DOUBLE COMBINATION. 19 



in water was used in the last experiment), whilst the comple- 

 mentary product iron chloride remains in solution. If the acid is 

 not too much diluted it is not necessary to heat the test-tube in 

 order to cause the action to take place. 



Expt. 14. To produce a Black Precipitate by means of a 

 Gas by Double Decomposition. Hold over the mouth of the 

 test-tube in the last experiment a piece of white blotting-paper 

 that has been dipped into a strong solution of silver nitrate (or of 

 lead nitrate). The paper will turn black because a precipitate 

 of black silver sulphide (or lead sulphide) is formed, just as in 

 the fifth glass in Expt. 12, hydrogen nitrate being produced as the 

 second product. 



Many substances give precipitates of characteristic colours and 

 special properties when treated with appropriate "reagents" in 

 this kind of way, and upon actions of this kind are based a large 

 proportion of the various " tests " used in analytical chemistry. 

 Thus the property of sulphuretted hydrogen gas to blacken paper 

 soaked in nitrate of silver solution, or other analogous compound, 

 is made use of in testing coal-gas to see if it is free from admix- 

 ture with that particular gas, which is not the case if the coal- 

 gas have been imperfectly purified during manufacture : if the 

 test-paper is blackened the gas is- still impure. 



In all such cases the total chemical change may be regarded as 

 the final result of a succession of actions of decomposition and 

 combination taking place between the various materials involved 

 in the reaction ; and in similar fashion the most complex chemical 

 actions known may be viewed as the final results of analogous 

 series of changes. 



Double Combination is the term applied to another kind of 

 chemical action where two different substances act upon one 

 another so as to produce two new products different from each 

 other and from each of the original substances, and where the 

 total action may be regarded as consisting of the breaking up of 

 one of the substances into two constituents, each of which thus 

 combines with part of the second substance. Thus in Expt. 243 

 the volatile fluid known as " carbon disulphide" and consisting 

 of a compound of the two materials carbon and sulphur, is burnt 

 (like spirits of wine), producing a flame and generating heat ; the 

 action of burning consisting in the chemical change taking place 

 between the carbon disulphide and the oxygen of the air, the 

 result of which is to form two new compounds, carbon dioxide and 

 sulphur dioxide, both of which are gases : the change may be 

 regarded as made up of two stages; first the carbon disulphide 

 breaks up into carbon and sulphur ; and secondly, each of these 



