DR. KANE ON THE CHEMICAL HISTORY OF ARCHIL AND LITMUS. 311 
equivalents, which can scarcely come within the limits of experimental error. The 
basic lead salt agrees well with C 26 H n 0 23 + 16 Pb O. 
It would be very wrong, with the limited data which were in my power to obtain, 
to fix decidedly upon one of these formulae to the absolute exclusion of the other. 
They both account for the existence of spaniolitmine completely, and the latter simply. 
For the elimination of the azote from bodies such as those now being described occurs 
but rarely, and should not consequently be supposed to occur in cases where it can 
be avoided. I leave, therefore, to future investigators in this field the determination 
of whether the equivalent of spaniolitmine should be considered C 18 H 7 G 16 , or C 26 
Hu O^. 
Section IV. Of the Decolorization of the Bodies which exist in Archil and in Litmus. 
The various coloured substances which have been described in the preceding sec- 
tions, will now require to be considered in some points of view, which being common 
in a great degree to all, would have led to useless repetition had their discussion been 
entered upon in the case of each individual substance. I have reserved, therefore, to 
the present stage of the investigation all general considerations, and shall now pro- 
ceed to the examination of such points of their history as yet remain unnoticed. 
The question as to whether the proper colour of litmus be red or blue, may now 
be considered as finally set at rest, and the theory of those changes of colour which 
have from so early an age in chemistry served for the recognition of alkaline or acid 
properties, is definitely laid down. Among the various suppositions made from time 
to time by chemists as to the nature of litmus, the idea of Perretti has ultimately 
been proved to be the most correct, although, as his views were not supported by any 
accurate experiment, no positive value could be attached to his opinion. Litmus is 
reddened by acids, because these acids remove the loosely combined ammonia by 
which the blue colour is produced, and the so-called hydrogen acids liberate the co- 
louring matter, inasmuch as they combine with the alkali to form bodies (chlorides or 
iodides) with which the colouring matter has no tendency to unite. Thus the my- 
sterious reddening of litmus paper, by which so many chemists of narrow ideas ima- 
gined that the most cogent philosophical proofs of the basic power of the compounds 
of hydrogen might be evaded, is found to be merely produced by the chloride of 
hydrogen, & c. forming on potash chloride of potassium, and with ammonia sal-am- 
moniac, and the proper red colouring matter becoming free. It is thus sufficiently 
shown that the reddening of litmus is no proof that chloride of hydrogen is an acid, 
but that the double decomposition which occurs is the same in principle, whether 
hydrogen or a fixed metal come into play, thus taking azolitmate of ammonia and 
chloride of hydrogen upon the one hand, and azolitmate of ammonia and chloride of 
copper on the other ; the decompositions are 
first, (C 18 H 10 N O 10 + N H, O) + Cl H . = (C 18 H 10 N O 10 + H O) + Cl N H 4 ; 
and second, (C 18 H !0 N O 10 + N H 4 O) + CICu = (C 18 H 10 N O 10 + Cu O) + Cl N H 4 . 
