240 FRAGMENTS OF SCIENCE. 



as the various movements of the earth's surface are exe- 

 cuted independently of the orbital revolution of our planet. 



The vapor molecules are kept asunder by forces which, 

 virtually or actually, are forces of repulsion. Between 

 these elastic forces and the atmospheric pressure under 

 which the vapor exists, equilibrium is established as soon 

 as the proper distances between the molecules have been 

 assumed. If, after this, the molecules be urged nearer to 

 each other by a momentary force, they recoil as soon as 

 the force is expended. If they be separated more widely 

 apart, when the separating force ceases to act they again 

 approach each other. The case is different as regards the 

 constituent atoms. 



And here let it be remarked, that we are now upon the 

 very outmost verge of molecular physics ; and that I am 

 attempting to familiarize your minds with conceptions 

 which have not yet obtained universal currency even among 

 chemists ; which many chemists, moreover, might deem un- 

 tenable. But, tenable or untenable, it is of the highest sci- 

 entific importance to discuss them. Let us, then, look men- 

 tally at our atoms grouped together to form a molecule. 

 Every atom is held apart from its neighbors by a force of 

 repulsion ; why, then, do not the mutually repellant mem- 

 bers of this group part company ? The molecules separate 

 from each other when the external pressure is lessened or 

 removed, but the atoms do not. The reason of this stabil- 

 ity is that two forces, the one attractive and the other re- 

 pulsive, are in operation between every two atoms; and the 

 position of every atom — its distance from its fellows — is 

 determined by the equilibration of these two forces. If the 

 atoms come too near, repulsion predominates and drives 

 them apart; if too distant, attraction predominates and 

 draws them together. The point at which attraction and 

 repulsion are equal to each other is the atom's position of 

 equilibrium. If not absolutely cold — and there is no such 



