rtlOXIMATE PRINCIPLES OF SOME OF THE LICHENS. 
79 
repeated so long as the addition of the hypochloride of lime causes the production of 
the red colour ; for this shows that the lime solution still contains unoxidized colour- 
ing principle. Towards the end of the process, the bleaching solution should be 
added by only a few drops at a time, the mixture being carefully stirred between 
each addition. We have only to note how many measures of the bleaching liquor 
have been required to destroy the colouring matter in the solution, to determine the 
amount of the colouring principle it contained. The following are the results of 
trials with the same test liquor upon four varieties of lichen. 
Measures. 
Angola lichen required 200= TOO 
American lichen 120=0’60 
Cape lichen 035=0'17 
Lecanora tartarea from Germany, near Giessen . . 025 = 0' 12 
The amount of colouring principle in a lichen may also be directly determined 
by extracting the lichen with milk of lime, by precipitating by means of acetic acid, 
collecting the precipitate on a weighed filter, drying it at the ordinary temperature, 
and then weighing it. 
Evernia Prunastri. 
This lichen was examined in 1843 by Messrs. Rochleder and Heldt, who found 
in it a substance which they regarded as identical with lecanoric acid. The results of 
my examination are so different that I am under the necessity of supposing, either 
that the proximate principles of the Evernia Prunastri which grows in Germany are 
quite different from those of the same plant in Scotland, or, what is much more 
probable, that these gentlemen had not examined the true Evernia Prunastri, but 
some other lichen in its stead. The lichen on which I operated was pronounced to 
be the true Evernia Prunastri by Dr. Scouler and by two other botanists to whom I 
submitted it. The lichen was extracted by milk of lime in the way already so often 
described. Its solution had a bright yellow colour, and on being neutralized by 
muriatic acid, it yielded an abundant pale yellow precipitate. This was washed, 
collected on a filter and cautiously dried. It was then digested, at a temperature 
much under that of boiling, in a quantity of very weak spirits, and this digestion 
was repeated several times till about two-thirds of the precipitate had dissolved. On 
the cooling of the solution a mass of small yellowish crystals was deposited. These 
crystals were rendered perfectly colourless by being treated with animal charcoal, 
and by being repeatedly crystallized out of weak spirits. They constitute a new 
acid, which I purpose to call Evernesic acid. The portion of the precipitate which 
did not dissolve in the weak spirits is usnic acid, which requires pretty strong boiling 
alcohol for its solution. The usnic acid will be again referred to at a subsequent 
part of this paper. 
