CHEMISTRY. 



way. Thus, we have spoken of sulphate of copper 

 as a compound of 



Oxide of Copper, 



that is, and Anhydrous Sulphuric Acid. 



Copper and Oxygen, 



Base. 



Acid. 



But just as a strong base, such as potash, will de- 

 compose this salt, driving out the oxide of copper, 

 and forming sulphate of potash, so a more positive 

 metal than copper for instance, zinc drives out 

 the copper, and forms sulphate of zinc, in which 

 the base is oxide of zinc. Here it is the metal 

 which is changed ; all the rest remains as it was. 

 We may therefore represent the salt as a com- 

 pound of metal with all the rest of the salt, thus : 



Copper and Oxygen and Anhydrous Sulphuric Acid. 



MetaL 



Salt-radical. 



The rest of the salt, that which in union with metal 

 forms the salt, is called the ' salt-radical.' In the 

 case of the sulphates, and indeed of the majority of 

 salts, the salt-radical has not been obtained in the 

 free state that is, by itself, and is only known in 

 combination with metals, forming salts, and with 

 hydrogen, forming the hydric salt or hydrated acid, 

 thus : 



Water. Acid. 



Hydrogen and Oxygen and Anhydrous Sulphuric Acid. 



* t t 



Salt -radical. 



There are, however, a number of substances known, 

 which, in combination, play exactly the part of the 

 salt-radicals we have just been considering that 

 is, they unite with metals to form salts, and with 

 hydrogen to form hydric salts, corresponding ex- 

 actly in general properties to the hydrated acids. 

 These substances are called 'halogens' (that is, 

 salt-producers). We shall illustrate their characters 

 by means of an example, and for this purpose we 

 shall select the most important of them namely, 

 chlorine. Chlorine is a simple substance in other 

 words, it has not been decomposed and thus 

 differs entirely from the salt-radicals of the sul- 

 phates, nitrates, &c which consist of anhydrous 

 acid and oxygen, and have not been obtained in 

 the free state. Chlorine, in the free state, is a 

 greenish yellow gas with an acrid smell. It 

 readily unites with hydrogen, forming the sub- 

 stance known as muriatic acid gas, or, from its 

 composition, hydrochloric acid gas. This is a true 

 hydric salt, possessing all the general properties 

 of the hydrated acids. It acts upon bases, pro- 

 ducing salts and water ; it drives out weaker acids 

 (in the hydrated form that is, as hydric salts) 

 from their salts. The salts thus formed we have 

 in the former pages called ' muriates,' or hydro- 

 chlorates f but we now see what they are ; they are 

 -compounds of metals and chlorine, the oxygen of 

 the metallic oxide (or base) uniting with the 

 hydrogen of the hydrochloric acid to form water. 

 They are hence called 'chlorides.' Thus, instead 

 of ' muriate of soda,' we say ' chloride of sodium ;' 

 and so on. They are, however, true salts, under- 

 going all the changes and reactions common to 

 salts. It will be noticed that all salts can be 

 represented as compounds of metal and salt-radical, 

 while some salts (such as the chlorides) cannot be 



represented as compounds of acid and base. This 

 has led to a modification of the old names of many 

 salts; thus, we often use the names sulphate of 

 sodium (or sodium sulphate) instead of the old 

 sulphate of soda. It must not be supposed that 

 sulphate of sodium means a compound of sodium 

 and sulphuric acid ; it is a compound of sodium 

 and oxygen and sulphuric acid (anhydrous). 



We may now place side by side the various 

 names given to the same salt : 



Sulphate of Soda Sodic Sulphate ... Sulphate of Sodium. 



Hydrated Sulphuric Acid. . Hydric Sulphate. . Sulphate of Hydrogen. 



Muriate of Soda Sodic Chloride. ... Chloride of Sodium. 



Hydrochloric Acid Hydric Chloride.. Chloride of Hydrogen. 



Nitrate of Oxide of Lead.. Plumbic Nitrate. .Nitrate of Lead. 

 &C. &c. &c. 



Sulphate of soda means the salt formed by the 

 action of sulphuric acid on soda ; sulphate of 

 sodium means the compound of sodium and the 

 salt-radical of the sulphates ; while sodic sulphate 

 may be used to indicate the same substance with- 

 out referring to either view of its constitution. 

 For this, as well as for other reasons, the names 

 in the middle column are to be preferred, although 

 this system sometimes involves us in the use of 

 novel and awkward adjectives, such as zincic, or 

 bismuthous. 



We have seen that many bases are oxides of 

 metals, and we must now attend to the relation of 

 the metals to their oxides. When a metal (or 

 other substance) is made to unite with oxygen, it 

 is said to be 'oxidised;' when the oxygen is 

 taken away from an oxide, and the original sub- 

 stance (whether metal or not) is reproduced, the 

 oxide is said to be ' reduced/ We have thus two 

 processes inverse to one another that is, the one 

 undoing what the other does, oxidation and reduc- 

 tion. Thus, there is an important ore of iron 

 called the magnetic oxide of iron ; when this is 

 smelted, the oxygen is taken from it, and we obtain 

 metallic iron this is a process of reduction. If 

 we heat iron strongly in the air, it becomes 

 covered with a crust or scale of a black, brittle 

 substance, which breaks off when the iron is 

 hammered this is an oxide of iron, and is iden- 

 tical in composition with the magnetic iron ore 

 this is a process of oxidation. It is obvious that 

 these two processes go in opposite directions. 



Most processes of oxidation are- accompanied 

 by the giving out of heat, sometimes in a very 

 marked degree ; thus, when charcoal is strongly 

 heated in air or oxygen (and it must be remem- 

 bered that about one-fifth of the atmospheric air 

 consists of oxygen), it unites with oxygen, forming 

 an oxide (carbonic acid, Black's fixed air), and in 

 doing so, gives out much heat Such an oxida- 

 tion we call a combustion, or burning. And 

 although different processes of oxidation are ac- 

 companied by the giving out of very different 

 quantities of heat, we may generalise the term, 

 and call all such processes cases of 'burning. 

 Similarly, all processes of reduction may be 

 grouped together as cases of unburning.* 



As in a case of ' burning' or ' oxidation,' heat is 

 given out, so in a case of ' unburning ' or ' reduc- 

 tion,' heat disappears, or is used up ; and we find 

 that exactly as much heat is used in effecting the 

 unburning of an oxide as was given out in pro- 

 ducing it It will thus be seen that the greater 



* This is here stated generally ; some of the few exceptions wiD 

 be mentioned among the compounds of Chlorine and Oxygen. 



