312 
DR. KANE ON THE CHEMICAL HISTORY OF ARCHIL AND LITMUS. 
The bodies C 18 H 10 N O 10 . H O, and C 18 H 10 N O 10 . Cu O, differ in solubility and co- 
lour, but are generated by reactions identical so far as principle is concerned. 
Are the coloured substances in archil and litmus themselves acids ? This is a ques- 
tion which I consider as being of very little consequence to science, particularly as 
the word acid has by the progress of research been deprived of almost all definite 
meaning in philosophic language, and can scarcely be used except for purposes of 
familiar illustration. These various substances do not redden litmus, but if browned 
turmeric paper be brought into contact with one of the red litmus bodies, the alkali 
is divided between the red and the yellow material, and the browning of the one dimi- 
nishes according as the other becomes blue. Whichever colouring matter is in ex- 
cess preponderates, and hence I consider that the usual characters of acidity are so 
slightly marked, as to render it more suitable to adopt the indifferent termination in 
ine, which I have used for all of the substances described in this paper except one. 
This is the erythroleic acid, and if chemists approve of the mode of distinction pro- 
posed for the two orceines, I should prefer indicating the difference between the ery- 
throleins in a similar way. The erythroleic acid should then be beterythrolein. and 
the other alpherythrolein. Whatever nomenclature chemists in general may adopt, 
I shall myself willingly accede to. 
One of the most singular and obscure properties of these colouring matters, is that 
of becoming colourless under a variety of circumstances, and of generating a series 
of compounds, which are white whilst they remain unchanged, but which, on expo- 
sure to the air, become coloured, and assume their original condition. These effects 
may be produced in many ways, of which I shall notice the more important. 
It is long known that sulphuretted hydrogen possesses the property of blanching 
litmus, but chemists are not agreed as to the precise manner in which it takes place. 
Some refer it to a deoxidation effected by the hydrogen, like that which the same gas 
produces on salts of the peroxide of iron ; others, and as I have found more accurately, 
look upon the sulphuretted hydrogen as combining unaltered with the colouring ma- 
terial ; these chemists being, however, in error in supposing the colouring matter 
to be blue. The grounds on which I base my opinion of the nature of this pheno- 
menon are the following. 
First. If a perfectly clear solution of an alkaline salt of a litmus or archil body be 
treated with pure sulphuretted hydrogen, a colourless and scarcely opalescent liquor 
is obtained, the quantity of sulphur deposited being nothing, or almost inappreciable. 
This liquor contains an alkaline sulphuret, as may be proved by its action on a salt 
of iron or of manganese. If it be boiled, the sulphuretted hydrogen is given off, and 
the liquor becomes reddish, but very long boiling, or else exposure to the air, is ne- 
cessary, that it may become fully red. It cannot be completely restored to its ori- 
ginal blue or violet colour without the addition of some more alkali. Taking azolit- 
mine as the standard, the reaction evidently is 
C 18 H 10 NO 10 .N H 3 + 2 S H = C 18 H 10 NO 10 .HS + SN H 4 . 
