Mr. F. D. Brown on Molecular Attraction. 255 



This fact seems to me to point to the conclusion that the atoms 

 which make up a molecule are as close together as their periodic 

 motions will permit, and are not merely held in certain posi- 

 tions of equilibrium by various opposing forces ; for if the 

 latter supposition were true, I fail to see how it would be pos- 

 sible for the same atom, together with its proportion of space, 

 to have always the same volume, surrounded, as it is, some- 

 times by atoms by which it is strongly attracted, and by which 

 therefore its volume would be diminished, sometimes by atoms 

 with which it has but little affinity, and which, therefore, 

 would leave it plenty of room in which to execute its vibra- 

 tions. The proximity of the several molecules in the liquid 

 and solid states must also be assumed, in order to account 

 for the invariability of molecular volumes. 



The assumptions contained in the preceding paragraphs are 

 in no way opposed to the views generally held concerning 

 molecular and atomic motion which we owe to the develop- 

 ment of the science of heat. They merely state that there is 

 no force of repulsion exerted between contiguous atoms, and 

 that the vibratory or other movements alone prevent their 

 absolute contact. 



Having thus fixed our ideas concerning the structure of 

 molecules, we may pass to the forces exerted between them, 

 neglecting the repulsion sometimes assumed, but which seems 

 to be wholly unnecessary. 



The theory of universal gravitation, as I understand it, asserts 

 that the mutual attraction exerted by any two bodies A and 

 B is dependent only on their respective masses and on the 

 distance between them, being entirely uninfluenced by the 

 presence of other bodies even in the immediate neighbourhood 

 of A or B. Thus at a given moment the Earth and Venus, 

 being in certain definite positions, exert upon each other a 

 certain force of attraction ; the attraction thus taking place 

 between the masses of the two planets would be unaltered by 

 the removal of the moon from the sphere of action ; the gra- 

 vitation of the earth and the moon does not therefore tie up 

 any portion of the attractive energy of the earth, and so dimi- 

 nish the force with which other bodies gravitate towards it. 



A totally different assumption is usually made with regard 

 to that form of attraction which gives rise to chemical pheno- 

 mena. Here it is supposed that two or more atoms, having 

 combined together, have thereby become incapable, at any 

 rate in the majority of cases, of attracting others to any appre- 

 ciable extent. Thus I imagine that most chemists hold the 

 view that when hydrogen and oxygen combine together to 

 form water, they thereby exhaust, or nearly exhaust, their com- 



