136 SCIENTIFIC AMUSEMENTS. 



known which have no action of either kind ; such substances are 

 said to be neutral. Thus, if some pure common salt, or some 

 " Epsom salts " (sulphate of magnesia), be dissolved in water, the 

 resulting liquid will neither redden water made blue with tincture 

 of litmus, nor turn blue such water reddened by addition of a drop 

 of weak acid. 



Fill a good-sized dry bottle with ammonia gas, as in Expt. 75 ; 

 set it on the table mouth upwards, and quickly pour in two or 

 three teaspoonfuls of distilled water to which a few drops of 

 diluted hydrochloric acid have been added; and which, conse- 

 quently, possesses (to begin with) a strong acid reaction. Cork 

 the bottle, and shake up ; the ammonia gas will be dissolved, and 

 will chemically react on the hydrochloric acid, without, however, 

 producing any visible change ; owing to the absorption, there will 

 be a partial vacuum formed in the bottle, as in Expt. 77, so that 

 the cork will be pretty firmly pressed inwards by the atmospheric 

 pressure, and the air will rush into the bottle when the cork is 

 extracted. Pour the liquid out of the bottle into a little eva- 

 porating basin, and test it with a blue litmus paper ; no reddening 

 will now ensue unless too much acid have been added to the water. 

 Should this be the case, fill another bottle with ammonia gas, and 

 shake up the liquid a second time with this gas also. Finally, 

 evaporate the fluid either over a small flame, or preferably on the 

 water-bath (Expt. 89) ; a white solid residue will be left, which (if 

 pure) will be neutral in character as regards its reaction on litmus 

 colouring matter. This white solid is called " salammoniac," or 

 " chloride of ammonium" and is the product of the chemical action 

 taking place between the ammonia and the hydrochloric acid. 



If a few drops of diluted sulphuric acid be used instead of 

 hydrochloric acid in this experiment, precisely the same results as 

 regards appearance will be produced; the neutral solid ultimately 

 obtained in this case, however, will be different in chemical 

 character, being the salt called sulphate of ammonium. 



Expt. 139. To restore the Colour of Clothes spotted by Acid. 

 The colouring-matter of litmus is by no means the only one affected 

 by acids and alkalies, many other substances being known which 

 behave in a similar fashion, amongst which. may ^.be mentioned 

 ordinary ink, and most of the somewhat analogous black dye- 

 stuffs used for dyeing clothes. In experimenting with acids, even 

 with the greatest of care, you are pretty sure sooner or later to get 

 some spilt about or splashed on to your clothes, with the result of 

 burning holes in them if the acids are strong mineral ones, like oil 

 of vitriol ; or of making red spots, if weaker. In this latter case 

 the original colour of the cloth can often be made to reappear, 



