II. 



PRINCIPLES OF CHEMISTRY. 



5. ELEMENT, COMPOUND, CHEMICAL AFFINITY, MODES OF 

 EFFECTING CHEMICAL CHANGE. 



HAVING considered some of the subjects of physics, we may now 

 pass to the field of chemistry. The nature of a chemical change and 

 the scope of chemistry have already been discussed on page 18. One 

 of the simplest means of bringing about a change in the composition 

 of matter is by applying heat. Let us see what may be learned 

 from the following experiment. 



Decomposition by heat. The results of the action- of heat upon 

 matter have been stated to be : Increased velocity of the motion of 

 molecules, increase in volume of the substance heated, and in many 

 cases a conversion of solids into liquids and of these into gases. Be- 

 sides these results there frequently may be noticed another. 



FIG. 35. 



Decomposition of mercuric oxide in A ; collection of mercury in B, and of oxygen in C. 



To illustrate this action of heat, we will select the red oxide of 

 mercury, a solid substance which is insoluble in water, almost taste- 

 , and of a brick-red color. When this oxide of mercury is placed 



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