430 PRINCIPLES QE CHEMISTRY 



Silver chloride may be decomposed, with the separation of silver 

 oxide, by heating it with a solution of an alkali, and if an organic 



or in the ratio 112 : 100 ; for the chloride the ratio is = 55 : 100 ; for the iodide it = 7 : 100 

 In the case of silver the oxide (ratio = 1350 : 100) is less stable than the chloride (ratio 

 = 804 : 100), and the iodide (ratio of the weight of metal to the weight of the halogen 

 = 85 : 100) is the most stable. From these and similar examples it follows that the most 

 stable compounds are those in which the weights of the combined substances are equal. 

 This may be partly explained by the attraction of similar molecules even after their 

 having passed into combination with others. This attraction is proportional to the 

 product of the acting masses. In silver oxide the attraction of Ag 2 for Ag 2 = 216x 216 

 = 46,656, and the attraction of Ag 2 for O = 216 x 16 = 3,456. The attraction of like mole- 

 cules thus counteracts the attraction of the unlike molecules. The former naturally 

 does not overcome the latter, otherwise there would be a. disruption, but it nevertheless 

 diminishes the stability. In the case of an equality or proximity of the magnitude of 

 the combining masses, the attraction of the like parts will counteract the stability of the 

 compound to the least extent in other words, with an inequality of the combined masses, 

 the molecules have an inclination to return to an elementary state, to decompose, which 

 does riot exist to such an extent where the combined masses are equal. There is, there- 

 fore, a tendency for large masses to combine with large, and for small masses to combine 

 with small. Hence' Ag 2 O + 2KI 'gives K 2 O + 2AgI. The influence of an equality of 

 masses on the stability is seen particularly clearly in the effect of a rise of temperature. 

 Argentic, mercuric, auric and other oxides composed of unequal masses, are somewhat 

 readily decomposed by heat, whilst the oxides of the lighter metals (like water) are not so 

 easily decomposed by heat. Silver chloride and iodide approach the condition of 

 equality, and are not decomposed by heat. The most stable oxides under the action of 

 heat are those of magnesium, calcium, silicon, and aluminium, since they also approach 

 the condition of equality. For the same reason hydriodic acid decomposesrwith greater 

 facility than hydrochloric acid. Chlorine does not act on magnesia or alumina, but it 

 acts on lime and silver oxide, &c. This is partially explained by the fact that by con- 

 sidering heat as a mode of motion, and knowing that the atomic heats of the free elements 

 are equal, it must be supposed that the amount of the motion of atoms (their vis viv.a) is 

 equal, and as it is equal to the product of the mass (atomic weight) into the square of the 

 velocity, it follows that the greater the combining weight the smaller will be the square 

 of the velocity, and if the combining weights be nearly equal, then the velocities also will 

 be nearly equal. Hence the greater the difference between the weights of the combined 

 atoms the greater will be the difference between their velocities. The difference between 

 the velocities will increase with the temperature, and therefore the temperature of de- 

 composition will be the sooner attained the greater be. the original difference that is, 

 the greater the difference of the weights of the combined substances. The nearer these 

 weights are to each other, the more analogous the motion of the unlike atoms, and con- 

 sequently, the more stable the resultant compound. 



The instability of cupric chloride and nitric oxide, the 'absence of compounds of fluorine 

 with oxygen, whilst there are compounds of oxygen with chlorine, the greater stability of 

 the oxygen compounds of iodine than those of chlorine, the stability of boron nitride, and 

 the instability of cyanogen, and a number of similar instances, T> here, j udging from the above 

 argument, one would expect (owing to the closeness of the atomic weights) a stability, 

 show that Beketoff's addition to the mechanical theory of chemical phenomena is still 

 far from sufficient for explaining the true relations of affinities. Nevertheless, in his 

 mode of explaining the relative stabilities of compounds,, we find an exceedingly interest- 

 ing treatment of questions of primary importance. Without such efforts it would be 

 unpossible;.to generalise the complex data of experimental knowledge. 



Fluoride of silver, AgF, is obtained by dissolving Ag. 2 O or Ag. 2 CO 3 in hydrofluoric acid. 

 It differs from the other halogen salts of silver in being soluble in water (1 part of salt in 

 0'55 of water). It crystallises from its solution in prisms, AgFH 2 O (Marignac), or AgF2H 2 O 

 (Pfaundler), which lose their water in vacuo. Guntz (1891), by electrolising a saturated 



