690 PRINCIPLES OP CHEMISTRY 



causing them to decompose according to the rise of temperature 8 

 therefore it is impossible to expect in the magnitude of the specific heat 

 the great simplicity of relation to composition which we see, for instance, 

 in the density of gaseous substances. Hence, although the specific heat 

 is one of the important means for determining the atomicity of the 

 elements, still the mainstay for a true judgment of atomicity is only 

 given by Avogadro-Gerhardt's law, i.e. this other method can only be 

 accessory or preliminary, and when possible recourse should be had to 

 the determination of the vapour density. 



Among the bivalent metals the first place, with respect to their 

 distribution in nature, is occupied by magnesium and calcium, just as 

 sodium and potassium stand first amongst the univalent metals. The 

 relation which exists between the atomic weights of these four metals 

 confirms the above comparison. In fact, the combining weight of 

 magnesium is equal to 24, and of calcium 40 ; whilst the combining 

 weights of sodium and potassium are 23 and 39 that is, the latter 



that they are distant from a liquid state, and do not undergo a chemical change when 

 heated that is, when no internal work is produced in them (B = 0). Therefore this 

 work may to a certain extent be judged by the observed specific heat. Thus, for instance, 

 for chlorine (Q = 0'12, Eegnault ; & = r88, according to Straker and Martin, and therefore 

 K = 0'09, MK = 6'4), the atomic heat (3'2) is much greater than for other gases containing 

 two atoms in a molecule, and it must be assumed, therefore, that when it is heated some 

 great internal work is accomplished. 



In order to generalise the facts concerning the specific heat of gases and solids, it 

 appears to me possible to accept the following general proposition : the atomic heat 

 (that is, AQ or QM/n, where>M is the molecular weight and n the number of molecules) is 

 smaller (in solids it attains its highest value 6'8 and in gases 3*4), the more complex the 

 molecule (\.e. the greater the number (n) of atoms forming it) and so much smaller, up 

 to a certain point (in similar physical states) the smaller the mean atomic weight M/n. 



8 As an example, it will be sufficient to refer to the specific heat of nitrogen tetroxide, 

 NjO^ which, when heated, gradually passes into NOj that is, chemical work of decom- 

 position proceeds, which consumes heat. Speaking generally, specific heat is a complex 

 quantity, in which it is clear that thermal data (for .instance, the heat of reaction) alone 

 cannot give an idea either of chemical or of physical changes -individually, but always 

 depend on an association of the one and the other. If a substance be heated from 

 to ti it cannot but suffer a chemical change (that is, the state of the atoms in the mole- 

 cules changes more or less in one way or another) if dissociation sets in at a temper- 

 ature ij. Even in the case of the elements whose molecules contain only one atom, 

 a true chemical change is possible with a rise of temperature, because more heat is 

 evolved in chemical reactions than that quantity which participates in purely physical 

 changes. One gram of hydrogen (specific heat = 8'4 at a constant pressure) cooled to the 

 temperature of absolute zero will evolve altogether about one thousand units of heat, 

 8 grams of oxygen half this amount, whilst jn combining together they evolve in the 

 formation of 9 grams of water more than thirty times as much heat. Hence the store 

 of chemical energy (that is, of the motion of the atoms, vortex, or other) is much greater 

 than the physical store proper to the molecules, but it is the change accomplished by 

 the former that is the cause of chemical transformations. Here we evidently touch on 

 those limits of existing knowledge beyond which the teaching of science does not yet 

 allow us to pass. Many new scientific discoveries have still to be made before this if 

 possible. 



