RICHARDS. — SIGNIFICANCE OP CHANGING ATOMIC VOLUME. 7 



vai'iable affinities, au approximate idea could be obtained concerning 

 the other from these data concei'ning atomic and molecular volume. 



A slight uncertainty is caused also by the possible varying intensity 

 of the '• crystal-making tendency " which determines the structure of 

 solids. The small differences caused by this uncertainty may be seen 

 from the following typical calculation. If solid rather than liquid 

 mercury had been chosen above, the atomic volume of the mercury 



would have become = 14.2 instead of 14.7, and the excess of 



14.1 



volume of the oxide would have been 5.2 instead of 4.7. These 



differences are unimportant compared with the larger values under 



consideration ; the precise state of the solids or liquids makes less 



difference than one would have supposed. 



Is there any direct method of determining either the mutual affinity of 

 the two elements or the affinity of the metal for itself? 



Countless attempts to measure the former have so continually resulted 

 in failure that many chemists are inclined to deny the existence of 

 chemical affinity. The electrometric method suggested by Ostwald * 

 clearly measures one of the ways in which chemical affinity may accom- 

 plish work, but it is limited in application and only represents a small 

 fraction of the possibilities. The thermal relations are complicated by 

 well-known thermodynamic irregularities, and would be fully significant 

 only at the imaginary absolute zero. 



The direct determination of the affinity of a substance for itself is an 

 easier matter, for many of the properties of a single substance, such as 

 volume, compressibility, tenacity, must be associated with this affinity. 

 Let us seek to study these relationships more closely. 



If one could only be sure that all substances, when relieved of their 

 self-affinity, would occupy the same volume, the atomic volume itself 

 would be the simplest and most direct means of comparing this property 

 in different substances. The smaller the actual atomic volume, the 

 greater must be the self-affinity. Such an assumption would at first 

 sight seem to be justified, for those elements which have the largest 

 atomic volumes have the least inclination to remain in the elementary 

 states. Deserting the elementary state means introducing other affini- 

 ties, however ; hence the assumption would be unsafe. 



It has been already pointed out that compressibility, if measured over 

 a wide range of pressures, might afford a clue to the extent of compres- 



* Ostwald, Tlie Chemometer, Z. phys. Chem. 15, 399 (1894). 



