34 
COHESION. 
gation. This, like chemical affinity, exerts its influ- 
ence only at insensible and infinitely small dis- 
tances, as we perceive when we attempt to unite 
the broken fragments of a piece of marble or a me- 
tallic wire. It diffejrs, also, from affinity in this, 
that while it tends to unite similar particles, chemi- 
cal attraction is exerted between dissimilar parti- 
cles. Lime and carbonic acid, which together con- 
stitute limestone, diifer from each other as well as 
from marble, and are united by chemical affinity ; 
while a piece of marble is an aggregate of smaller 
portions, attached to each other by cohesion, and 
the parts so attached are called integrant particles. 
The lime and the carbonic acid are called the com- 
fonent or constituent parts ; thus we perceive that 
the former are aggregated by cohesion, the latter by 
affinity. 
The hardness and toughness of minerals is pro- 
portionate to the strength with which their particles 
cohere. Cohesion may be destroyed by mechanical 
division or by the application of heat; and both 
these means are employed in the reduction of ores, 
and the conversion of limestone into lime. The 
former is useful by increasing the extent of surface 
exposed to the influence of heat, and heat is essen- 
tial in bringing bodies into that state of liquefac- 
tion necessary for chemical union. A larger portion 
of salts is dissolved in a certain amount of warm 
water than in the same quantity of cold, because 
the cohesion of the salts is diminished by the heat. 
Cohesion may often be restored when lost, as when 
melted sulphur or lead is again cooled, or the wa- 
ter containing salts in solution has been evapora- 
ted. If we dip an iron wire in water, we find on 
taking it out that some particles of water adhere to 
it. If we dip it in quicksilver, we find there is no 
adhesion of its particles. If we turn quicksilver 
into a bowl of glass or earthen, the surface will 
appear convex, because there is no attraction be- 
