OXYGEN AND ITS SALINE O OIlilN.XTK >NS 187 



practice. If we take, for instance, insoluble nui^m-sium oxide, it i> 

 easily dissolved in sulphuric acid, and on evaporation ^m-s a saline 

 substance, bitter, like all the salts of magnesium, and familiar to 

 all under the name of Epsom salts, used as a purgative. If a solu- 

 tion of caustic soda which is obtained, as we >a\\ , by the action of 

 water on sodium oxide be poured into a flask in which charcoal has 

 been burnt ; or if carbonic anhydride, which is produced under so many 

 circumstances, be passed through a solution of caustic soda, then sodium 

 carbonate or soda, Na 2 C(X, is obtained, of which we have spoken several 

 times, and which is prepared on a large scale and often used in manu- 

 factures. This reaction is expressed by the equation, 2NaHO + CO 2 = 

 Na 2 CO 3 + H.)O. Thus, the various bases and acids form an innumer- 

 able number of different salts. 52 Salts constitute an example of definite 

 chemical compounds which, both in the history and practice of science, 



sulphuric anhydride and potassium oxide. In this, as in the reaction of hydrates, an 

 intermediate oxide is separated out namely, alumina. Its separation will be very 

 evident in this case, as alumina is insoluble in water, whilst its compounds with the 

 acid and alkali, like the compound of an alkali with an acid i.e.,& salt are soluble 

 in water, and therefore on mixing the solutions of alumina in an acid and an alkali, it is 

 precipitated as a gelatinous hydrate. 



5 - The mutual interaction of hydrates, and their capacity of forming salts, may .be 

 taken advantage of for determining the character of such hydrates as are insoluble in 

 water. Let us imagine that a given hydrate, whose chemical character is unknown, is 

 insoluble in water. It is therefore impossible to test its reaction on litmus. It is then 

 mixed with water, and an acid for instance, sulphuric acid is added to the mixture. If 

 the hydrate taken be basic, reaction will take place, either directly or by the aid of 

 heat, with the formation of a salt. In certain cases, the resultant salt is soluble in 

 water, and this will at once show that combination has taken place between the 

 insoluble basic hydrate and the acid, with the formation of a soluble saline substance. In 

 those cases where the resultant salt is insoluble, still the water loses its acid reaction, 

 and therefore it may be ascertained, by the -addition of an acid, whether a given 

 hydrate has a basic character, like the hydrates of oxide of copper, lead, &c. If 

 the acid does not act on the given insoluble hydrate (at any temperature), then 

 it has not a basic character, and it should be tested as to whether it has an acid 

 character. This is done by taking an alkali, instead of the acid, and by observing 

 whether the unknown hydrate then dissolves, or whether the alkaline reaction dis- 

 appears. Thus it may be proved that hydrate of silica is acid, because it dissolves in 

 alkalis and not in acids. If it be a case of an insoluble intermediate hydrate, then it 

 will be observed to react on both the acid and alkali. Hydrate of alumina is an 

 instance in question, which is soluble both in caustic potash and in sulphuric acid. 

 But it must be remarked that intermediate oxides, in an anhydrous state, often 

 evince great resistance to the formation of saline compounds. Thus alumina or 

 aluminium oxide, in the anhydrous form in which it is met with in nature, and which 

 forms a crystalline substance, is insoluble in this form both in solutions of alkalis and 

 of acids. In order to convert it into a soluble form, it must be ground into a fine 

 powder and fused together with certain acid compounds, which are unchanged by 

 heat, such as acid potassium sulphate. 



The degree of affinity or chemical energy proper to oxides and their hydrates is very 

 dissimilar ; some extreme members of the series have it to a great extent. When acting 

 on each other they evolve a large quantity of heat, and when acting on intermediate 

 hydrates they also evolve heat to a considerable degree, as we saw in the coinbi- 



