Chemistry and Physics. 65 



SCIENTIFIC INTELLIGENCE. 



I. Chemistry and Physics. 



1. Osmotic Pressure and Electrolytic Dissociation. — The well- 

 known Van't Hoff theory of osmotic pressure supposes that a 

 substance in solution exercises a pressure equal to that which 

 would be produced if the solvent were entirely removed and the 

 substance left in the state of gas in a volume equal to that orig- 

 inally occupied by the solution. It has been shown by Arrhenius 

 that if P is the osmotic pressure of a solution containing n mole- 

 cules of a dissolved substance and N molecules of the solvent, d 

 being the density of the solvent, D that of the vapor of the solu- 

 tion and T the absolute temperature, then P is proportional to 

 wc?T/ND. If n molecules are contained in unit volume of the 

 solution (or of the solvent if the solution is very dilute) d/M. may 

 be substituted for N, where M is the molecular mass of the sol- 

 vent. If the liquid solvent and its vapor have the same value 

 for M, then at any constant temperature T, this value is a simple 

 constant multiple of D for all solvents. Hence if n is constant, 

 the osmotic pressure is constant, and it follows that n molecules 

 of a dissolved substance in unit volume of any solution have, 

 when the solution is dilute, the same osmotic pressure at constant 

 temperature. Compton has now pointed out that the above 

 assumption, i. e. that the liquid solvent and its vapor have the 

 same molecular mass, is not justifiable. While it is true that 

 many liquids are normal or " monomolecular," the molecules of 

 the liquid being the same as those of its vapor, yet in quite as 

 many other cases, liquids are found to be " associated," their 

 molecules being built up of several gaseous molecules. In this 

 case if M is the normal monomolecular mass, the liquid molecular 

 mass will be scM, the value x being the factor of association, and 

 representing the number of normal molecules going to form one 

 molecule of the associated liquid ; n normal molecules forming 

 n/x liquid molecules. Hence the question arises, if association 

 takes place in either solvent or dissolved substance, what effect 

 will be produced on the osmotic pressure ? Evidently this will 

 be no longer constant but will vary proportionally to x. Now 

 the hydrocarbons and their halogen derivatives, the ethers, most 

 etherial salts, most aldehydes and ketones and probably the 

 acid anhydrides, may be considered monomolecular ; while the 

 alcohols and allied compounds, the organic acids, the oximes, 

 some nitro-compounds, and compounds containing amidogen, may 

 be viewed as associated. In place of using the osmotic pressure 

 itself to test the question, certain relations between this value 

 and certain other properties of solutions were used for the pur- 

 pose. Thus, if r be the heat ol liquefaction of a solvent whose 

 density is d and absolute fusing point T, then for a monomolecu- 

 lar liquid having a normal vapor density, the value rd/T is con- 



Am. Jour. Sci. — Fourth Series, Vol. V, No. 25. — January, 1898. 

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