6 PROCEEDINGS OP THE AMERICAN ACADEMY. 



"While in the first part of this paper no atomic hypothesis is assumed, 

 the words atomic volume, atomic weight, and atomic heat will be used in 

 a purely matei-ial sense, as the actual constants pertaining to quantities 

 chemically consistent. 



The results recorded in this table are typical of the variety of degrees 

 of contraction which take jjlace when substances combine with oxygen. 

 It is evident that in some cases the product occupies considerably more 

 space than the metal from which it was formed, and that in others 

 (typified by magnesium and sodium above) the oxide occupies consid- 

 erably less space than the metal. This last remarkable circumstance 

 at once emphasizes the absurdity of estimating the atomic volume of an 

 element in a compound by discovering the volume-change which takes 

 place when that element is replaced by another. Oxygen cannot be 

 said to occupy a minus quantity of space, — the only possible outcome 

 of the false assumption in this particular case. The false method gives 

 fairly consistent results among carbon compounds only because of the 

 great similarity of their composition. This consideration leads to the 

 first law underlying the change of volume in chemical or physical 

 change, namely. The atomic volume is not a constant, but is dependent 

 upon the environment. This law was first suggested by Horstmann,* 

 but he looked upon it rather as the absence of a law than as the 

 presence of one. 



If the affinity of oxygen for the metal were the only variable entering 

 into the figures given above, it is obvious that the total contraction, 

 the difference between the volumes of factors and product, would be at 

 once a comparative measure of the attractive forces which produce the 

 compression. This reasoning of course rests upon the plausible ground 

 that a state of being which resists pressure, such as liquid oxygen or 

 solid metal, may be compressed only by the application of pressure. 

 In this case pressure may be supposed to be applied by the mutual 

 affinity. But unfortunately the case is not so simple. 



It is clear that in each case recorded above at least three affinities are 

 concerned : first, the affinity of the metal for itself; second, the 

 affinity of oxygen for itself ; and third, affinity of the metal for 

 oxygen. The second of these is constant throughout the series, hence 

 for the present comparison it may be considered as a known quantity. 

 Therefore each change of volume may concern at least two unknown 

 quantities. Hence if it were possible to measure either of the two 



* Horstmann, Ostwald's Lehrbuch, I. 389 (1891). 



