IV.] PRIMARY FORCES. 41 



conlined to the liquid state of matter; and affinity gene- 

 rally, if not always, requires a certain degree of eleva- 

 tion of temperature before it will act. Combustion, for 

 instance, wliicb is the most energetic of all chemical 

 actions, in most cases will only commence at a very high 

 temperature, and never, so far as I know, at a very 

 low one. 



Gravity is incapable of saturation ; that is to say, what- 

 ever be the quantity of matter that any mass of matter is 

 attracting, it is capable of attracting any additional quan- 

 tity with exactly the same force as if it had no other to 

 attract. In briefer language, all matter attracts all other 

 matter. Capillarity, on the contrary, is capable of satu- 

 ration: as capillarity acts between molecules only when 

 they are in contact, the capillarity of a molecule of (for 

 instance) water or mercury for other molecules of the same 

 liquid is in a state of saturation so long as it is surrounded 

 by other molecules of the same liquid ; all its capillarity 

 is employed in attracting them, and it cannot attract, or 

 be attracted by, any more. Affinity also is capable of 

 saturation. One volume of hydrogen, for instance, does 

 not attract all oxygen, but only one half-volume of oxygen; 

 and when these are combined to form water, their affinitv 

 for each other is saturated; the water has no further 

 affinity for either hydrogen or oxygen. 



Gravity is not an elective force ; that is to say, it acts on 

 all matter alike : all matter attracts all other matter with 

 a force directly as the mass. Capillarity, on the contrary, 

 is elective ; that is to say, some, probably all, liquids have 

 a stronger capillarity for some substances than for others. 

 Mercury, for instance, has a strong capillarity for itself, as 

 is shown by the way it runs into globules ; but it has very 

 little capillarity for other substances, except metals. Oil, 

 on the contrary, has a stronger capillarity for most solids 

 than for itself, as is shown by its rising in a wick and 

 spreading over surfaces, in opposition to gravity. But oil 

 and water, or water and mercury, have hardly any capil- 

 larity for one another, and refuse to mix, or even to adhere. 

 Affinity also is elective : oxygen, for instance, has a very 



