38 PRINCIPLES OF CHEMISTRY. 



thousands, the number of elements is comparatively small, but 

 sixty-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. 



Chemical affinity. There must be some cause which enables 

 or even forces the different elements to unite with each other so 

 as to form compound bodies. There must be, for instance, a 

 cause which enables oxygen and mercury to combine and form 

 a red powder. 



This cause is to be found in the existence of another form of 

 the general attraction, which causes the smallest particles of 

 different elements to unite to form new substances with new 

 properties. This kind of attractive power is called chemical 

 force, affinity, or chemism, and bodies possessing this capacity of 

 uniting with each other are said to have an affinity for each 

 other. 



There is a great difference between chemical attraction and 

 the various forms of attraction spoken of heretofore. Cohesion 

 simply holds together the molecules of the same substance, 

 adhesion acts between the molecules of solid and liquid sub- 

 stances, gravitation acts between masses. But all these forces 

 do not change the nature, the external and internal properties 

 of matter ; this is done when chemical force or affinity is operat- 

 ing, when a chemical change takes place. 



For instance : In a piece of yellow sulphur the molecules are 

 held together by cohesion, and we can counteract this cohesion 

 by mechanical subdivision, converting it into a fine powder; 

 or by the application of heat we can further subdivide the sul- 

 phur, melt, and finally volatilize it; or we can throw a piece of 

 sulphur into the air, when it will fall back upon the earth in 

 consequence of gravitation ; or we can dip it into water, when 

 it becomes moist in consequence of surface-action. Yet in all 

 these cases sulphur remains sulphur. 



It is entirely different when sulphur enters into chemical 

 combination exerting chemical attraction, for instance, when it 

 burns, this means, when it combines with the oxygen of the 

 atmospheric air. In this case a new substance, a disagreeably 

 smelling gas, a compound of oxygen and sulphur, is formed. 



It is consequently a complete change in the properties of 



