RECENT ADVANCES IN SCIENCE 189 



Tammann {Zeit. Anor. Chem., 1919, 107, 1-239) marks a con- 

 siderable advance in the study of isomorphism. Tammann 

 has examined the resistance to chemical reagents of a large 

 number of mixed crystals, and finds that, whereas their phy- 

 sical properties vary continuously with the composition, this 

 is not the case for the galvanic and chemical properties. 

 Those reagents which attack only one of the components, such 

 as nitric acid on the Au-Cu alloys, are shown to be without any 

 action on the mixed crystal when the unattackable component 

 exceeds a definite concentration. A series of mixed crystals 

 can be divided into (i) the series containing 100 to gx per cent, 

 of attackable component, which is completely disintegrated by 

 the action of the reagent ; (2) the series ^1 to ^a, which is 

 partly attacked ; and (3) the series g^ to o per cent., which, 

 except for the molecules on the surface layer, is unattackable. 

 The resistance limits of normal mixed crystals, i.e. those which 

 have been well tempered, lie at molecular fractions which are 

 multiples of |. Thus boiling nitric acid removes the whole 

 of the copper from a gold alloy if the gold content is less than 

 f mol ; if the composition lies between | and | mol of gold, 

 the material is unattackable. 



These reaction limits can occur only at temperatures where 

 there is no appreciable diffusion of the atoms in the crystal 

 {i.e. in the Au-Cu series of mixed crystals the copper and gold 

 atoms are not interchangeable) ; if the temperature be raised 

 and the diffusion of the atoms becomes appreciable, then the 

 protective action of the noble metal disappears, and the chemi- 

 cal properties vary continuously with the composition. The 

 sharpness of the limits of protective action depends on the 

 previous history of the alloy ; crystals which are prepared at 

 ordinary temperatures by electrical deposition or by precipita- 

 tion from solution are more reactive and give less sharp 

 reaction limits than those prepared by fusion of the components 

 and tempered at temperatures not far below the melting- 

 point. Thus alloys with the same molar composition, pre- 

 pared in different ways, may behave differently to chemical 

 reagents. Tammann ascribes these differences to the manner 

 in which the atoms are distributed in the space lattice. He 

 concludes that the arrangement of the atoms in those mixed 

 crystals which are prepared at ordinary temperatures, though 

 fulfilling the requirements of symmetry, is irregular, whereas 

 in the well-tempered material there is a uniform distribution of 

 the atoms. During the process of tempering the " unsym- 

 metrical " mixed crystals are converted into the " symmetri- 

 cal " variety. 



The work has been extended to non-metallic mixed crystals, 

 and provided that these have been well tempered, they yield 



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