7 he Molecular Volume of Solids. 



457 



(i The Molecular Volume of Solids." By Edward Wilson, M.A. 

 Communicated by Professor Stokes, Sec. U.S. Received 

 May 19. Read June 16, 1881. 



The object of the present paper is to trace the relation between the 

 molecular volume of any solid snbstance and its chemical constitution. 

 This subject has engaged the attention of several previous inquirers, 

 notably of Kopp, Schroder, and Hermann, but a review of their 

 labours will be postponed till an exposition of the principles of this 

 paper has been given. Such an arrangement, it is thought, will 

 very much facilitate a comparison between the views and results of 

 the present writer, and those of his predecessors in the same field of 

 inquiry. 



The molecular volume of any solid substance may be defined to be 

 the weight of the molecule divided by the specific gravity of the 

 substance. The weight is known in terms of the standard unit, if the 

 chemical composition be known. The specific gravity may be deter- 

 mined by experiment. Then we may form either of the equations — 



TT t pit weight of molecule 



Volume oi molecule = —^7; : — 



specific gravity 



.f, ., weight of molecule 



bpecinc gravitv= — — ^ . 



volume of molecule 



Another definition might be given as follows : — Every solid and 

 liquid substance may be conceived as made up of molecules separated 

 from one another by intervals of space, and kept apart by the repul- 

 sive forces which each molecule exerts on the molecules adjacent to it. 

 At a certain distance the molecules cease to repel, and beyond that 

 distance they attract one another ; consequently, any molecule may be 

 regarded as situate at the centre of a sphere within which any similar 

 molecule would be repelled. This sphere may be called the sphere of 

 repulsion, and the volume of the molecule may be defined as the 

 volume of its sphere of repulsion. This definition is more suitable to 

 the gaseous forms of matter, where the molecular volume is determined 

 by observations on the interdiffhision of gases according to the plan 

 adopted by Professor Loschmidt of Vienna. It is easy to show that 

 according to this definition the molecular volume of any solid sub- 

 stance is equal to its molecular weight divided by its specific 

 gravity.* 



Each atom which enters into the composition of a molecule has a 

 known and invariable atomic weight, and the weight of the molecule 

 is the sum of the weights of the component atoms. In like manner 



* Tide note. 



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