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enormously high (as thermoelectric forces go), as in the case of half a dozen different metals I observed 3 volt, about the same as that given by a junction of bismuth and antimony at that temperature, did the ordinary formulæ hold so far. At lower temperatures these irregular effects are not so great, being more than proportionately smaller, but still on several occasions I found them distinctly marked even as low as 200° C.

In any series of observations the relation between the observed values of the temperature T and E.M.F. e is of the form ea(T-To) +b(T-T), where To is the temperature of the cold junction and a and b are constants to be determined, b being positive or negative, according as the lines of the two metals being examined intersect below or above To. In each, a and b were determined by the method of least squares. e having been calculated again in each case from the values of a and b thus obtained and the observed values of T, the means of the differences between the values of e thus calculated and those observed are given below in each case as the mean error in absolute units. To compare the error in different cases, I have also expressed it as the mean error in temperature. It will be noticed that frequently the discrepancies between different sets of observations on one junction are enormously greater than the mean error in one set, in spite of the fact that, as far as one could tell, they were in exactly the same condition in each case and were not even touched between the experiments. In view of the experiments referred to above and these discrepancies, one is led to the suspicion that the thermoelectric constants are not really constant, but that they vary in a given specimen in a manner which, if not arbitrary, yet arises from changes in condition which are inappreciable. I usually made three sets of observations on any pair of metals I examined, but with copper and lead I made more,-first to find out if the constants tended to become more constant, or, if not, to see if they varied in any regular manner; and, secondly, because I had taken copper as a base-line for some metals which lay close to it, and it was therefore necessary to determine its position with more than usual accuracy. The following are the thermoelectric heights observed; the metal with the higher line being in each case put first:

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The column headed "mean error "shows the mean error of each set of observations, expressed in absolute units of E.M.F., and also in fractions of a degree. The last column is the calculated E.M.F. of a pair one of whose junctions is at 0° C., the other at 100° C. The discrepancies between the values determined from different sets of observations are very much greater than the mean error of a single set, which appears to indicate, as already mentioned, that the thermoelectric "constants" are not absolutely constant. In deter

mining these constants, it is an open question whether they should be found from a short range of low temperatures, or whether a long range of temperatures should be observed. In the former case the most accurate observations can be taken, and are practically uninfluenced by the irregular effects mentioned above; but in this case the Thomson effect comes in as a quantity of a much smaller order of magnitude than the absolute height. If the range of temperature be greater, the Thomson effect is a much more important quantity, and can consequently be determined more accurately. In this case, however, the accuracy of the observations is very seriously interfered with by the irregularity of the effect referred to above. Different ranges of temperature with different junctions might be advisable, but I considered that a range of 85° or 90° would give sufficiently accurate results. I was disappointed in some of them, e. g. antimony and cadmium, but in these the mean result is probably not very far from the truth, especially in calculating electromotive forces; the error in the constants considered apart from one another is of course greater. In writing down the thermoelectric height of each relatively to lead, some metals will have two values; that is, in cases of three metals where the three pairs were examined separately, as for instance in lead, copper, and zinc, the height of the zinc line might be taken as the observed height above lead, or the sum (or difference) of the lead-copper and copper-zine heights. In this case, where the zinc line lies between the copper and the lead lines, the former of the two values will be the more accurate, and in estimating the position I have given the two values weight in the ratio of three to one. In the case of tin, which lies very close to lead compared with copper, I have taken the directly observed height as the true one, as the other is the difference of two large and nearly equal quantities-a very inaccurate method of observing the value. The following table is the result of all the observations, which is also shown graphically on the diagram with the exception of the antimony line :

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Metals Used.

Lead. This was very pure, and was specially prepared for me by the method of Stas, by Mr. N. T. M. Wilsmore, M.Sc., F.C.S. Lead acetate was dissolved in water and digested for a week with sheet lead. The solution was filtered into hot dilute sulphuric acid, and the precipitated sulphate treated with water, dilute sulphuric acid, and again with water till perfectly free from acid. The sulphate was then stirred for some days in a beaker with solution of large excess of ammonium sesquicarbonate and ammonia, and washed till free from soluble ammonium sulphate. This treatment was repeated till the lead sulphate was nearly all converted into carbonate. A little of this was converted into monoxide by heating in a platinum dish, and the remainder nearly dissolved in dilute nitric acid, and the oxide added little by little, thus precipitating any iron which might be present. The solution was again filtered and added to solution of excess of pure ammonium sesquicarbonate and stirred, thus forming pure lead carbonate. This was dried and reduced under potassium cyanide in an unglazed porcelain crucible, the contents of which were poured into a polished steel mould. The lead button was washed with water, alcohol, and ether, and immediately placed in a glass tube with a narrow tube sealed on to the end. This was alternately exhausted and filled with pure hydrogen about a dozen times, and finally exhausted with a Sprengel and hermetically sealed. The lead was again melted and run into the narrow tube, a compact rod being thus obtained, which was drawn through draw-plates down to a wire of the desired size. All the reagents used were as pure as could be obtained, and were tested for impurities, especially metallic impurities, and when necessary were repurified. Mr. Wilsmore considered that the lead was free from all other metals, and from impurities of all kinds except perhaps a trace of oxide formed during the washing in water.

Silver. This was prepared by Mr. W. Percy Wilkinson, Government Analyst, and considered by him to be "fairly pure." Silver chloride was reduced with wood charcoal and pure sodium carbonate. The ingot was washed with boiling water slightly acidulated with hydrochloric acid, and finally scrubbed with clean sand. This was re-dissolved in pure nitric acid, largely diluted, precipitated with dilute hydrochloric acid, washed with water, boiled with nitric acid,

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