CONCLUSIONS. 6 1 



First of all, the phase rule tells us at once that we can have no true 

 compound here between the components albite and anorthite, for such 

 a compound would mean one more component and an additional phase 

 in every solution before equilibrium could be established. Moreover, 

 if the mixture had been eutectic in character, the component (albite or 

 anorthite) which happened to be in excess would have crystallized out 

 in each case, causing a continual change in the composition of the re- 

 maining glass until the eutectic proportion was reached and the result- 

 ing charge would have contained only crystals of one (or, in case of 

 hysteresis, both) of the components and the eutectic. Our curve is 

 continuous and the resulting charges homogeneous for all proportions 

 of the components. Lane's suggestion* that the triclinic feldspars 

 form a eutectic series in which the eutectic proportion is at or near 

 Ab 2 An 3 is, therefore, not borne out by our experiments. 



Laying aside the eutectic mixture, and passing over to solutions of 

 components which are miscible in many or all proportions, we find a 

 small number of examples, chiefly organic compounds, which have 

 been studied as types by Roozeboom, Kuster, Bodlander, Garelli, 

 Bruni, Van Eyk, and others, among which our series appears to fall. 



APPLICATION OF THE LAWS OF SOLUTIONS. 



From the physico-chemical standpoint, the case we now have in 

 hand closely resembles Kuster's problem of 1891.! His measurements 

 were made upon mixtures of organic compounds of low melting point, 

 while ours reached a maximum temperature of 1532 , but we have, 

 between albite and anorthite, an exactly similar series of solid solutions 

 the melting pointsj of which change in nearly linear relation to the 

 percentage of the two compounds which enter into their composition. 



This simple linear relation was called by Kuster perfect isomor- 

 phism, and he formulated the ' ' Rule " which has since borne his name, 

 that the solidifying point of an isomorphous mixture lies on a straight 

 line joining the melting points of the components and can be calcu- 

 lated from the percentage composition of the mixture. If this line 

 proved to be slightly concave or convex, as it did in most cases, 

 imperfect isomorphism was assigned as the cause. To this rule an 



* Lane, Journal of Geology, xu, 2, p. 83, 1904. 



f F. W. Kuster, Zeitsehr. fiir Phys. Chem., 8, p. 577, 1891. 



X Kuster measured solidifying points, but we have pointed out above that such 

 measurements lead to no positive result in liquids of such viscosity as the feld- 

 spars, in which equilibrium is not established during solidification. Undercooling 

 rarely appeared at all in Kuster's cases. 



