Section 6. Method of Procedure. 25 



The bomb is next brought into the 281 bath. It is now necessary to 

 keep constant watch of the conductivity between the upper auxiliary elec- 

 trode and the walls of the bomb, so as to be sure that too much solution 

 has not been put into the bomb. The reasons for putting in solution 

 enough at the start to so nearly fill the bomb at the highest temperature 

 are first, to reduce the vapor space at all the temperatures as much as pos- 

 sible, since a correction has to be made for the amount of solvent in this 

 space ; and secondly, to see that the bomb is absolutely tight even at the 

 highest temperature, when the solution is in contact with the upper elec- 

 trode. This latter is important, since a leak, if it took place above the 

 liquid level, would cause a loss of pure solvent and a consequent increase 

 in the concentration of the solution. 



After completing the measurements at 281, the bomb is returned to the 

 218 bath, then to the 140 bath, and finally it is brought back to 26. In 

 going from a higher temperature to a lower much time is saved by cooling 

 the bomb, in front of a fan outside the bath, to a temperature which is at 

 least as low as that next desired ; for while heating in a vapor bath is 

 rapid, the cooling in it of a hotter body is very slow. During the first half 

 of the experiment, where the bomb is introduced each time into a hotter 

 bath, stirring inside the bomb takes place of itself, it being accomplished 

 by the rising vapor bubbles and the rapid convection currents caused by 

 the bottom of the bomb being so much hotter than the top during the heat- 

 ing. During the second half it is necessary to provide for this by shaking 

 the bomb before putting it into each bath. How much shaking is neces- 

 sary can be determined by repeating the operation and seeing whether the 

 conductivity has been affected by it. 



The advantages derived from cooling the bomb down through the same 

 series of temperatures and again taking measurements are that these fur- 

 nish a check on the accuracy of the preceding ones, and especially that 

 they show whether or not there has been any contamination, and if so, 

 between what temperatures it took place and to how much it amounts. 



The bomb is opened as soon as the experiment is completed, since other- 

 wise there may be trouble in getting the cover off because of the strong 

 adhesion of the gold packing-ring to the platinum lining below. This 

 effect increases with use, since a small amount of gold from the ring 

 adheres to the platinum each time, and subsequent rings will adhere more 

 firmly to this gold than they would to a clean platinum surface. The effect 

 can easily be reduced, however, as soon as it grows troublesome, by rub- 

 bing the platinum cover, where it comes in contact with the ring, with a 

 burnisher and by marking in the groove with a lead pencil. The trace of 

 graphite which adheres to the surface is very effective. 



