68 VAPORIZATION. 



pansible to an indefinite extent, in the proportion that pres- 

 sure upon them is diminished, and to be contractible under 

 increased pressure exactly in proportion to the compressing 

 force the well-known law of Mariotte. The bulk of atmos- 

 pheric air has been found rigidly to correspond with this law, 

 when it was expanded into 300 volumes, and also when com- 

 pressed into l-25th of its primary volume. But there is reason 

 to doubt whether the law holds with absolute accuracy, in the 

 case of a gas either in a state of extreme rarefaction, or of the 

 greatest density. Thus atmospheric air does not appear to 

 be indefinitely expansible ; for there is certainly a limit to 

 the earth's gaseous atmosphere, and it does not expand into 

 all space. Dr. Wollaston supposed that the material particles 

 of air are not indefinitely minute, but have a certain magnitude 

 and weight. These particles are under the influence of a power- 

 ful mutual repulsion, as is always the case in gaseous bodies, 

 and, therefore, tend to separate from each other ; but as this 

 repulsive force diminishes as the distance of the particles 

 from each other increases, Dr. Wollaston imagined that the 

 weight of the individual particles might come at last to balance it, 

 and thus prevent their farther divergence. On this view, which 

 is exceedingly probable, the expansion of a gas, caused by 

 the removal of pressure, will cease at a particular stage of rare- 

 faction, and the gas not expanding farther, will come to have 

 an upper surface, like a liquid. The eartlr's atmosphere has 

 probably an exact limit, and true surface. 



The deviation from the law of Mariotte, in the case of gases 

 under a greater pressure than that of the atmosphere, has been 

 distinctly observed in the more liquefiable gases. Thus, Pro- 

 fessor Oersted, of Copenhagen, found that sulphurous acid gas 

 diminishes, under increased pressure, more rapidly than com- 

 mon air. The volumes of atmospheric air and of the gas were 

 equal at the following pressures : 



Pressure upon air in Pressure upon sulphurous 



atmospheres. gas in atmospheres. 



1 .... 1 



1.175 .... 1.173 



2.821 .... 2.782 



3.319 .... 3.189 



It will be observed that less pressure always suffices to reduce 

 the sulphurous acid gas to the same bulk than is required by 



