THE VALENCY AND SPECIFIC HEAT OF THE METALS 575 



history of chemistry, especially since the development given to this law 

 by the researches of Regnault, and since Cannizzaro (1860) showed the 

 agreement between the deductions of this law and the consequences 

 arising from Avogadro-Gerhardt's law. 



Dulong and Petit, having determined the specific heat of a number 

 of solid elementary substances, observed that as the atomic weights of 

 the elements increase, their specific heats decrease, and that the product 

 of the specific heat Q into the atomic weight A is an almost constant 

 <l>Kintity. This means that to bring different elements into a known 

 thermal state, an equal amount of work is required if atomic quantities 

 of the elements are taken ; that is, the amounts of heat expended in 

 heating equal quantities by weight of the elements are far from equal, 

 but are in inverse proportion to the atomic weights. For thermal 

 changes the atom is a unit ; all atoms, notwithstanding the difference of 

 weight and nature, are equal. This is the simplest expression of the 

 fact discovered by Duloiig and Petit. The specific heat measures that 

 quantity of heat which is required to raise the temperature of one unit 

 of weight of a substance by one degree. If the magnitude of the 

 specific heat of elements be multiplied by the atomic weight, then we 

 obtain the atomic heat that is, the amount of heat required to raise 

 the temperature of the atomic weight of an element by one degree. It 

 is these products which for the majority of the elements prove to be- 

 approximately, if not quite, identical. A complete identity cannot be 

 expected, because the specific heat of one and the same substance varies 

 with the temperature, with its passage from one state into another, and 

 frequently with even a simple mechanical change of density (for in- 

 stance by hammering), not to speak of allotropic changes, c. We will 



of others if? generally had recourse to, and is quite necessary, because phenomena of dis- 

 sociation, polymerisation, &c., may complicate the individual determinations by each 

 method. 



It will be well to observe that a number of other methods, especially from the province 

 of those physical properties which are clearly dependent on the magnitude of the atom 

 (or equivalent) or of the molecule, may lead to the same result. I may point out, for 

 instance, that even the specific gravity of solutions of the metallic chlorides (Chapter VII. 

 p. 822) may serve for this purpose. Thus if beryllium be taken as trivalent that is, if 

 the composition of its chloride be taken as BeCl 3 (or a polymeride of it), then the specific 

 gravity of solutions of beryllium chloride will not fit into the series of the other metallic 

 chlorides. But on ascribing to it an atomic weight Be = 7, or taking Be as bivalent, the 

 composition of its chloride as BeCl 2 , we arrive at the general rule given on p. 818. Thus 

 W. G. Burdakoff determined in my laboratory, that the specific gravity at 15/4 of the 

 solution BeCl 2 + 200H. 2 O = 1'0138 that is, greater than the corresponding solution 

 KC1 + 200JL.O (=1-0121), and less than the solution MgCl.^ + 200H 2 O (= T0203), 

 as would follow from the magnitude of the molecular weight BeClo = 80, because 

 KC1 = 74'5 and MgCLj = 95 (see my work Investigation of Aqueous Solutions, 1887, 

 p. 425). 



