CONTRIBUTIONS FROM THE CHEMICAL LABORATORY 

 OF HARVARD COLLEGE. 



THE SIGNIFICANCE OF CHANGING ATOMIC VOLUME. 



IV. — THE EFFECTS OF CHEMICAL AND COHESIVE INTERNAL 



PRESSURE. 



By Theodore AVilliam Richards. 



Presented February 10, 1901. Received February 3, 1904. 



In the previous papers* upon this subject, it has been shown that 

 there are many facts pointing toward the existence of a causal relation- 

 ship between the volume occupied by solids and licpiids and their internal 

 pressures of cohesion and chemical affinity. 



If a relation between the volume and internal pressures really exists, 

 the volume occupied by the gram-equivalent of a solid becomes as im- 

 portant and essential a property of material as any other property. 



In attempting to explain the facts with the help of the atomic hypothe- 

 sis, there are two possible alternatives, — either a relatively incompressible 

 atom exists within a compressible space, or else the atom itself is com- 

 pressible, and in a solid or a liquid comes into contact with its neighbors. 

 The former hypothesis is the one usually accepted; but it seems to me 

 to be hampered by several insuperable difficulties. The nature of the 

 supposed empty space between the molecules of a solid is not easy to 

 imagine ; for if it is due to the impact of the thermal vibrations of a small, 

 hard particle, the difference between a solid and a gas is not suifieiently 

 explained. Moreover, even a,t the lowest attainable temperatures, where 

 the thermal vibration must be scarcely perceptible, ice occupies only 

 about half the volume of the solid oxygen and hydrogen from which it 

 may be made.f Such a large change in volume seems incredible, if it is 



* Richards, These Proceed in^rg, 37 ; 1 (1901), 399 (1902) ; 38, 293 (1902) ; also 

 Zeitschr. phys. Cliem., 40, 169, 597 (1902) ; 42, 129 (1902). 



t Dr. Frederick Soddy has kindly called my attention to this circumstance, the 

 facts having been reported by Professor Dewar in an Address at the British Asso- 

 ciation, 1903. 



