by Displaceme?it in their own Mother-liquor. 35 



The Crystal. — Table IV contains four compartments. In 

 the first (a) we have the values of T, the temperature at which 

 the crystals and mother-liquor of each salt were in equilibrium, 

 and that at which the various displacements were observed. 



Under the experimental conditions, which have been minutely 

 described above, it is impossible to fix in advance the exact 

 temperature of equilibrium of the crystallizing liquid. This 

 is given by the meteorological conditions, modified by the 

 structural features of the laboratory and of the apartment or 

 enclosure where crystallization takes place. 



In .the second compartment (b) we have the values of D, or 

 the specific gravity of the salt in crystal at T, referred to that 

 of distilled water of the same temperature as unity. The 

 data in this compartment are in most cases for different, but 

 always neighboring, temperatures. The differences of the 

 values of T are, however, so small and those of D are so great 

 that we may discuss the specific gravities as if they had been 

 made at one common temperature. 



On examining the values of D, we see that they increase 

 with those of MR in Table III ; but the increase is not con- 

 tinuous, it is remittent. It takes place triad-wise ; and this 

 holds whether we take the triads in column or in line. Com- 

 paring salts in the same line, we see that replacing Rb by Cs 

 causes a rise of specific gravity which is twice as great as that 

 caused by the substitution of Rb for K. Comparing salts in 

 the same column, the replacement of CI by Br causes more 

 than double the rise caused by the substitution of I for Br. 

 However we regard it, we see that the specific gravity of the 

 salts is a periodic function of their molecular weight, within 

 the ennead. 



In the third compartment (c) we have the values of -=— or 



the displacement of one molecule (MR) of salt stated in grams 

 of water, and in compartment (d) the same constant is stated 



in gram molecules of water (-— ^ . In dealing with the spe- 

 cific gravities, we saw that, whether we follow the columns or 

 the lines, they increase with increase of molecular weight. In 

 the case of the molecular displacements this holds for the 

 columns but not for the lines. In these the salts of rubidium 

 have the greatest molecular displacement, the potassium salts 

 have the least, and the caesium salts occupy an intermediate 

 position. As we shall see later, this irregularity is due to a 

 specific peculiarity of the caesium salts. Meantime it may be 

 noted that the volumetric equivalent of one gram molecule of 

 any of the salts of the ennead varies from 2*124 H„G to 3 - 204 

 H 2 0, the iodides having the highest and the chlorides the 



