42 DR. LAUDEK LINDSAY ON CHEMICAL BEACTION 



lichenologist ; or a solution may be made by shaking up the chlori- 

 nated powder with water, and employing the filtered fluid. On ex- 

 posure to the air, however made, the solution undergoes somewhat 

 rapid decomposition, the active principle, the hypochlorous acid, 

 being given off, and the inert carbonate of lime formed by ab- 

 sorption of carbonic acid. The liquid should therefore always be 

 freshly made when used. 



The reaction of bleaching-solution* with the colorific principles 

 of Lichens was pointed out by chemists more than 20 years ago. 

 In particular, its applications as a colorimetric test were dwelt 

 upon by Stenhouse. Following him I used this reagent very 

 largely in my first series of experiments on lichen dyes (1850-53). 

 The novelty of its present application consists, therefore, in its 

 being regarded as a means of discriminating hotanical species. 

 "What is its value in this respect the sequel will show. 



In 1853 I wTote thusf of the bleaching-solution test, my con- 

 clusions being based on several hundred experiments : — " This 

 test requires the greatest nicety and caution in applying it ; for, 

 from its strong bleaching or decolorizing power, the least excess 

 destroys the colour of any lichen-dye in solution in whatever 

 menstruum. The red colour is generally so delicate and fugitive 

 that, if an excess of the test have been originally added, no red re- 

 action may be perceived at all- Hence, from carelessness or 

 inaccuracy in manipulation on the part of the experimenter, a 

 very erroneous opinion may be formed of the colorific quality of 

 a given lichen. Perhaps the safest mode of using the test is in 

 the form of a very weak solution, which will admit of being added 

 in appreciable quantity. As the depth of tint of the red colour 

 struck is to the eye a measure of the quality of colorific mate- 

 rial contained in the lichen, so the amount of any given strength 

 of bleaching-solution required to destroy this red and convert it 

 into a pale wine-yellow, has been recommended by Stenhouse as 

 an easily appreciable and suflBciently accurate mode of estima- 

 ting the same thing quantitatively J. The strength of the bleaching- 



r 



* I have used throughout the present paper the term ** bleaching-solution " 



a convenient synonym 

 t MSS. inedit. 



placed 



archil-manufacturers, which goes to show that no trustworthy argument regard- 

 in*' the dye-yielding properties of Lichens, either qualitative or quantitative. 



can 



