XIII. On the constitution o f the atmosphere. By John Dalton, 
Esq. F. R. S. &c. Communicated January 12, 1826. 
Read February 24, 1826. 
The 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- 
