CALCULATION OP THE COMPOSITION OF HYDRATES. 



27 



and alcohol as soon as ice began to separate, in order that they might not 

 be surrounded by the very cold freezing mixture while temperature equi- 

 librium was being established. The proper correction for the increase in 

 concentration due to the separation of the ice was introduced, except in the 

 more concentrated solutions. It w y as always a perfectly simple matter to 

 ascertain when pure ice separated from the solution, or when some of the 

 dissolved substance was deposited. This could usually be determined by 

 the gritty or sand-like character of the solid that separated, which was easily 

 detected by rubbing the stirrer up and down in the freezing-tube. In all 

 cases it could be detected by the behavior of the thermometer as more and 

 more of the solid was allowed to separate. When the cryo- 

 hydric point was once reached, the thermometer remained 

 stationary as more and more of the solid separated. Other- 

 wise, the thermometer would fall as more and more pure ice 

 separated and increased the concentration of the solution. 

 That solid solutions were formed seems highly improbable. 



The conductivities of the solution were measured at 0, in 

 order to be able to ascertain, as nearly as possible, the dissocia- 

 tion at that temperature. The most concentrated solutions 

 were placed in the form of cell seen in fig. 18. The value of 

 /too was determined directly, whenever it was practicable to 

 do so, simply by increasing the dilution of the solution until 

 the molecular conductivity reached a maximum constant 

 value. In other cases the value of /co was obtained from the 

 velocities of the ions in question and Kohlrausch's law, /*<x 

 =a + c. 



It is well known that the conductivity method is not an ac- 

 curate measure of dissociation in concentrated solutions. It is, however, 

 an approximate method of measuring such dissociations, and is the best we 

 can do at present. All that can be said is, that it probably gives values 

 of the right order of magnitude. 



Since concentrated solutions contain, in a liter of solution, less than 1,000 

 grams of water, it is necessary to determine the specific gravities of all such 

 solutions, in order to correct the freezing-point lowering actually observed 

 for the difference between the amount of water really present in the solu- 

 tion and 1,000 grams of water. The observed freezing-point lowering, thus 

 corrected, can then be compared directly with the freezing-point constant 

 of water, increased in terms of the percentage dissociation of the solution 

 in question. 



FIG. IS. 



