16 SCIENTIFIC AMUSEMENTS. 



materials, solid zinc and liquid solution of hydrochloric acid ; we 

 end with two products, gaseous hydrogen and a liquid, which is a 

 solution of the substance called chloride of zinc. In this experi- 

 ment, as in the preceding one, the total action may be regarded as 

 the sum of two changes ; firstly, the hydrochloric acid, being a 

 compound of chlorine and hydrogen, may be regarded as broken 

 up into these two constituents ; and secondly, the chlorine may 

 be viewed as combining with the zinc to form chloride of zinc as 

 fast as it is produced, just as it was regarded as combining with 

 the iron in the last experiment. The result of the two actions 

 jointly may be regarded as a displacement of one substance from 

 combination with a second by a third, which combines with the 

 second and forms a new product in so doing, whilst the first sub- 

 stance is set free. Thus in Expt. 9 the iron displaces the copper 

 from combination with chlorine, forming a compound of iron and 

 chlorine and setting the copper free ; and in Expt. 10 the zinc dis- 

 places the hydrogen from combination with the chlorine, forming 

 a compound of zinc and chlorine and setting the hydrogen free. 

 Chemical changes of this kind are accordingly spoken of as actions 

 of displacement or substitution. 



A somewhat more complex kind of chemical action is that 

 spoken of as double decomposition or double displacement. 



Expt. 11. To produce a Yellow Solid by Double Displace- 

 ment from two Colourless Fluids. Boil up in a test-tube (a thin 

 glass tube made for the purpose, sealed up at one end, obtainable 

 at the instrument dealers) as much chloride of lead as will lie on a 

 three-penny piece with a tablespoonful of distilled water ; let the 

 whole stand for an hour or two, and decant off some of the clear 

 colourless liquid into a wine-glass, and add to it a few drops of a 

 solution of iodide of potassium (also colourless). The liquid will 

 become yellow, and on standing a yellow solid substance will 

 gradually subside to the bottom, being said to be " precipitated " 

 from the liquid. 



In this experiment the two materials employed are respectively 

 chloride of lead, made up of the two constituents chlorine and 

 lead ; and iodide of potassium, made up of the two constituents 

 iodine and potassium. The total action may be regarded as made 

 up of two pairs of subordinate ones, viz., the chloride of lead is 

 decomposed into chlorine and lead, and the iodide of potassium is 

 similarly decomposed into iodine and potassium ; whilst the lead 

 and the iodine thus formed combine to produce iodide of lead, 

 which being solid and not dissolving easily in cold water, makes 

 its appearance in the solid state ; simultaneously the chlorine and 

 the potassium set free combine to form chloride of potassium, 



