CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the quantity of heat given out during an oxida- 

 tion, the greater is the amount of work required 

 to be done, in order to effect the corresponding 

 reduction. Now, the difficulty of undoing a union, 

 or of breaking up a combination, may be taken as 

 a measure of its stability, so that the most stable 

 oxides are those during the formation of which 

 the most heat is given out 



Many oxidations can be effected directly by the 

 action of oxygen, and some reductions can be 

 effected directly by means of heat or electricity ; 

 thus, hydrogen unites with oxygen, or is burnt, 

 producing water ; and water can be decomposed 

 by heat or by electricity into hydrogen and oxygen. 

 There are, however, many oxidations which can- 

 not be effected by the direct action of oxygen, but 

 require the use of an ' oxidising agent ' that is, a 

 substance or mixture containing oxygen in combin- 

 ation, and ready to part with it to a body capable 

 of being oxidised. Thus, nitric acid contains a 

 large quantity of oxygen, and readily gives up 

 part or all of this oxygen to oxidisable substances ; 

 accordingly, nitric acid is a powerful oxidising 

 agent, and is used to oxidise bodies in cases where 

 the action can either not be produced at all, or not 

 so conveniently by oxygen alone. As an instance 

 of an oxidising agent which is a mixture, we men- 

 tion the mixture of chlorine and water. This 

 mixture will oxidise in many cases in which water 

 alone would be without action the tendency of 

 chlorine to unite with the hydrogen of the water 

 increasing the readiness of the oxygen to leave the 

 hydrogen, and unite with the substance to be 

 oxidised. 



Just as we have oxidising agents, so we have 

 also reducing agents that is, substances or mix- 

 tures which remove oxygen : these are generally 

 bodies having a tendency to combine with oxygen, 

 in other words, they are oxidisable ; and the most 

 powerful reducing agents are, as might be ex- 

 pected, those which give out most heat in uniting 

 with oxygen. By far the most practically import- 

 ant reducing agents are hydrogen and carbon, 

 either singly as hydrogen gas, and as charcoal or 

 coke, or combined, as they are in coals, coal-gas, 

 wood, and other ordinary combustibles. A large 

 number of metallurgical operations are cases of 

 reduction, in which coal, or coke or charcoal, is 

 used to take away the oxygen of a metallic oxide, 

 and leave the metal uncombined. It is in this 

 way, as is more fully explained in the article 

 METALLURGY, that iron, tin, zinc, and some other 

 metals are reduced from such of their ores as con- 

 tain the metals as oxides. 



THE DOCTRINE OF EQUIVALENTS, AND THE 

 ATOMIC THEORY AND NOTATION. 



We have already seen what is meant by an 

 'equivalent' of an acid, or of a base; we shall 

 now consider the question of ' equivalence ' some- 

 what more fully. As has been already stated, 

 zinc can turn copper out of a salt, such as sul- 

 phate of copper, and take its place ; this is techni- 

 cally called 'replacement,' and zinc is said to 

 'replace' copper.* As the salts may be considered 



* ' Replace ' is here used in the sense of the French remplacer, 

 and not in its ordinary English sense ; but as we have no one 

 English word which can be used to mean 'to be a substitute for,' 

 it seems better to keep the word at present in use, than to try 

 another, such as ' displace ' (as has been suggested), which ex- 

 presses the meaning no better. 

 314 



' as compounds of sulphuric acid (anhydrous) witb 

 oxide of copper and with oxide of zinc respectively,, 

 it is evident that equivalents of these oxides con- 

 tain the same quantity of oxygen, for the oxygen- 

 remains as it was, thus : 



Oxide of Copper. 



Before ...... Copper) , ,-. 



Afte^ ........ zir ^ and Oxygen and Sulphuric Aad.- 



Oxide of Zinc. 



And in the same way it may be shewn, by means 

 of other replacements, that an equivalent of any- 

 basic oxide contains the same quantity of oxygen. 

 By an extension of the meaning of the word 

 equivalent, this quantity of oxygen is called an- 

 equivalent of oxygen, and the quantity of metal 

 united to it is called an equivalent of the metal. 

 Those metals which form more than one basic 

 oxide have, of course, more than one equivalent ; 

 and when we speak of the equivalent of a metal of 

 this kind, we must state -which basic oxide or 

 which set of salts we are referring to. Thus, the 

 black oxide of mercury contains twice as much 

 mercury as the red oxide for the same quantity of 

 oxygen ; and therefore the equivalent of mercury 

 in the black oxide and its salts is twice as great as 

 the equivalent of mercury in the red oxide and its 

 salts. The phenomena of 'electrolysis' furnish an 

 excellent illustration of the equivalence of metals 

 in their salts. When an electric current from a 

 galvanic battery is passed through a solution of 

 a salt, the salt is decomposed, the metal being 

 deposited at the end of the wire coming from 

 the zinc of the battery, and the salt-radical at 

 the end of the wire coming from the copper or 

 the battery.* This decomposition, by means of 

 an electric current, is called 'electrolysis.' Now r 

 a very remarkable law of electrolysis is, that <t 

 given amount of electricity decomposes (or electro- 

 lyses) equivalent quantities of different salts. Thus r 

 if a current from a battery be made to pass through 

 a series of vessels, each vessel containing a 

 solution of a salt, the quantity decomposed of any 

 one salt will be equivalent to the quantity decom- 

 posed of any other. If, now, one of these vessels- 

 contain a solution of a salt of the red oxide of 

 mercury, and another a solution of a salt of the 

 black oxide of mercury, we shall find that twice 

 as much mercury is separated in the second as 

 in the first. 



In order to express these equivalent quantities 

 in numbers, it is necessary, in the first place, to- 

 fix upon a number which shall, by convention,. 

 represent one of them, and then, by experiment, 

 find out the relation between the equivalent of 

 that substance and the equivalent of every other 

 element, radical, or substance. For this purpose,. 

 it has been found most convenient to select hydro- 

 gen as the element with which to compare all 

 others, and to fix the equivalent of hydrogen as. 

 unity. The equivalent of hydrogen is thus settled 

 by convention to be I, and by experiment we 

 determine the equivalents of other elements or 

 radicals. Thus, when hydric sulphate (hydrated, 

 sulphuric acid) acts on metallic zinc, for every 



* Where the metal is one which acts on water (such as sodium), 

 we_ have, of course, not the metal itself separated, but the products 

 of its action on water ; and similarly if the salt-radical is incapable 

 of separate existence, we obtain the products of its decomposition* 



