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 differs, also, from affinity in this, 

 that w^hile it tends to unite similar particles, chemi- 

 cal attraction is exerted between dissimilar parti- 

 cles. Lime and carbonic acid, which together con- 

 stitute limestone, differ 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- 

 poneni 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- 



