232 Mr. A. T. Waterman on the Positive 



3. Lead Chloride. 



(a) The salt used for this test was primarily very pure, 

 but it had undergone considerable exposure to the air. Its 

 melting point is 500°. 



The salt was moistened with distilled water and placed 

 on the same strip, mounting (a), on which Cu 2 Cl 2 and AgOl 

 had been heated. The strip gave no significant emission 

 below 1000°. 



Emission was first detected at 460°, but it was too small 

 for measurement and almost immediately died away. No 

 emission was then found until 600°. This steadily increased 

 with rise in temperature and readings were taken at 820°. 

 The increase of emission with temperature was much less 

 than with any of the other salts. 



In making observations with the slit method when the 

 emission was fairty large, curves were found with more than 

 one maximum. In this case it seemed probable that this 

 arose from different parts of the salt emitting rather than 

 from there being more than one kind of ion present in any 

 quantity. This was concluded partly from the shape of the 

 curves ; partly from the fact that the balance method gave 

 results that were fairly steady ; and from the fact that the 

 values of m/H, calculated for a possible two kinds of ions 

 (there being two distinct maxima), seem very unlikely. 

 For instance, in a typical curve, the ions would have to be 



m/H = 18-6 and m/H = 357'5 



on this supposition. 



The mean values for twenty readings follow : — 



V. 



H. 



M. 



T. 



ejm. 



wi/H. 



170 







820° 







203 



2030 



B&S 



to 

 940° 



250-9 



37-0 



The balance method gave results varying from 35*5 at first 

 to 39 at higher temperatures. The slit method, if interpreted 

 as above, gave about 39. 



Lead Bromide. 



(a) This salt was of about the same purity as the PbCl 2 

 just described. Its melting point is 400°. It was moistened 

 with distilled water and placed on the strip just used for 

 PbCl 2 . The emission from the strip was negligible below 

 925°. 



