196 Dr Thomson on Parietin, 



gests its utility in the laboratory. An alcoholic solution may be 

 kept for use, as the addition of a drop or two of the solution 

 to a considerable quantity of an alkaline liquor, will be imme- 

 diately followed by a change to red ; or the process may be 

 reversed, by placing a few drops of the alcoholic solution in a 

 test-glass, and adding to it a drop or two of the alkaline liquor. 

 The alcoholic solution may be prepared simply by digesting 

 the lichen in cold alcohol, of sp. gr. '840, as I have found that 

 a small portion of lichen will impart a colour to a large quan- 

 tity of alcohol, sufficiently intense to serve as a very delicate 

 test for alkalies. Observing the strong colour that the alco- 

 holic solution imparted to the filtering paper which was used 

 to purify the solution when first prepared, I cut these into test 

 papers, and found that, when properly impregnated with the 

 solution, they were little, if at all inferior to turmeric paper, 

 in their delicate detection of ammonia. Test paper may be 

 prepared extemporaneously from the alcoholic solution, when 

 it is wished to detect ammonia, by dipping a piece of paper 

 into the alcoholic solution, and then applying it in its wet state 

 to the ammoniacal vapour. The yellow colour is immediately 

 transformed into a reddish purple, but more distinct than the 

 colour that becomes apparent in turmeric paper of old prepa- 

 ration, under similar circumstances, which is a dirty brown. 

 One of the principal recommendations of the liquid test already 

 noticed, is the circumstance of its being capable of preserva- 

 tion without undergoing deterioration, while the test papers 

 which have been frequently recommended although possessing 

 most delicate testing powers when freshly prepared, gradually 

 lose their value by preservation. I believe this to be the ex- 

 planation of the failure in this country of some continental 

 test papers, which have been recently recommended. It would 

 therefore appear, that the best test paper being that which is 

 of fresh preparation, the most convenient source for its pro- 

 duction is that from which it can be most rapidly procured in 

 an efficient state. The observations which have been made 

 upon parietin, in reference to its colouring powers, tend to 

 show that it may be employed with advantage for the most 

 delicate purposes to which turmeric is applied. Parietin, how- 

 ever, is not acted on by acids ; the natural yellow colour merely 



