OF NATURAL PHILOSOPHY. 231 



factory knowledge of the general law of the equili 

 brium of the air under the influence of greater or less 

 pressures. These discoveries have since been ex- 

 tended to all the various descriptions of aerial fluids 

 which chemistry has shown to exist, and to main- 

 tain their aeriform state under artificial pressure, 

 and even to those which may be produced from 

 liquids reduced to a state of vapour by heat, so long 

 as they retain that state. 



(248.) The manner in which the observed law of 

 equilibrium of an elastic fluid, like air, may be con- 

 sidered to originate in the mutual repulsion of its 

 particles, has been investigated by Newton, and the 

 actual statement of the law itself, as announced by 

 Mariotte, " that the density of the air, or the quan- 

 tity of it contained in the same space, is, cceteris 

 paribus, proportional to the pressure it supports," has 

 recently been verified within very extensive limits 

 by direct experiment, by a committee of the Royal 

 Academy of Paris. This law contains the principle 

 of solution of every dynamical question that can 

 occur relative to the equilibrium of elastic fluids, 

 and is therefore to be regarded as one of the highest 

 axioms in the science of pneumatics. 



Hydrostatics. 



(249.) The principles of the equilibrium of 

 liquids, understanding by this word such fluids as do 

 not, though quite at liberty, attempt to dilate them- 

 selves beyond a certain point, are at once few and 

 simple. The first steps towards a knowledge of them 

 were made by Archimedes, who established the 

 general fact, that a solid immersed in a liquid loses 

 Q 4 



