AFFINITY. 



33 



freezing mixture, when the mercury falls to a fixed 

 point, and this point is marked ; then the interme- 

 diate space is graduated, according as convenience 

 or fancy may dictate. In Fahrenheit's thermome- 

 ter, the freezing-point is marked at 32, and the boil- 

 ing-point of water at 212, making the intermediate 

 space 180. The reason he did this was, that as he 

 thought, by mixing sal ammoniac and snow, he could 

 produce the greatest degree of cold, he marked this 

 by 0, and from this point divided his scale into 212 

 degrees, of which 32 extend to the freezing-point of 

 water, and the remaining 180 to the boiling-point of 

 water. 



Before concluding our remarks on the chemistry 

 of the globe, it is necessary to consider some of 

 the laws of chemical combination, those laws which 

 bind the materials of this earth into a consolidated 

 mass. 



All chemical phenomena are owing to affinity or 

 chemical attraction. It is this which causes the mi- 

 nutest particles of different kinds of matter to com- 

 bine, so as to form new bodies endowed with entire 

 new properties. It acts only at insensible distances, 

 and in this respect differs from the physical laws de- 

 scribed in the last chapter. In order to chemical 

 union, the particles of matter must actually be in 

 contact ; for, if removed to ever so small a distance, 

 they will not unite. If two bodies brought togeth- 

 er do not unite, they form a mechanical mixture. 

 Thus, if we mix iron filings and powdered sulphur 

 together, they do not combine, but form a mixture ; 

 if we apply heat to them, however, they melt, and, 

 by a chemical combination, produce a substance dif- 

 ferent from either of the former. So, also, oil and 

 water mix^ but do not chemically combine ; while 

 water and sulphuric acid combine readily. That 

 power which draws the particles of matter together 

 so as to form large masses, like rocks, is called co- 

 hesion^ cohesive attraction, or the attraction of aggre- 



