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SCIENCE 



[N. S. Vol. XXXII. No. 817 



ultimate constitution of matter," but is firmly 

 grounded upon the cardinal principles of con- 

 servation, dissipation and transformation of 

 energy, which have not been invalidated by 

 any single fact of recent science. In the Tale 

 professor's exhaustive memoir on chemical 

 equilibrium we see the huge fabric of theo- 

 retical chemistry developed like a plant out of 

 these single germs, and not one of its seven 

 hundred equations and formula has been dis- 

 credited or disproved by any result of labora- 

 tory investigation. Eather do the physical 

 chemists tend more and more to look up to 

 Gibbs as the theoretical founder of their sci- 

 ence, and each year has brought forth some 

 new and interesting application of his ideas, 

 from the card-diagram by which the engineer 

 tests the heat wastes of an engine up to indus- 

 tries so various as the manufacture and re- 

 pair of steel rails, the chemical investigation 

 of soils and their constituents, the artificial 

 manufacture of rocks and precious stones, the 

 liquefaction of obstinate gases like helium, 

 the testing of such " materials of engineer- 

 ing " as Portland cement, the complete re- 

 vision of analytical chemistry and the most 

 recent aspects of colloidal chemistry — the 

 capillary and dispersoid chemistries and the 

 interfacial chemistry (Oberfidchenchemie) of 

 the Germans. The Gibbs theory of osmosis, 

 in particular, is strong above all others in the 

 simplicity of its ideas, and in stating his 

 theorems that the osmotic pressure and the 

 surface-tension are both of them functions of 

 the temperature and the chemical potentials 

 of the component substances involved, we 



- Possibly one reason why some chemists have 

 neglected Gibbs's theory of osmosis is that many 

 years after he published it, he seems to have fallen 

 under the sway of the van't Hoff hypothesis and 

 in 1896 published an independent proof of the 

 latter based upon the assumption that the mole- 

 cules of the solute " should not be broken up in 

 solution nor united to one another in more com- 

 plex molecules." {Nature, LV., 461.) This tour 

 de force, while completely at variance with the 

 chemical hypothesis of the formation of hydrates 

 in solution, does not in the least impair the value 

 of Gibbs's earlier and more comprehensive argu- 

 ment of 1874. 



know that the " chemical potential " of a sub- 

 stance is no mere fanciful concept, but means 

 the measurable surface energy of the sub- 

 stance (per unit mass) that is available for 

 mechanical effect. A substance of higher 

 chemical potentiality than another would 

 therefore be one having a greater surface 

 energy, that is, a greater immediate intensiiy 

 (as distinguished from capacity) for dis- 

 tributing, diffusing or dissipating energy, i. e., 

 for doing work, and such a substance would 

 obviously have a greater power for chemical 

 combination over substances more inert. In 

 connection with the application of his ideas 

 to experimental pathology by Traube and 

 Ascoli, these theorems of a physicist, whom 

 Boltzmann declared the greatest synthetic 

 mathematician since Newton, should have 

 some interest. E. H. Garrison 



Abmy Medical Museum 



SPECIAL ARTICLES 

 ^' THE LUMINOSITY OF COMETS 



The return of HaUey's Comet has given rise 

 to increased interest in these " heavenly wan- 

 derers," and it has also induced much specula- 

 tion as to the cause of their luminosity. 



It is known from the revelations of the 

 spectroscope that comets are composed of mat- 

 ter in its various phases. Some comets may 

 be wholly gaseous during most of their jour- 

 ney, but when remote from the sun the gas 

 condenses to liquid and finally freezes, owing 

 to the low temperature of interstellar space. 



The spectra of comets change as they pass 

 to and from the sun. Near the sun they show 

 the Fraunhofer lines indicating that the light 

 from them is reflected sunlight. At other 

 times, when nearer the sun, they show emis- 

 sion spectra of sodium, calcium, iron, etc., 

 indicating that the sun's heat has volatilized 

 these metals; but during most of the comet's 

 journey the only light emitted is similar to 

 that produced by the ionization of gases 

 through electric influence. 



The nucleus of a comet may be solid, liquid 

 or even more or less dense gas. In any case 

 the nucleus is surrounded by a gaseous en- 

 velope which is more attenuated the greater 



