DENSITIES OF THE PLANETS. 279 



serious, is the low specific gravity of the Sun. If when the 

 matter of the Sun filled the orbit of Mercury, its state of 

 aggregation was such that the detaclied ring formed a planet 

 having a specific gravity equal to that of iron ; then the Sun 

 itself, now that it has concentrated, should have a specific 

 gravity greater than that of iron ; whereas its specific 

 gravity is not much above that of water. Some other inter- 

 pretation must therefore be sought. 



Differences in the specific gravities of the members of our 

 Solar System have several possible causes ; which may act 

 singly or in co-operation. 1. The one above supposed — 

 differences among the substances composing them, in respect 

 of their natures. 2. Differences among the quantities of sub- 

 stance : mutual gravitation of parts in large masses, produc- 

 ing, under conditions otherwise alike, gTeater density than 

 in small masses. 3. Differences among internal structures, 

 as dependent on the stage of concentration reached ; which 

 differences will be determined primarily by bulk (small 

 bodies cooling faster than large ones), and, secondarily, 

 by the ratio of centrifugal force to gravity (centrifugal force 

 impeding concentration). A glance at the foregoing list, 

 showing an immense contrast between the low specific 

 gravities of the great planets and the high specific gravities 

 of the small ones, raises the suspicion that the last cause of 

 difference is the chief cause. In advancing from the gaseous 

 state to the molten state, there have to be passed through 

 all intermediate states — states in which the gaseous and 

 liquid matters, mingled in some manner or other, bear con- 

 tinually changing ratios to one another. Beginning with an 

 envelope of precipitated cloud from which drops of metallic 

 rain fall inwards; going through stages in which this 

 metallic rain, getting ever thicker, fills the interior; coming 

 to a stage at which a molten nucleus begins to be formed ; 

 there has to be reached the other extreme, in which all the 

 condensible matter is aggregated into a molten spheroid. 



