proceedings: philosophical society 75 



The 795th ineetiiij>- was held at the Cosmos Club November 24, 1917; 

 President Buckingham in the chair; 34 persons present. 



Messrs. H. E. Merwin and L. H. Adams gave a paper on Poly- 

 morphism of the oxides of lead. PbO appears in a red, tetragonal form 

 stable below about 500°, and in a yellow, orthorhombic form stable at 

 higher temperatures. Inversion with rising temperature is rapid, but 

 with falling temperature so sluggish that it is the yellow form that is pro- 

 duced conunercially from molten lead oxide. From hot solution in con- 

 centrated alkali both forms can be obtained. The yellow crystals 

 frequently come out first and definitely orient the subsequent red crys- 

 tals which are found attached to them. Pressure with a point upon 

 certain faces of the yellow form causes immediate transformation to 

 the red form at the point of pressui'e and along planes radiating in 

 definite crystallographic directions. Pressure upon other faces causes 

 cleavage, with little or no inversion. Heating causes the yellow crystals 

 to become red before they begin to glow, owing to a large temperature co- 

 .efficient of light absorption. In powder the red for n is dull j^ellowish. 



Discussion: The paper was discussed by Messrs. Buckingham, 

 White, Swann, and Burgess. 



Messrs. G. W. AIorey and E. D. Williamson gave a paper on Quanti- 

 tative applications of the phase rule. The subject of heterogeneous equi- 

 librium is usually developed by the aid of the phase rule, together with 

 the Le Chateliei' principle of mobile equilibrium. The phase mle is 

 a qualitative corollary of a perfectly general and widely applicable 

 equation, equation 97 in Willard Gibb's paper ' 'On the equilibrium of 

 heterogeneous substances," 



vdp = ■r]dt + midij.i + niodix-i-^- .... Wnrf/in 

 in which v, p, -q, and t denote volume, pressure, entropy, and tempera- 

 ture, resp'ectivelv ; n is defined as being {—') , e being the energy, 



vii the mass of component 1, and the subscripts denote constanc}^ of 

 volume, entropy, and the other masses respectively. 



The application of this equation to several problems was discussed. 

 Plrst, the method of derivation of the approximations known as the laws 

 of dilute solutions was exemplified by the derivation of Raoult's law, 

 stress being laid on the assumptions that it is necessary to make in order 

 to derive these appi'oximations fi-om the exact relations. The appH- 

 cation of equation 97 to some purely phase rule problems was then dis- 

 cussed. First, theorems were developed enabling the determination of 

 the sequence of the P-T curves that intersect at an invariant point, then 

 th? question of th? change in slope of the P-T curve of a univariant 

 equililjrium with change in composition of phases of variable composition 

 was discussed. 



The subject mattei' of this paper has appeared in amplified form in the 

 January number of the Journal of the American Chemical Society. 



Discussion: The paper was discussed by Mr. Sosman. 



Donald H, Sweet, Secretary. 



