1428 
that had been treated in this way, the resistance disappeared in the 
same way as with pure mercury *); much of the time spent on the 
preparation of pure mercury by distillation with liquid air, might 
therefore have been saved, without the experiments on the sudden 
disappearance of the resistance which were made with mercury 
prepared in the ordinary way with double distillation giving other 
results. 
Even with the amalgam that is used for the backing of mirrors, 
the resistance was found O at helium temperatures. (Later Dee. 1912) 
it was found that it disappeared suddenly, as with the pure mercury 
but at a higher temperature. *) 
Where the influence of impurities, in the form of mixed crystals 
in the solid mercury, seems to retire into the back ground, the 
next most natural supposition is that less conductive particles, sepa- 
rated out of the mercury during the freezing, or coming amongst 
the mercury crystals in some other way, bring a resistance into the 
path of the current. But if we do not assume that a thread of per- 
fectly pure mercury can possess a residual resistance itgelf, this theory 
of the origin of the potential differences is not very probable, because 
in a resistance-free path of current, only by a closing of the whole 
section by an ordinary conductor resistance is produced. Particles 
of the sort we mean, as also other casual circumstances, for instance 
the manner of freezing and small cracks, can influence the magnitude 
of the threshold value of the current density derived from the ex- 
periments, but the values found for this quantity. although they vary, 
differ so little, that in addition to the causes mentioned we must 
assume for a thread of pure mercury the existence of a residual resis- 
tance which we will call a “microresidual” resistance, to distinguish it 
from the “additive mixture” resistance to be attributed to impurities. 
§ 10. Erperiments on the possible influence of contact with ar 
ordinary conductor upon the superconductivity of mercury. In the 
reasoning that we have just given it is assumed that the laws of 
current division between two conductors which touch each other 
also hold when one of the conductors consists of mereury below 
4°19 K. But this assumption might not be correct. In the line of 
1) Perhaps nol even a quantity of the order of a thousand millionth of zine or 
gold is absorbed in solid mercury. The application of the sensitive test of the 
disappearance of the resistance may be of value for the theory of solid solutions. 
Of course in our argument we only deal with absorption in a form which comes 
into consideration for the resistance (mixed crystals). 
2) This part of the text is changed in accordance with the facts see § 13 y in - 
Vill of this series. 
