1864.] 



Prof. Guthrie on Drops, 



457 



the size of the drop is partially recovered. There is a stage of dilution 

 when the specific gravity is r0680, where the drop-size is a minimum. 

 Further, it is seen from colum.n 5 that the quantity of nitre in a drop in- 

 creases continually as the strength of the solution increases, although both 

 the weight and the volume of the drop vary. 



Inversely, the regularity of the variation of drop-size, in the case of nitre, 

 points to the absence of hydrates of that body. 



It would be delusive to endeavour to construct a formula connecting the 

 specific gravity with the drop-size or drop-weight of the solution, but, as 

 before, a graphic representation serves to show the connexion between the 

 variables. In curve D, fig. 2, the abscissae represent the quantity of 

 nitrate of potash in solution, the ordinates show the corresponding drop- 

 sizes. As with chloride of calcium, it is seen that the drop-size of water is 

 larger than that of any solution of nitre. Curve E, fig. 2, having the 

 same abscissae as D, has ordinates which represent the drop-weights. 



It is confessedly a matter of great interest, and still greater diflSculty, to 

 determine exactly the relation which exists between a dissolved solid and 

 its solvent ; that is, to find out whether or when a solid should be viewed 

 as being in combination with a portion of the liquid in which it is dissolved. 

 Such questions may perhaps receive additional Hght from experiments 

 similar to the above, but more extensive, and performed with this special 

 object in view. Comparing the curves C and D, for instance, there can be 

 little doubt that the secondary maxima and minima of C are owing to the 

 existence of hydrates of chloride of calcium in solution. The only known 

 hydrates of chloride of calcium are Ca CI, 2 HO and Ca CI, 6 HO, the latter 

 of which contains 50*7 per cent, of Ca CI. Solution S contains about 42*5 

 per cent. It is noteworthy that, while the six-water chloride in the solid 

 state absorbs heat on solution, the solution S evolves heat on dilution, as 

 already mentioned. In the case of nitre we have in the drop-sizes evidence 

 only of the opposite efforts of two cohesions, that of the water and that of 

 the nitre. By pursuing this direction of experimental inquiry, evidence 

 may probably be got concerning the truth of BerthoUet's hypothesis of 

 reciprocal recomposition in the case of the mixture of the solutions of two 

 salts, AX and BY, where AY and BX are also soluble in water. 



