236 Chemistry. [Lecture 32. 



aulvca a part of the metallic oxide, especially 

 when pure or caustic, that is, uncombinecl with 

 carbonic acid, in which state it shows a high de- 

 gree of activity with regard to the metals, espe- 

 cially if applied to them when melted. 



The alkaline carbonats, or, as they were for- 

 merly called, mild alkalies, have not, however, 

 this last effect : they and some of the neutral 

 salts only promote their fusion without dissolv- 

 ing them. Borax, for instance, is much used in 

 soldering metals. Its effect in promoting their 

 fusion seems to arise from its forming a conti- 

 nued fluid medium between the sides of the ves- 

 sels and the surface of the metal ; through this 

 medium the heat is transmitted from the one to 

 the other more easily than it would otherwise be. 

 Borax is of use too in promoting the union of 

 the particles of metals, by making their surfaces 

 clean and bright. Globules of mercury, when 

 covered with dust, can scarcely be united ; but 

 when made quite bright, they unite as soon as 

 brought into contact. Borax preserves the par~ 

 tides clean by dissolving any particles of dust, 

 or of their own oxides, which may cover them ; 

 for borax is a very powerful solvent. By spread- 

 ing over the internal surface of crucibles, it also 

 gives a small glazing, which makes the vessels 

 pour put better; hence the goldsmiths use it 

 much. The other neutral salts act only by the 

 Assistance of heat : by a considerable degree of 



