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 ammoniap 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 consohdated 
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 nlelt, 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- 
hesiony cohesive attraction^ or the attraction of aggre- 
