26 CHEMISTRY, AS APPLIED TO THE 



Affinity is not exerted between the atoms of all bodies ; some cannot be 

 made to unite, while others seize upon each other with such energy that their 

 union is accompanied by an explosion, heat and light being generated. 



We observe something analogous in mixing some of the common fluids ; 

 oil and water cannot be made to unite, while sulphuric acid and water do so 

 readily, with the production of considerable heat. 



But what distinguishes chemical combination (the result of the operation 

 of affinity) from mere mixture, is that the latter takes place between sub- 

 stances possessing similar properties, while the former increases in energy 

 with the difference in the properties of the combining substances. 



Affinity only acts at insensible distances ; in other words, each atom of the 

 combining substances must be in contact before affinity can be exerted be- 

 tween them. 



It is obvious, then, that the physical state of a body considerably modifies 

 the action of affinity ; between solids, for instance, where the atoms are con- 

 gregated together into a hard mass, affinity cannot be exerted, since the in- 

 terior atoms of the one are prevented from being brought into contact with 

 the other. In gases the repulsion which exists between their atoms is equally 

 fatal to combination ; we may mix two gases, but no combination takes place 

 until we have compressed them, or taken other steps to overcome this repul- 

 sion and bring the atoms into contact. The liquid state is most favourable 

 to combination ; the ultimate particles of a liquid are sufficiently mobile to 

 allow it to mix with the substance with which we wish it to combine, while 

 they are sufficiently approximated to be within the sphere of affinity. The 

 old chemical axiom still obtains, " Corpora non agunt nisi sint soluta." 



The physical state of bodies depends on two antagonist forces, Cohesion 

 and Repulsion. Cohesion acts on the atoms of bodies, tending to bring them 

 into contact. In solids, then, this force preponderates over repulsion. In 

 elastic fluids the contrary is true ; since we find such repulsion among their 

 particles, that, were it not for the containing vessels, these particles would be 

 indefinitely separated. In non-elastic fluids, or liquids, the forces are about 

 balanced. It is generally admitted that heat, or, more properly speaking, the 

 cause of heat, caloric, is the cause of repulsion. If we consider caloric as a 

 subtle fluid, we may imagine it to produce the effects of repulsion, by insinu- 

 ating itself between the atoms or molecules of bodies, and thus counteracting 

 the effects of cohesion. 



