NOYES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 185 



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 heating. 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 necessary 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 furnish a check on the accuracy of the preceding ones, and espe- 

 cially 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 

 otherwise 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 trouble- 

 some, by rubbing 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. 



When the bomb is not in use, good water is left standing in the 

 lower part. The cover is inverted and the upper chamber similarly 

 kept filled with water. If for the next experiment a more dilute 

 solution is to be employed, the bomb must first be heated with good 

 water to perhaps 218° for some little time. No amount of rinsing or 

 soaking out at ordinary temperatures will answer the purpose. There 

 is on this account a great saving of time effected by beginning with the 

 dilutest solution to be investigated, and afterwards measuring those 

 more and more concentrated. 



Our measurements at 300° were carried out, for the most part, after 

 complete experiments up to 281° had been made, so that they usually 

 consisted merely of measurements at 2G°, 30G°, and agaiu at 26 . 



