Crystallised Normcd Sulphates of Rubidium, $r. oul 



The employment of the compensated method has proved highly suc- 

 cessful, extremely concordant results being afforded with crystals not 

 necessarily more than 5 mm. thick. The twenty-nine different parallel- 

 faced crystal-blocks employed varied in thickness from 4*8 to 107 mm. 

 The main conclusions from the work are given in the following 

 summary. 



The coefficients of cubical expansion exhibit a progression, corre- 

 sponding to the progression of the atomic weights of the three respective 

 metals. This is true of both the constants a and b in the general ex- 

 pression for the coefficient of cubical expansion, a = a + 2bt, for any 

 temperature t. The actual values are shown in the following table : 



a. J). 



K.,SO 4 0-000 104 75 O'OOO 000 069 8 



Kb 2 SO 4 0-000 103 14 O'OOO 000 076 7 



Cs 2 S0 4 0-000 101 70 0-000 000 081 



The order of progression of the two constants is inverted ; , the 

 coefficient for 0, diminishes with increasing atomic weight of the metal, 

 while >, half the increment of the coefficient per degree of temperature, 

 increases. Consequently, the coefficients of cubical expansion of the 

 three salts converge, with rise of temperature, and attain equality in 

 pairs. Identity is attained for potassium and rubidium sulphates at 

 114, for potassium and caesium sulphates at 136, and for rubidium 

 ind caesium sulphates at 168. At 136 equality for all three is only 

 deviated from by one unit in five hundred. Beyond the temperature 

 of identity divergence occurs, and an increase of atomic weight is now 

 accompanied by an increase of expansion. 



The thermal deformation is of the nature of an expansion in all 

 directions in the crystals of all three sulphates. 



The differences between the coefficients of linear expansion along 

 the three axial directions of any one salt, although only amounting to 

 one-eighth of the total coefficient, are large compared with the differ- 

 ences between the values for the same direction of the three salts. 

 This, together with the fact that the replacement of one metal by 

 another is accompanied by considerable modifications of the relations of 

 two of the three values for the original salt, those corresponding to the 

 axes a and c, prevent the coefficients of linear expansion for any one 

 direction of the three salts from exhibiting any progression correspond- 

 ing to that of the atomic weights of the three metals. These direc- 

 tional perturbations are, however, mutually compensative, so that the 

 effect of interchange of the metals is clearly exhibited by the solid 

 deformation, the cubical expansion, in the progressive manner already 

 indicated. 



The increment of the linear coefficient of expansion along the axis c 

 of each salt is about twice as large as the increments for the other two 



