NEUTRALISATION. 155 



and the other a little starch paste with a drop or two of iodide of 

 potassium solution. Chlorine water dropped into the first, and 

 stirred up with the liquid, will discharge the blue colour, but will 

 cause a very similar tint to appear in the colourless fluid in the 

 second glass, thus apparently causing the liquids to change places. 

 The blue colour produced in the second glass can be again 

 discharged by dropping in a little solution of sulphurous acid, or of 

 the salt called sodium thiosulphate, sometimes termed sodium 

 hyposulphite, or hyposulphite of soda.. 



CHAPTER XII. 

 INTERMIXTURE OF LIQUIDS ACCOMPANIED BY CHEMICAL ACTION. 



The preceding experiments have shown that when two liquids 

 (solutions) are intermixed, visible change of state may occur, 

 through the formation of a precipitate of solid matter formed by 

 the chemical action and insoluble in the fluids present ; or through 

 the formation and evolution of a gas or vapour. But besides 

 actions of these kinds, liquids may act chemically upon one another 

 in such fashion that two dissimilar liquids unite to produce one 

 single resultant fluid, or may form by double decomposition two 

 different liquid products. The following experiments illustrate 

 these kinds of action. 



Expt. 165. To neutralise Sulphuric Acid. We have already 

 seen, in Expt. 138, that when a solution of hydrochloric acid or of 

 sulphuric acid is shaken up with air containing ammonia gas, the 

 ammonia is absorbed and neutralises the acid, forming a saline 

 combination, chloride or sulphate of ammonium. In just the same 

 kind of way sulphuric acid, dissolved in a moderately large quantity 

 of water, can be neutralised or converted into a neutral saline 

 combination by cautiously adding to it a solution of caustic soda or 

 potash. Place some diluted sulphuric acid in an evaporating 

 basin, add to it a few drops of .tincture of litmus so as to redden 

 the fluid, and then drop in with continual stirring a solution of 

 caustic potash until the red colour turns purple ; if too much 

 potash be added the fluid will become quite blue, in which case a 

 few drops more of dilute acid should be added. Evaporate the 

 liquid, and set by to crystallise ; potassium sulphate will deposit 

 in small crystals as the liquid cools. These may be collected on 



