000 PHILOSOPHICAL TRANSACTIONS. [ANNO 1784. 



being mixed with any alkaline liquor. The blue infusion of litmus is also a test 

 of the presence of fixed air in water, with which it turns red, as it does with 

 other acids. 



The great degree of sensibility of this test would leave very little reason to 

 search for any other, were there reason to believe that it is always a test of the 

 exact point of saturation of acids and alkalis, which the following fact seems to 

 call in question. I have observed, that a mixture of phlogisticated nitrous acid 

 with an alkali will appear to be acid, by the test of litmus, when other tests, 

 such as the infusion of the petals of the scarlet rose, of the blue iris, of violets, 

 and of other flowers, will show the same liquor to be alkaline, by turning green 

 so very evidently as to leave no doubt. 



At the time I made this discovery, the scarlet roses and several other flowers, 

 whose petals change their colour by acids and alkalis, were in flower. I stained 

 paper with their juices, and found that it was not affected by the phlogisticated 

 nitrous acid, except in so far as it acted the part of a neutralizing acid; but I 

 found also, that paper, stained in this manner, was by no means so easily affected 

 by acids of any kind as litmus was, and that in a short time it lost much of that 

 degree of sensibility it possessed. Having occasion in winter to repeat some ex- 

 periments, in which the phlogisticated nitrous acid was concerned, I found my 

 stained paper almost useless. I was therefore obliged to search for some substi- 

 tute among the few vegetables which then existed in a growing state; of these 



1 found the red cabbage (brassica rubra) to furnish the best test, and in its fresh 

 state to have more sensibility both to acids and alkalis than litmus, and to afford 

 a more decisive test, from its being naturally blue, turning green with alkalis, 

 and red with acids; to which is joined the advantage of its not being affected by 

 phlogisticated nitrous acid any further than it acts as a real acid. 



To extract the colouring matter, take those leaves of the cabbage which are 

 freshest, and have most colour; cut out the larger stems, and mince the thin 

 parts of the leaves very small; then digest them in water, about the heat of 120 

 degrees, for a few hours, and they will yield a blue liquor, which, if used im- 

 mediately as a test, will be found to possess great sensibility. But as this liquor 

 is very subject to turn acid and putrid, and to lose its sensibility, when it is 

 wanted to be preserved for future use, the following processes succeed the best. 

 1. After having minced the leaves, spread them on paper, and dry them in a 

 gentle heat; when perfectly dry, put them up in glass bottles well corked; and 

 when you want to use them, acidulate some water with vitriolic acid, and digest, 

 or infuse, the dry leaves in it till they give out their colour; then strain the 

 liquor through a cloth, and add to it a quantity of fine whiting or chalk, stirring 

 it frequently till it becomes of a true blue colour, neither inclining to green or 

 purple; as soon as you perceive that it has acquired this colour, filter it immedi- 



