138 SCIENTIFIC AMUSEMENTS. 



as to give one colour with an acid liquid and another with an 

 alkaline one. The colour obtained by mashing up a red cabbage, 

 treating the pulp with alcohol, and squeezing out the fluid through 

 a calico bag, becomes green with alkalies, red with acids, and 

 purplish when the liquid is neutral. Violets and purple dahlias 

 contain similar colouring matters. The liquid obtained by 

 similarly treating cochineal (a kind of insect) with alcohol furnishes 

 when diluted with water a colour which is orange if the solution 

 is ever so faintly acid, but of a beautiful purple if alkaline. A 

 substance termed phenol-phthalei?i, when dissolved in spirits of 

 wine and used to test an alkaline watery solution, develops a fine 

 red or brilliant pink colour, which becomes absolutely colourless 

 on adding enough acid to neutralise the alkali. Turmeric test- 

 paper is prepared by digesting powdered turmeric root with 

 alcohol, straining off the fluid, and soaking fine white blotting- 

 paper in the dark orange-yellow solution. Alkalies turn this 

 paper brown, whilst acids restore the original yellow tint. 



As a general rule, a test-paper soaked in a strong solution 

 of an alterable colouring matter like those above mentioned, can 

 be used as a test for acids by previously adding to the coloured 

 fluid a minute quantity of alkali before soaking the paper; or, 

 conversely, as a test for alkali by adding a little acid to the 

 liquor. Red and Hue litmus papers are thus prepared, the 

 former serving as a test for alkalies, becoming blue when dipped 

 therein ; the latter as a test for acids, becoming red when brought 

 in contact therewith (Expt. 57); and similarly with other colouring 

 matters. 



If a test-paper suitable for distinguishing alkalies (like red 

 litmus paper) be moistened with water and exposed to air contain- 

 ing only a very minute quantity of ammonia gas, the alkaline 

 colour will soon appear, as the ammonia will dissolve in the water, 

 moistening the paper and neutralising the trace of acid present. 

 In the same kind of way acid vapours and gases in the air will 

 speedily redden a moistened blue litmus paper ; e.g., when such a 

 paper is held over hot vinegar, or over the mouth of a bottle of 

 fuming hydrochloric acid. Putrefying urine liberates ammonia, as 

 also does decaying horse-dung, wherefore a red litmus paper hung 

 up in a urinal or in a stable will speedily become blue. Decaying 

 fish produces an alkaline vapour closely resembling ammonia, 

 termed methylamine, which will similarly turn a delicate test-paper 

 blue. In all such cases the chemical action taking place is pre- 

 cisely parallel with that occurring in Expt. 138, viz., that the 

 gaseous or vaporous alkaline matter in the air is dissolved by the 

 moisture present in the paper, and reacts on the acid also present, 



