ON THE PHYSICAL STRUCTURE OF THE EARTH 203 



THE MECHANICAL AND PHYSICAL PROPERTIES OP THE MATTER 

 COMPOSING THE EARTH. 



(1) The materials of the Earth must manifestly influence its general 

 structure, and no inquiries with this structure can be usefully made if 

 the physical properties of these materials are not ke[)t in view. If the 

 interior of the Earth is in a fluid state it is reasonable to believe that 

 the fluid is not the ideal substance called by mathematicians a perfect 

 liquid, namely, a substance not only endowed with perfect mobility 

 amonfT its i)articles, but also absolutely incompressible. It is more rea- 

 sonable to believe that the fluid in question resembles the liquid out- 

 pourings of volcanoes, or at least some real and tangible liquid whose 

 properties have been experimentally studied. I have already shown 

 that by overlooking this simple principle certain untenable conclusions, 

 which assert the exclusively solid character of the Earth, have been 

 deduced. Here I propose to develop some additional arguments rela- 

 tive to one of the properties of liquids which has an essential bearing 

 upon the internal structure of the Earth. 



(2) In a former paper, on the limits of hypotheses regarding the 

 properties of matter composing the Earth's interior,* I find that hav- 

 ing referred to published statements where the facts were not clearly 

 put forward, I underrated the compressibilitj' of liquids as compared 

 with solids. The influence of the imperfect experiments of the Aca- 

 demia del Cimento has long injuriously operated in defining liquid and 

 solid matter, and has ]>roduced a remarkable conflict of opinions. 



On taking the results of the best experimental investigations it 

 appears that, although liquids are but slightly compressible as com- 

 pared with gases, they are highly compressible as compared with 

 solids. In many treatises on physics and mechanics which have a high 

 reputation, matter is divided into solids, elastic fluids or gases, and 

 incompressible fluids or liquids. Hence the erroneous inference seems 

 to have arisen that liquids are incompressible, not only in comparison 

 with gases, but also in comparison with solid bodies. I was surprised 

 to find this remarkably misleading proposition formally seated, long 

 after the decisive experiments of Oersted, Colladon, and Sturm, Keg- 

 nault, Wertheim, and Grassi, in such a work as Pouillet's Elements de 

 Physique^ and also in the German translation by Miiller. The great 

 compressibility of liquids as compared with solids is seldom affirmed as 

 a distinct general proposition in books on physics. It occurs, however, 

 in Deschanel's treatise, both in the original and in the English edition. 

 Daguin states, in vol. I of his Traite de Physique, 2d edition, p. 40, that 

 the compressibility of liquids was long considered doubtful, but never- 

 theless they are more compressible than solids. 



Lame also pointed out the great compressibility of liquids as com- 



* Philosophical Magazine for October, 1878, p. 265. 



