882 DR S. M. JOHNSTON ON THE BOILING AND FREEZING POINTS OF 



into a final computation be remembered, not to speak of the calculations. Into every 

 final computation there entered at least five experiments and two curve readings or 

 interpolations. The agreement above mentioned is sufficient to suggest, for the chloride 

 iodide, and bromide of lithium, also for" strontium bromide, that up to a concentration 

 of 4 or 5 gramme molecules per litre the same quantity has been measured, or in 

 other words, that for these salts the hydration is both molecular and ionic. 



For each of these salts — that is, lithium chloride, etc., as above — the hydration 

 found per ion on the assumption that ions only hydrate increases with concentra- 

 tion up to the highest concentration. This would seem improbable, and is sufficient 

 to suggest that there is hydration other than ionic. For each of the salts the 

 degree of hydration per molecule, on the assumption that molecules only hydrate, 

 diminishes with increase of concentration, and no indication is given from this 

 standpoint as to whether hydration is molecular, or ionic, or both. Consequently, 

 for some salts, notably some of those showing the larger hydration capacity, there is 

 evidence that both molecules and ions hydrate ; for others which hydrate less the 

 figures are indeterminate in their significance, as there is almost equal constancy, 

 whether we view hydration as ionic or both ionic and molecular. A third class, 

 as some of the ammonia salts, indicate that the ions only hydrate, as hydration 

 ceases with ionization. To these perhaps another class should be added, such as 

 zinc * sulphate and magnesium sulphate, for which the values of the elevation constant 

 found are smaller than the theoretical value, which indicates that the molecules of these 

 salts associate or enter into combination with each other. 



Observations were made at very high concentrations for calcium chloride and 

 bromide, for lithium chloride and bromide, and for strontium bromide, not only to find 

 out how the maximum hydration would vary and what it would become for such 

 concentrations, but also to see what it would become per molecule of dissolved substance 

 or per molecule in solution, with a view to ascertaining whether the molecules of water 

 of hydration at high concentration might or might not become less than the water of 

 crystallization of the salt. The results showed that for calcium chloride the molecules 

 of hydration water per molecule of dissolved substance fell off to 1'9. The salt takes 

 up six molecules of water of crystallization. It thus appears that in this instance 

 the water of hydration per molecule may be very much less than the water of 

 crystallization. The minimum hydration per molecule of dissolved substance for 

 calcium bromide obtained was 3 '3. The salt crystallizes with six molecules of water. 

 The minimum hydration per molecule for strontium bromide found was 4*0 ; the salt hav- 

 ing six molecules of water of crystallization. Lithium chloride and lithium bromide each 

 crystallize with two molecules of water of crystallization, and the smallest hydration 

 values obtained per molecule were 1*9 and 2 "3 respectively. 



Consequently the results obtained for these several salts as to their hydrations per 

 molecule indicate that, for those salts which hydrate with six molecules of water of 



* Pa<'e 871. 



