ATTRACTION, 157 



2. Cohesion. It has been already stated that this power is the im- 

 mediate antagonist of chemical action, which rarely takes place till it is 

 overcome. Hence the great advantage of solution and fusion, which 

 are the most common means of inducing chemical action. In a few 

 cases, the energy of attraction is so great as to overcome the cohe- 

 sion of two solids, and cause them to unite, and to become fluid in 

 the act of combining. Muriate of lime and snow, and caustic fixed 

 alkalies and snow are examples. Even fluids may have their ener- 

 gy exalted by increased temperature, as is the case with nitric acid and 

 alcohol or oils, and with sulphuric acid and water :* if these fluids are 

 hot it is scarcely safe to mingle them in any considerable quantity. 

 Heat always promotes chemical combination, when cohesion is an 

 obstacle, and often, it is sufficient that one of the substances should 

 be fluid. Mechanical division favors chemical action, principally by 

 increasing the surface. Cohesion resulting from chemical action 

 often modifies the results of experiments. A mixture of sulphuric 

 and muriatic acids, with a solution of baryta, will result in the form- 

 ation of sulphate of barytes ; in part, no doubt, on account of its in- 

 solubility, but the effect must depend also upon a superior affinity. 



3. Insolubility. This depends upon cohesion, and has reference 

 to the solvent power of the liquid in which the cohesive power is ex- 

 erted. It removes the body, newly formed from the sphere of action, 

 and thus leaves the remaining principles free to act upon each other. 



4. Gravity. So far as there is a great difference in the gravity 

 of bodies that are mixed, it goes to retard chemical action. Thus, 

 salt at the bottom of water dissolves much more slowly and unequal- 

 ly than if it is agitated ; and if allowed to remain quiet, the solution 

 will be most dense at bottom, and the least so at top. If metals of 

 widely different specific gravity are melted together to form an alloy, 

 a larger proportion of the heaviest metal will be found at the bottom, 

 and agitation is necessary, in order to bring the particles into prox- 

 imity, so that the union may be effected. 



5. Elasticity. This power, under different circumstances, both op- 

 poses and favors chemical action. In general, gases are not prone to 

 combine, because their ponderable particles are too far removed from 

 each other by the caloric, with which they are united. Thus oxygen 

 and hydrogen gases may be retained in mixture, without combining : 

 till flame causes them to unite explosively. Ammonia and the acid 

 gases unite readily, and even precipitate solid matter, and one gas 



* Dr. Turner remarks, (Chem. 2d Ed. p. 137,) that " fluids commonly act upon 

 each other as energetically at low temperatures, or at a temperature just sufficient 

 to cause perfect liquefaction, as when their cohesive power is still farther diminished 

 by caloric." The familiar instances mentioned in the text show that this remark 

 needs to be qualified. 



