RICHARDS. — SIGNIFICANCE OF CHANGING ATOMIC VOLUME. 11 



Thus the contraction of the oxygen is really less in the case of 

 mercuric oxide, although it appears to be the same. 



Without going further, one can explain by means of these considera- 

 tions the behavior of zincic and mercuric oxides when subjected to high 

 temperatures. The sixteen grams of oxygen in mercuric oxide occupies 

 a larger space than an equal weight in the case of zinc, hence one 

 infers that it is less compressed by its affinity, hence the affinity must be 

 less. This smaller affinity siiould be more easily overcome by rising 

 temperature, a prediction which agrees with facts, TJius there appears 

 to he in this case a connection between the compression of substances 

 and their tendency to combine one with another. 



The case under consideration is typical. In the case of sodium and 

 magnesium, the affinity of the metal for oxygen is so enormous as to 

 overcome easily the large affinity of the metal for itself, and besides this 

 to compress both metal and oxygen together into a space smaller than 

 that previously occupied by the metal. This fact corresponds with the 

 great difficulty of decomposing sodic and magnesic oxides. Metallic 

 magnesium probably has as energy-quotient a stress more than four times 

 as great as sodium (see table on p. 10) ; hence the total contraction on 

 combination with oxygen is less than in the case of sodium. Compari- 

 son with the cases of mercury and zinc will show that this small con- 

 traction does not necessarily conflict with the fact that magnesium 

 decomposes sodic oxide at high temperatures. Again, the contraction 

 involved in the formation of argentic oxide is very slight. In this case 

 the large volume of oxygen is not concealed by the contraction of the 

 metallic element, as it was in the case of mercury, for silver is not par- 

 ticularly compressible. Hence one can infer that the affinity of silver for 

 oxygen is smaller than that of magnesium for oxygen, — an inference 

 which agrees with fact. Moreover, since the relation is nearly additive, 

 that is, neither silver nor oxygen change much in volume on com- 

 bination, their combination is easily shifted, that is to say, silver oxide 

 is easily decomposed by heat. 



Of course many tables comparing the molecular volumes of solids and 

 liquids might be drawn up, since a very great number of specific gravi- 

 ties have been determined. A table containing chlorides of the metals 

 already considered may be of interest. 



Here the variations in contraction are less than they were before. 

 Chlorine evidently possesses more equally distributed affinities than 

 oxygen does, and apparently somewhat weaker ones. The two most 

 interesting features of this table, which may be seen without the elimi- 



