal. 



Three different portions, prepared by different processes 

 and more or less perfectly washed, left on burning metallic 

 silver equivalent to 



4.1-780, 42-822, 4.3*585 per cent. 



