ELEMENT, COMPOUND, CHEMICAL AFFINITY, ETC. 89 



While the number of known compounds exceeds many thousands, 

 the number of elements is comparatively small, about seventy-six of 

 these simple substances being known to exist on our earth. And yet 

 this small number of elements, by combining with each' other in many 

 different proportions, form all that boundless variety of matter which 

 we see in nature. 



In the case of oxide of mercury heat has evidently caused a weak- 

 ening of the attractive force which held the two elements, mercury 

 and oxygen, together, thus permitting them to part company. Such 

 a change is known as decomposition. But, in other cases, heat 

 increases the attraction between elements, so that they unite to form 

 more complex bodies, which would not occur at ordinary tempera- 

 ture. Such a change is known as combinatioyi . For example, mag- 

 nesium metal does not unite with the oxygen of the air at ordinary 

 temperature, but when heated sufficiently, it unites with oxygen 

 with great vigor. In fact, mercury unites with oxygen when heated 

 to a temperature a little below its boiling-point, and forms the red 

 oxide, but if the latter is heated to a higher temperature it is decom- 

 posed. 



The student must not think that elements are obtained in* all cases 

 of decomposition by heat. In some instances the new products ob- 

 tained are themselves compounds, while in others an element and a 

 new compound result. For example, when calcium carbonate is 

 heated (see page 18), calcium oxide, a solid compound, and carbon 

 dioxide, a gaseous compound, are obtained. When potassium chlor- 

 ate, a compound of potassium, chlorine, and oxygen, is heated, the 

 element oxygen is evolved, while a new compound composed, of po- 

 tassium and chlorine, and known as potassium chloride, remains as a 

 solid in the vessel. 



The quantity of heat required for decomposition differs widely 

 according to the nature of the substance. Some substances can be 

 produced only at a temperature below the freezing-point of water, 

 a higher temperature causing their decomposition ; other substances 

 may be decomposed at temperatures between the freezing- and 

 boiling-points; others again, and to these belong the majority of 

 inorganic compounds, may be raised to red or white heat before decom- 

 position sets in; and still another number of compounds have never 

 yet been decomposed by heat. Theoretically, however, we assume 

 that all compounds may be decomposed by heat, should it be possible 

 to raise it to a sufficiently high degree. 



