ACIDS AND ALKALIES. 135 



do damage. The mixture must be made cautiously and gradually 

 (see Expt. 98) in a stoneware vessel, such as a white jam-pot. The 

 black carbon thus formed is prepared on a large scale for making 

 ordinary blacking, treacle being used instead of sugar, and the 

 product being treated by certain processes to remove the acid, 

 which otherwise would destroy the boots to which it is applied. 



CHAPTER XI. 



SOLUTION OF GASES IN LIQUIDS AND SEPARATION OF GASES FROM 



LIQUIDS. 



Just as in the case of solution of solids by processes involving 

 chemical change, so with gases may the change produced be one 

 invisible to the eye, or one producing a visible alteration, either 

 in colour, or owing to the formation of a precipitate, &c. Of in- 

 visible changes, the following experiments furnish illustrations. 



Expt. 138. To produce a neutral Solid by the Action of Am- 

 monia Gas on an Acid Fluid. In Expt. 57 an illustration is 

 given of the action of acids on certain kinds of colouring matter, 

 viz., that derived from the litmus lichen; if a little tincture of 

 litmus (alcoholic solution of the colouring matter) be added to some 

 distilled water, and one drop of diluted hydrochloric acid be then 

 poured in and the whole stirred up, the liquid will turn bright 

 red, owing to the action of the acid on the colouring matter. 



This is a general property of all acids ; but there exists another 

 class of bodies, of which lime-water (Expt. 152), ammonia solution, 

 and common washing soda are examples, which possess exactly 

 the opposite property ; i.e., whilst acids turn the litmus-colouring 

 matter from blue to red, these other substances, or antacids, will 

 destroy this effect, and restore the original blue colour on addition 

 to the reddened fluid ; this may be easily verified by dividing the 

 red liquid obtained as above into three portions, and adding some 

 lime-water to one, some ammonia solution to the second, and a 

 little solution of washing soda to the third, when in each case, the 

 liquid will turn blue, if too much acid have not been added in the 

 first instance. Lime-water, ammonia, washing soda, and other 

 bodies that produce this effect, are said to exert an alkaline re- 

 action on colouring matters, whilst hydrochloric acid and similar 

 bodies exert an acid reaction. Many substances, however, are 



