-1859] WRITINGS OF JOSEPH HENRY. 155 



In order to simplify the conditions and to give precise ideas 

 of the mechanical equilibrium of the atmosphere we will at 

 first suppose it to be a body consisting of simple atoms, 

 which though they obey the attraction of the earth repel 

 each other. This repulsion increases, as we have said in our 

 exposition of the atomic theory, with a diminution of the 

 distance of the atoms — a fact which may, perhaps, be best 

 illustrated by a portion of air confined by a movable pis- 

 ton in a tube closed at the bottom, as in the case of the ordi- 

 nary fire syringe, the well known instrument used for igniting 

 tinder by means of the condensation of a portion of air. If 

 such an instrument be placed under the receiver of an air- 

 pump, and the pressure of the atmosphere be removed from 

 it, the air which is contained under the piston will expand; 

 and if the tube be sufiiciently large this expansion will con- 

 tinue until the repulsive energy of the atoms under the pis- 

 ton is just equal to the weight of the piston itself. If we 

 now double the weight of the piston it will descend until 

 the air is compressed into half its first volume. At this 

 point a new equilibrium will take place between the weight 

 of the piston and the repulsive energy of the atoms. If 

 another addition be made to the weight of the piston it will 

 descend through another distance, and in all cases the com- 

 pression will be inversely proportioned to the weight applied; 

 but the density of the air, that is, the weight for a given 

 quantity increases as the bulk diminishes, and therefore in 

 all cases of a gas the density or the number of ponderable 

 atoms in a given space will be inversely proportioned to the 

 pressure applied. 



This fact was discovered independently by an English and 

 a French philosopher, and is generally known by the name 

 of the discoverers, namely, the law of Boyle and Mariotte, 

 but perhaps more frequently it bears the name of the latter. 



The same law applies to all other gases within certain 

 ranges. In the case of atmospheric air, within the limit of 

 experiment it appears to hold without variation, or if any, 

 with a very minute one, when great pressure is applied in 

 connection with a great reduction of temperature. In the 



