774 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



direction of principal absorj^tion common to all the bands, but the 

 other two principal rectangular directions of diverse bands of absorp- 

 tion may be variably orientated with the plane of symmetry, g^. In 

 certain crystals these directions depart very little from the principal 

 axes of optical elasticity, in others they may make with these axes 

 very great angles, reaching sometimes 45°. Becquerel gave to these 

 directions the name, principal directions of anomalous absorption, 

 and inferred from them important consequences. 



These phenomena occur in crystals containing the rare earths and 

 are probably due to the complexity of the bodies which form the 

 crystals. De Senarmont had already shown that if we crystallize 

 mixtures in variable proportions of the component substances, geo- 

 metrically isormorphic but with the optic axes differently orientated 

 with reference to geometrically like directions, we can obtain a crys- 

 tal having any optical properties whatever, the resultant emergent 

 vibration being due to the resultant of the partial vibrations travers- 

 ing the various crystals, the symmetry of the total system depending 

 upon the portions of each component. So in apparently like crystals 

 the absorption may be wholly different, each component crystal ab- 

 sorbing certain radiations independent of its neighbors and there may 

 be no relation betw^een the axes of absorption corresponding to these 

 bands and the directions of symmetry of the crystal. If certain crys- 

 tals have principal directions of anomalous absorption, it is because 

 they are such mixtures of crystals. Crystals containing didymium 

 show the necessity of admitting its division into neodidymium and 

 prsesodidymium. Demarcay was able to separate the distinct ele- 

 ments in j)ra^sodidymium, the existence of which Becquerel had thus 

 shown the necessity. In neodidymium there are bands which char- 

 acterize complex crystals. 



This method of analysis can indicate bodies existing in a crystal 

 which are destroyed by its solution. If, having noted all the absorp- 

 tion bands in a crystal of sulphate of didymium, we dissolve it, the 

 spectrum of the solution is notably different, certain bands have dis- 

 appeared, others have suffered displacement, while yet others have 

 remained unchanged. The bands modified are those which in the 

 crystal w^ere marked by these anomalies and the variations may be 

 explained if we admit that there exists in the crystal such a mixture 

 which is completely destroyed and transformed by the water. 



The separation of the rare earths is very difficult. They are very 

 numerous and distinguished from each other only by extremely small 

 variations in their physical and chemical properties. It is generally 

 almost impossible to purify them. It would be very valuable to be 

 able to seize, by some optical process, in the heart itself of a mix- 

 ture, a crystalline body which shows this anomalous absorption and 

 which, the moment the body is dissolved, disappears to take place as 



