166 Royal Society. 



of the colour-making substance in a given species of lichen, and 

 trace the stages through which it passes before the coloured sub- 

 stance is developed ; secondly, to determine the nature of the various 

 colouring substances which exist in the archil of commerce ; and 

 thirdly, to examine the colouring materials of ordinary litmus. He 

 finds in the lichen Roccella tinctoria the following bodies, either pre- 

 existing in the plant, or formed during the processes employed for 

 its analysis: 1. Erythryline ; 2. Erythrine (the Pseudo-erythrine of 

 Heeren); 3. Erythrine bitter ; 4. Telerythrine ; and 5. Roccelline 

 (the Roccellic acid of Heeren). TJie properties and constitution of 

 these substances are then described, and the chemical formulae given, 

 M'hich are deducible from their respective analyses. The author 

 finds the archil of commerce to consist essentially of three ingre- 

 dients, namely, oi'ceine, erythroleic acid, and azoerythrine ; of each 

 of the two former there exist two modifications, and there is, in ad- 

 dition, a yellow matter. After comparing his results with those ob- 

 tained by Heeren, by an examination of the products evolved by his 

 erythrine in contact with air and with ammonia, and stating reasons 

 for some changes in nomenclature, the author gives the chemical for- 

 mulas resulting from his own analysis of these diflPerent substances. 



His inquiries into the constitution of ordinary litmus, which form 

 the last division of his subject, lead him to the conclusion that that 

 substance contains the principles designated by him as Erythrolein, 

 Erythrolitmine, Azolitmine, and Spaniolitmine; and that the colour- 

 ing constituents of litmus are, in their natural condition, red ; the 

 blue substances being produced by combination with a base, the 

 bases in that of commerce being lime, potass, and ammonia ; and there 

 is mixed up in the mass a considerable quantity of chalk and sand. 

 The details of the analyses of these several substances, and the re- 

 sulting chemical formulae representing their constitution, are then 

 given. 



The concluding section of the paper is occupied by an inquiry 

 into the decoloration of the bodies which exist in archil and in lit- 

 mus. The latter of these, the author concludes, is reddened by acids, 

 in consequence of their removing the loosely combined ammonia by 

 Avhich the blue colour is produced ; and the so-called hydrogen-acids 

 liberate the colouring matter by thoir combining with the alkali to 

 form bodies (either chlorides or iodides), vvith which the colouring 

 matter has no tendency to unite. Hence it appears that the redden- 

 ing of litmus is no proof that chloride of hydrogen is an acid, and 

 that the double decomposition which occurs is the same in principle, 

 whether hydrogen or a fixed metal come into play. After detailing 

 the blanching efi'ects of other deoxydizing agents on the colouring 

 matter of litmus, and the action of chlorine on orceine and azolitmine, 

 the author remarks, that in these actions chlorine is subjected to con- 

 ditions difierent from those which determine the nature of the results 

 with the generality of organic bodies, and that the displacement of 

 hydrogen, so marked in other cases, does not exist in the class of 

 substances under consideration ; but that, in reality, the ])ro(lucts of 

 the bleaching energy of chlorine resemble in constitution the com- 

 pounds of chlorine which possess bleaching powers. 



