616 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of ail ohm ; and from 230° upward, it appeared to have practically the 

 same resistance." As also noted by Faraday the application of external 

 heat is not necessary, as by the use of a source of current of sufficient 

 voltage the requisite current can be obtained to heat the specimen 

 sufficiently and " break down " its initial resistance. 



2nd Condition. After the initial high resistance has been broken 

 down and a current of some strength has passed, I have found no case 

 where the specimen attained its original high resistance. Usually, 

 after cooling, the resistance is but a fraction of that which originally 

 existed but is practically always several times greater than that which 

 exists when the specimen has become heated by a flow of current 

 through it. This stage, where the specimen shows a gradual lowering 

 of apparent resistance with increasing current or an increase in apparent 

 resistance with a diminishing current, but never returns to the abnor- 

 mally high initial resistance, is termed the second condition of the test. 



Srd Condition. After the current has passed through the specimen 

 for some time, — or usually more rapidly if the direction of the current 

 has been reversed, — the characteristics of the sulphide of silver speci- 

 men in its second condition are no longer found, and the resistance of 

 the specimen changes little with changes in current strength or with 

 heat. This peculiarity of what is here termed the " third condition " 

 was described by Faraday, who also noted that the properties of the 

 specimen described under the second condition " could not be renewed 

 until a fresh surface of the sulphuret had been applied to the positive 

 pole. This was in consequence of peculiar results of decomposition." 



Streintz's tests were made upon small cylinders of compressed silver 

 sulphide approximately two centimeters in length and fifteen hundredths 

 of a square centimeter in cross-section. Electrodes of platinum foil 

 were placed at the two ends of the cylinder and two supplementary 

 electrodes were embedded within the specimen. In my experiments 

 plates of compressed sulphide were used, which were rectangular in 

 shape and roughly three-quarters of an inch in length by five-sixteenths 

 of an inch in width ; the average thickness of the plates was about one 

 thirty-second of an inch. Electrodes of platinum, silver and copper 

 were used both in the form of tine wires or of foil. The terminals were 

 pressed into the plates of silver sulphide and the tests conducted both 

 while the specimen and its terminals were in the press and after they 

 had been removed. Many of the specimens studied by me had been 

 prepared by subjecting them to a pressure of 500,000 pounds to the 

 square inch. The pressure used in forcing the terminals into the speci- 

 men was approximately 25,000 pounds. 



The electrical measurements were made by the use of a Weston 



