OXYGES AM) ITS SALINE COMIMNATInNs IS;") 



property of restoring the blue colour to litmus which has been reddened 

 by the action of acids. The hydrates of the oxides of sodium and 

 potassium, NaHO and KHO, are examples of basic hydrates easily 

 soluble in water. They are true alkalis, and are termed caustic, because 

 they act very powerfully on the skin of animals and plants. Thus 

 NaHO is called ' caustic ' soda. 



Thus, the saline oxides are capable of combining together and with 

 water. Water itself is an oxide, and not an indifferent one, for it can, 

 as w r e have seen, combine with basic and acid oxides ; it is a represen- 

 tative of a whole series of saline oxides, intermediate oxides, capable of 

 combining with both basic and acid oxides. There are many such 

 oxides, which, like water, combine with basic and acid anhydrides for 

 instance, the oxides of aluminium and tin, &c. From this it may be 

 concluded that all oxides might be placed, in respect to their capacity 

 for combining with one another, in one uninterrupted series, at one 

 extremity of which would stand those oxides which do not combine 

 with the bases that is, the alkalis while at the other end would be 

 the acid oxides, and in the interval those oxides which combine with 

 one another and .with both the acid and basic oxides. The further 

 apart are the members of this series the more stable are the compounds 

 they form together, the more energetically do they act on each other, 

 the greater the quantity of heat evolved in their reaction, and the 

 clearer is their saline chemical character. 



We said above that basic and acid oxides combine together, but 

 rarely react on each other ; this depends on the fact that the majority 

 of them are solids or gases that is, they occur in the state least prone 

 to chemical reaction. The gaseo-elastic state is with difficulty destroyed, 

 because it necessitates overcoming the elasticity proper to the gaseous 

 particles. The solid state is characterised by the immobility of its 

 particles ; whilst chemical action requires contact, and hence a dis- 

 placement and mobility. If solid oxides be heated, and especially if 

 they be melted, then reaction proceeds with great ease. But such a 

 change of state rarely occurs in nature or in practice. In a few furnace 

 processes only is this the case. For example, in the manufacture of 

 glass, the oxides contained in it combine together in a molten state. 

 But when oxides combine with water, and especially when they form 

 hydrates soluble in water, then the mobility of their particles increases 

 to a considerable extent, and their reaction is greatly facilitated. Re- 

 action then takes place at the ordinary temperature easily and rapidly ; 

 so that this kind of reaction belongs to the class of those which take 

 place with unusual facility, and are, therefore, very often taken advan- 

 tage of in practice, and also have been and are going on in nature at 



