R. UooJce — Law of Densities of Planetary Bodies. 393 



mixtures, every constituent being a substitution derivative of 

 normal aluminum poly- or ortho-silicate. To the latter compound 

 a structure may be assigned somewhat different in form from 

 the one I have chosen, without affecting in any notable way the 

 general system adopted. Upon this basis all the minerals 

 named are reducible to the same general type, which accounts 

 for observed isomorphisms, and for the relations of the micas 

 to other species, with fewer assumptions of hypothetical com- 

 pounds than are necessary under other known schemes of 

 interpretation. In most cases the evidence is clear, direct and 

 conclusive ; in other cases, few in number, it is at present 

 somewhat obscure. It may be claimed, without extravagance, 

 that the formulae have the merit of suggestiveness, and that 

 they form a scientific basis for future research. ^ff 



Washington, July 15, 1889. , M} *" 



Art. XLIX. — The Probable Law of Densities of the Plan- 

 etary Bodies; by Robert Hooke. 



As far as the writer is aware no inquiry has ever been made 

 into the connection between the diameters and mean densities 

 of the planetary bodies, or if such inquiry has been made we 

 have no record of the result arrived at ; yet it is an interesting 

 subject of investigation, and in the light of modern science it 

 is to be wondered at that the subject has not attracted the 

 attention of some investigator long before this. Some three 

 years ago the attention of the writer was accidentally directed 

 to this subject through the reading of Professor Newcomb's 

 Popular Astronomy, and the result of the investigation, which 

 is now given publicity for the first time, was quickly arrived 

 at, owing to its very simple character. 



In the absence of knowledge respecting the law of compres- 

 sibility of matter as it exists in the planets, it was impossible 

 to formulate the law of relative density of planetary bodies, of 

 different diameters, from theoretical considerations. It was 

 therefore necessary to determine the law by experiment alone, 

 and the experiments must, from necessity, be the great natural 

 ones presented by the planets themselves. It was necessary at 

 the very outset of the investigation to adopt the hypothesis — 

 that the planets were formed of the same material, or to put it 

 more accurately — that the material which forms the principal 

 part of the masses of the planets and their satellites would, 

 when subjected to the same conditions of temperature and 

 pressure, have the same density. Under this hypothesis the 

 difference between the mean and surface densities of the earth 

 was due alone to the compression to which the matter in the 



