C 174 3 



XIII. On the constitution o^ the atmosphere. By John Dalton, 

 Esq. F, R. S. &c. Communicated January 12, 1826. 



Read February 24, 1826. 



1 HE fact discovered by Boyle and Marriotte, that the 

 space occupied by air is in the inverse ratio of the pressure, 

 is one of great importance in the doctrine of elastic fluids. 

 It may probably not be mathematically true in extreme 

 cases ; but in those where the condensations and rarefactions 

 do not exceed 50 or 100 times, there is reason to believe the 

 above ratio is a very near approximation to the truth. 



Sir Isaac Newton has shown in the 23d prop, book ii. of 

 the Principia, that if homogeneous particles of matter were 

 endued with a power of repulsion in the inverse ratio of their 

 central distances, collectively they would form an elastic 

 fluid agreeing with atmospheric air in its mechanical proper- 

 ties. He does not infer from this demonstration that elastic 

 fluids must necessarily consist of such particles ; and his 

 argument requires that the repulsive power of each particle 

 terminate, or very nearly so, in the adjacent particles. From 

 the scholium to this proposition, Newton was evidently aware 

 of the difficulty of conceiving how the repulsive action of 

 such particles could terminate so abruptly as his supposition 

 demands ; but in order to show that such cases exist in nature, 

 he finds a parallel one in magnetism. 



Whatever we may think of the constitution of an elastic 

 fluid, it is clear, that for the purpose of ascertaining its mecha- 



