Till-: VALENCY AND SPECIFIC HEAT OF THE METALS 581 



sible 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. Therefore, although the specific heat 

 gives one of the important means of judging the atomicity of the 

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

 given by Avogadro-Gerhardt's law. All other means can only be 

 accessory or preliminary, until it be possible to have direct recourse 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 

 are one unit less than the former. 9 They all belong to the number 

 of light metals, as they have but a small specific gravity, in which 

 they differ from the ordinary, generally known heavy, or ore, metals 

 (for instance, iron, copper, silver, and lead), which are distinguished by 

 a much greater specific gravity. There is no doubt that their low 

 specific gravity has a significance, not only as a simple point of dis- 

 tinction, but also as a property which determines the fundamental 

 properties of these metals. Indeed, all the light metals have a series 

 of points of resemblance which approximates them to the metals of 

 the alkalis ; thus both magnesium and calcium, like the metals of the 



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 t 

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

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

 ature /[. 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 = 3'4 at a constant pressure) cooled to the 

 temperature of absolute zero will evolve altogether about one thousand units of heat, 8 

 irnuns of oxy^.-n half this amount, whilst in 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 movement of the atoms, vortex, or other) is much 

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

 by this store that is the cause of chemical transformations. Here we evidently touch on 

 those limits of existing knowledge beyond which the discipline of science does not yeiz 

 allow us to pass. A number of new scientific conquests are necessary before this can 

 become possible. 



9 As if NaH = Mg and KH = Ca, which is in accordance with their valency, KH 

 including two monovalent elements is a bivalent group like Ca. 



