244 Mr. A. T. Waterman on the Positive 



almost entirely K-emission. The readings obtained with 

 the commutated direct current are near those given by the 

 simple direct current regardless o£ its direction. The 

 steadiness of conditions with the former was by no means 

 all that could be desired, and this was probably one of the 

 reasons for the exceedingly wide range of values obtained 

 at the higher temperatures. There was at one place good 

 evidence of the appearance of Na-emission (m/H = 23). In 

 order to make the calculated value of m/H change from 

 around 40 to 80, assuming that the character of the emission 

 remains the same, it would be necessary for dto be decreased 

 by about 2*5 mm., which is extremely unlikely, even for a 

 temperature difference of 1000° C. Besides, such a change 

 could be observed without difficulty from outside the 

 apparatus. At the highest temperatures the strip was easily 

 seen on account of its luminosity, and no bending of the 

 strip was noticed. Since with the present arrangement ions 

 from different parts of the strip should undergo equal de- 

 flexions, the only explanation for these results seems to be 

 that the emission at any temperature is composed of Mo + 

 ions, together with impurity emission consisting principally 

 of K + . The value of ?w/H obtained by the balance method 

 varies with the proportion of impurities present. Inasmuch 

 as the taking of these readings and the consequent heating 

 were carried on continuously for a period of twenty-six 

 hours, it is most probable that the impurity emission had 

 been largely driven off by the end of the experiment, the 

 permanent characteristic emission remaining. 



After the experiment the distance d was measured and 

 found to agree within *08 of 1 per cent, with the value 

 determined at the outset. 



The curves of increase of emission with decreasing resist- 

 ance (rising temperature) resembled those of an ordinary 

 thermionic current. The decay of the thermionic current 

 with time is quite rapid, particularly at the start. This 

 would of course point to the presence of an impurity 

 emission. But as the emission has seemed to be largely due 

 to impurities in most cases, it is impossible to say whether 

 the emission which appears to be characteristic of MoS 2 

 suffers a decay or not. The experiments just described, and 

 others made with direct current heating, indicate that this 

 emission becomes more prominent after long heating, a fact 

 which strengthens the conclusion that it is a characteristic 

 emission. 



As a means of further investigation, the method naturally 

 suggests itself of heating the molybdenite on a platinum 



