907 



and quiclvHme (in the proportion of 25 parts of water, 1-lOth lirae, 

 and l-20th sal ammoniac for every part of lichen), for a period vary- 

 ing from eight to fourteen days ; and by this process, he says, he 

 never failed to develope all the colour which the plant was capable of 

 yielding. 



" Dr. Stenhouse, of London, one of our latest and best authorities 

 on the chemistry of the lichens, adds to an alcoholic infusion of the 

 lichen a solution of common bleaching-powder (chloride of lime), 

 whereby, if it contain certain colorific principles capable of develop- 

 ing, under the joint action of air, water, and ammonia, red colouring 

 matters, a fugitive, but distinct, blood-red colour will be exhibited. 

 The amount of this colorific matter may be estimated quantitatively 

 by noting the quantity of the chloride of lime solution required to 

 destroy this blood-red colour in diflferent cases ; or the same result 

 may be obtained by macerating, for a short period, in milk of lime, 

 filtering, precipitating the filtered liquid by acetic or muriatic acid, 

 collecting this precipitate on a weighed filter, drying at ordinary tem- 

 peratures and again weighing." 



The author entered into a full analysis of these tests and processes, 

 pointing out their respective advantages and disadvantages, and show- 

 ing their practical value and applications. He stated that he had 

 made use of these and various other tests in upwards of 300 experi- 

 ments (the details of which he at present reserves) ; and the one which 

 he employed to the greatest extent, because most uniformly appli- 

 cable, was Helot's ammonia test. The following combination is that 

 most favourable for the development of the colouring matter of the 

 lichens, viz., the presence 



1. Of water, as a solvent menstruum ; 



2. Of atmospheric oxygen ; 



3. Of ammonia, in the state of vapour, or in solution ; and 



4. Of a moderate degree of heat. 



And according as the proportion of these combining elements varies^ 

 so do the kind and amount of colour educed by them. This combi- 

 nation is the foundation of all the processes for the manufacture of the 

 lichen-dyes throughout the world, however different they may appear 

 to be in detail or results. 



" I believe it may come to be a matter of great commercial impor- 

 tance to discover, at home or abroad, some cheap and easily pro- 

 curable substitute for the Roccellas, which are gradually becoming 

 scarce, and consequently valuable in European commerce, having 

 sometimes fetched, in times of scarcity, no less than .£1000 per ton. 



