172 CORRELATION OF PHYSICAL FORCES. 



equilibrium or quantitative equality of force, a remarkable 

 relation between chemical affinity and heat is that discovered 

 in many simple bodies by Dulong and Petit, and extended to 

 compounds by Neumann and Avogadro. Their researches 

 have shown that the specific heats of certain substances, when 

 multiplied by their chemical equivalents, give a constant quan- 

 tity as product or, in other words, that the combining weights 

 of such substances are those weights which require equal ac- 

 cessions or abstractions of heat, equally to raise or lower their 

 temperature. To put the proposition more in accordance with 

 the view we have taken of the nature of heat : each body has 

 a power of communicating or receiving molecular repulsive 

 power, exactly equal, weight for weight, to its chemical or 

 combining power. For instance, the equivalent of lead is 104, 

 of zinc 33, or in round numbers, as 3 to i: these numbers are 

 therefore inversely the exponents of their chemical power, 

 three times as much lead as zinc being required to saturate 

 the same quantity of an acid or substance combining with it ; 

 but their power of communicating or abstracting heat or re- 

 pulsive power is precisely the same, for three times as much 

 lead as zinc is required to produce the same amount of expan- 

 sion or contraction in a given quantity of a third substance, 

 such as water. 



Again, a great number of bodies chemically combine in 

 equal volumes, i.e. in the ratios of their specific gravities ; but 

 the specific gravities represent the attractive powers of the 

 substances, or are the numerical exponents of the forces 

 tending to produce motion in masses of matter of equal 

 volume towards each other ; while the chemical equivalents 

 are the exponents of the affinities or tendencies of the mole- 

 cules of dissimilar substances to combine and saturate each 

 other ; consequently, here we have in certain cases an equi- 

 valent relation between these two modes of force gravitation 

 and chemical attraction. 



Were the above relations extended into an universal law, 

 we should have the same numerical expression for the three 

 forces of heat, gravity, and affinity ; and as electricity and 

 magnetism are quantitatively related to them, we should 

 have a similar expression for these forces ; but at present the 



