58 
PROFESSOR R, THRELFALL AND MR. J. H. D. BREARLEY 
to Mr. Cook, who made the instrument in the Laboratory workshop. Most of the 
work in connection with mounting the moving parts and in installing the instrument 
was done by Mr. Brearley. 
A great, if not the chief, difficulty, in attempting to form a theory of electro¬ 
chemistry lies in the uncertainty surrounding the data. The memorable researches of 
Kohlrauscii on the conductivity of water, as well as those of Dixon on the effect of a 
small impurity on the mode of combination of gases, together with the quite allied 
and equally peculiarly large variations produced in thermoelectric power and contact 
force phenomena by minute differences of chemical composition, have had a most de¬ 
structive effect on the theories of twenty years ago. It is not too much to say that 
the electrical action of most bodies in a pure state is entirely unknown at present, 
although the fact of the continuity of the electrical properties of certain substances— 
e.g. mercury, gold, and copper—as they are more and more purified, raises a hope 
that the properties of substances as at present investigated may only differ quantita¬ 
tively from their properties when pure ; but whether this be so or not can only be 
decided by experiment. The question as to the effect of purification is one which 
must be discussed in each separate case, and probably over and over again, for the 
word “ pure ” has no significance except with respect to a definite state of the art of 
chemistry. It is to be noted that the final stages in a process of purification must 
always depend more or less upon hypothesis—for a point is soon reached where the 
impurities become too small in amount to be amenable to ordinary chemical examina¬ 
tion. Theoretically this can be got over by operating on large quantities of substances, 
but in practice such is not found to be the case, for the greater the bulk of substance 
to be prepared, the more time is required for its preparation* and the greater is the 
chance of small and unnoticed accidents occurring. In the case of sulphur this is par¬ 
ticularly well marked, for the chief source of impurity is in dust from the air—which 
leads to a contamination almost proportional to the time required in making the 
preparations. We look forward to the time when the exclusion of dust during 
chemical operations of precision will be regarded as of as much importance as its ex¬ 
clusion in bacteriological research. The sulphur we have used is probably as pure as 
it can be got without taking special precautions to avoid all contact with dusty air. 
The absence of such precautions vitiates to a great extent many observations of dis¬ 
charge in air, and led Quincke! to explain away the diminution of the resistance of 
benzene and other liquids of small conductivity with increased voltage per unit 
length. 
In the case we have studied, the physical action of dust particles is not to be 
feared, and appropriate means were adopted for minimising the possible chemical 
action between dust and sulphur. 
One of the results we have attained with sulphur in a certain condition—is a 
* Stas, “ Recherclies.” 
f Quincke, “Wied. Ann.,” 1S86. vol. 28, p. 546. 
