60 ILLINOIS STATE ACADEMY OF SCIENCE 



ed solutions in the laboratory, and if our observations 

 were limited to a modest range of dilution, we might con- 

 clude that the state oi the salt in the solution was but 

 little affected by a change in the dilution, for such would 

 be the results of our experiments. Actually, however, we 

 are able to carry the concentration to a point where a 

 rather abrupt change sets in and crystals appear. A 

 similar error would result if our experiments in tempera- 

 ture were limited to but a few degrees. We would not 

 recognize that a change in the physical state accurs when 

 the temperature reaches a certain definite point. 



These analogies should be useful to us in considering 

 the effects of pressure. Such pressures as we can obtain 

 in the laboratory affect the melting point, the rigidity and 

 the density of a substance, but in general we think of a 

 substance as being unaltered by pressure. One hundred 

 thousand atmospheres is perhaps the upper limit of our 

 experiments, and this is a very modest range as compared 

 with the tremendous pressures that are developed in the 

 interiors of the astronomical bodies by the force of gravi- 

 tation. The pressure at the center of the earth can 

 scarcely be less than three millions of atmospheres. For 

 astronomical bodies of the same densities the pressure at 

 the centers varies directly as the square of the radius ; so 

 that a planet of twice the diameter of the earth and of 

 the same density would be subject to twelve millions of 

 atmospheres pressure at the center. But if such a planet 

 were made of the same materials as the earth — an alto- 

 gether reasonable assumption — then the extra amount 

 of pressure would result in an increase of density and 

 therefore a still further increase in the pressure at the 

 center. As we do not know the law connecting pressure 

 and density we cannot compute just what the pressure 

 would be, but it is certain, at least, that in astronomical 

 bodies in the same solid or liquid state and composed of 

 the same materials, the pressure at the center increases 

 more rapidly than the square of the radius, provided the 

 pressures are great enough to affect the densities, and the 

 larger bodies will have greater densities than the smaller 

 bodies. 



