INFLUENCE OF HEAT. 137 



magnesia come together, they unite and form sulphate 

 of magnesia, or epsom salt. But the stimulus of heat 

 is often required, particularly when the acid, as well 

 as the oxide, is a solid substance. The affinity be- 

 tween acids and bases, is in accordance .with the gene- 

 ral law, that chemical attraction between substances is 

 strongest, in proportion as they are most unlike, or op- 

 pos3cl to each other, in their properties. 



324. PROPERTIES OF ACIDS AND BASES. 



What are the , 



properties of I he properties of these two classes of 

 acids and ba- com p Oun( j Sj are opposite, and when brought 

 together, they neutralize each other. Thus, 

 when acid and soda are brought together, the acid taste 

 of the former and the alkaline taste of the latter, both 

 disappear. Acids change certain vegetable blues to 

 red. Bases restore the color. The experiment may 

 be made with an infusion of litmus* in water. A leaf 

 of purple cabbage answers the same purpose. Acids 

 color it red, while potash, and the alkalies, change 

 the red to green. 



325. EFFECT OF HEAT TO PRODUCE COM- 



What is the . 



effect of heat BINATION. It IS 86611 from the foregoing, 



on chemical ^^ h t j ft essential to chemical 



combination ? 



combination. This is almost always the 

 case where both substances are solid. Beside height- 

 ening their chemical affinity, heat has the effect of 

 bringing the particles into closer and more general con- 

 tact, and, within the range of affinity, by the melting 



*Litmus is a blue vegetable pigment much used by chemists, for the 

 purpose mentioned in the text. 



