Dr. Macbride exposed a piece of rag dipped in lixivium tartari, and another tinged blue by the scrapings of raddishes, to the vapour arising from a large vat of melasses wash in high fermentation, without any change being effected in either, which could be supposed to proceed from an acid vapour. But as water impregnated with fixed air has evidently an acidulous taste, and it seemed probable that some of the vitriolic acid might be volatilized during the effervescence which proceeds from its admixture with the alkaline body, when the air is procured from these substances, it was apprehended that the antiseptic and sweetening powers of air thus obtained, might depend on the acid contained in it. In order to evince how far this conjecture was just, Doctor Percival was so obliging to assist me in suggesting and making the following experiments.
EXPERIMENT XXXIV.
Twenty drops of syrup of violets mixed with a glass-full of water were changed into a lively red by the addition of one drop of dilute spirit of vitriol. The season of the year did not allow us to use the fresh juices of vegetables, but this trial shews the genuineness of the syrup of violets, and that it was a sufficiently delicate test of acidity.
EXPERIMENT XXXV.
A paper besmeared with this syrup, was placed over a vessel which contained an effervescing mixture of chalk and oil of vitriol. No change of colour took place except in one small point[aa], which had probably been accidentally touched by the vitriolic acid.
EXPERIMENT XXXVI.
Twenty drops of the syrup of violets were added to a glass-full of water strongly impregnated with fixed air, after the method directed by Dr. Priestley[ab], but without any variation in the colour of the violets.
EXPERIMENT XXXVII.