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In.1886 I began to make some experiments on the specific resistance of certain Australian gums, with a view to finding new insulating material. In connection with this I constructed and tested various galvanometers. In 1887 I was joined by Mr. J. A. Pollock, and in 1889 we published conjointly in the ‘Philosophical Magazine’ a paper on the “Specific Resistance of Imperfectly Purified Sulphur,” and on the “Clark Cell, as a source of Small Constant Currents.” I also published in the same volume a paper “On the Measurements of High Specific Resistances.” All these papers may be considered as preliminary to the present investigation of the behaviour of pure sulphur under electric stress, which has gone on steadily since June, 1889. The remainder of that year, the whole of 1890, and most of 1891, were occupied in preliminary experiments on galvanometers and in making and testing various arrangements of electrodes between which the sulphur to be examined was placed. This work was carried out entirely by Mr. Pollock. Early in 1891 I was joined in the work by Mr. J. H. D. Brearley, who has worked continuously at the subject with me up to the present time. I began the study of sulphur originally with the object of discovering the exact electrical properties of a pure, non-metallic substance, and chose sulphur because it appeared to offer advantages in melting at a moderate temperature, in being capable of existing in several allotropic forms, and, above all, in being capable of being brought to a high degree of purity with comparatively little trouble. With respect to the galvanometer, which has enabled us to perform the experiments described, so far as its general features are concerned, it was designed by me, in 1890, on the lines laid down in the ‘Philosophical Magazine,' series 5, vol. 28, p. 473; but the detailed design, as well as the actual construction, I owe 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 electrochemistry lies in the uncertainty surrounding the data. The memorable researches of Kohlrausch 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 destructive 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 quantitatively 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 cf 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 examination. 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 particularly 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 exclusion 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 discharge 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.

IV. Researches on the electric properties of pure substances.—No. I. The electrical properties of pure sulphur