NATURE OF CHEMICAL CHANGES. 15 



if hydrogen and oxygen prepared in any convenient way are 

 mixed together in certain proportions and then heated, a violent 

 explosive action takes place (Expt. 208), much heat being developed; 

 and, as a result, instead of the two dissimilar gases, hydrogen and 

 oxygen, we have steam produced, which on cooling becomes liquid 

 water. Here " combination " has taken place. 



Besides these two fundamental kinds of chemical action, others 

 are also known (in some cases, but not always, leading to change 

 of state) where the number of products is the same as the number 

 of materials ; all these changes, however, may be regarded as be- 

 ing really the nett result of changes of decomposition and of 

 combination taking place in succession : thus, for example 



Expt. 9. To cover a Steel Knife with Copper. Dissolve in 

 water a little of the substance termed chloride of copper, and in 

 the liquid place a steel knife blade or other polished steel object. 

 In a short time a red coating of metallic copper will be visible on 

 the surface of the steel ; and after some length of time all the 

 copper originally contained in the solution will be thus deposited, 

 so that no coating of copper can be any longer obtained on placing 

 another bright steel blade in the liquid. When this is the case, 

 the liquid will contain a substance termed chloride of iron. Here, 

 then, there were two original substances, steel (or iron) and 

 chloride of copper ; and there result the same number of products, 

 viz., copper and chloride of iron. The total chemical action may 

 be viewed as the sum of two actions taking place in succession ; 

 firstly, an action of decomposition whereby the chloride of copper 

 becomes broken up into two constituents known as copper and 

 chlorine (just as water gives rise to oxygen and hydrogen) ; and 

 secondly, an action of combination, whereby the chlorine thus 

 produced instantaneously combines with the iron forming chloride 

 of iron, the copper being left in the free state as metal. 



Expt. 10. To produce a Combustible Gas from a Watery- 

 Fluid and Zinc. In the last experiment no visible change of 

 state on the whole occurred, as we started with a solid and a 

 liquid (steel and chloride of copper solution) and ended with a 

 solid and a liquid (copper and chloride of iron solution). In the 

 following case, however, a solid becomes apparently liquefied and 

 a liquid gasefied as the result of chemical change. 



On to some small fragments of granulated zinc (Expt. 16) pour 

 a little diluted hydrochloric acid ; the zinc will rapidly dissolve, 

 and a copious effervescence or bubbling will take place, owing to 

 the escape of bubbles of a peculiar kind of gas which will take 

 fire on bringing a light to them. This gas is in fact the hydrogen 

 already referred to as a constituent of water. We start with two 



