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n=500:9, a=59.4, b=47:5, r=0.292 millim.

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n=591.5, a=10:3, b=43.0, r=0.319 millim.

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Of these numbers the mean of the values , for each of the metals has been taken, and considered to hold good for the mean of the temperatures T. In this way we find, the conducting power of silver at 0° C. being = 100, that of Sodium

at 21.7 C. = 37.43 Magnesium 17.0 = 25.47 Calcium

16.8

= 22:14 Potassium

20:4

= 20.85 Lithium

20.0 = 19.00 Strontium

20.0 6.71 The potassium and sodium used for the experiments were commercial. The calcium, strontium, magnesium, and lithium were obtained electrolytically, as described in the Chem. Soc. Quart. Journ. vol. viii: pp. 107, 143.

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In the case of potassium, sodium and lithium, it was to be feared that a conducting power a little too small would be obtained, on account of their unavoidably oxidizing a little before the observations could be made. The oxidation of the wires took place gradually, and had some influence on their resistances, as some experiments on this point showed; the resistance of a wire immediately after being secured in the wire-holder was determined, and again after 5', 10, 15', &c. had elapsed. In the following Table the values of L, and the calculated values of , are given :

a

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In the alteration of the resistance with the time, as is shown by the Table, lies in all probability the chief reason why the absolute values of L, and L, in the former Table for potassium and sodium

vary

more than in the case of the other metals. A circumstance, owing to which several experiments failed, may be mentioned: sometimes a perfect metallic contact between the wire and the wire-holder was not obtained, in which case the resistance appeared too great, and always varying, so that the needle of the galvanometer could not be brought to remain at the zero-point; its movements were rapid and violent, especially when the trough was slightly shaken.

The conducting power of potassium and sodium at different temperatures was next determined for the solid as well as for the liquid state. For these experiments the metal was introduced into a wide thermometer-tube, which was blown out at two points, a and b_(fig. 6), before the glassblower's lamp, and at both of which places a short platinum wire was fused in; a tube of wider bore was joined on at one end. Through this tube hydrogen was passed for a short time, and then a piece of metal was pushed in at c; the whole length of it was heated until the metal was fused, when hydrogen was passed in at c, forcing the metal to the other end of it, upon which both ends were sealed by fusion; over a and b two pieces of caoutchouc tubing were fitted, one end of which was closed with pieces of glass rod, the other containing glass tubing. These were filled with quicksilver, and formed the quicksilver cups in which the connecting

a

wires were dipped. The whole tube was then brought into a trough, which was filled with water or a solution of chloride of calcium, according to the temperature desired. The liquid in the trough, which was kept continually agitated, was gradually heated by lamps placed underneath it, observations being made from time to time after the method used in the former experiments. The temperature was read off from a thermometer in the trough. The distance between the two platinum wires was treated as the length of the wire, and the diameter was measured by weighing a quantity of quicksilver which occupied a known length in the tube. From these data and the observed resistances the conducting power was calculated as above.

In these results an interesting fact was observable, viz. that at some distance from the point of fusion, as well in the liquid as in the solid state, the decrements in the conducting power witn the increase of temperature were almost in proportion, but near the point of fusion the decrease in the conducting power became much more rapid; with sodium this change appears to be very sudden, whereas with potassium it seems gradual

. This difference in these metals corresponds with their different behaviour in fusion ; namely, potassium does not become suddenly liquid like sodium, but first passes through a semifluid state*.

The values obtained can be expressed by the following formulas, in which 1 = the conducting power compared with silver

, a at 0° C. = 100, and t the temperature in Centigrade degrees. 0° ;

Potassium. For + between 0° and 46°.8,

a=2014 -0.08191 +0:000235t2; for t between 46°.8 and 56o.8,

1=668.26-40:402t+0.83801t? -0.00581521; for t between 56° 8 and 100°,

=13:35 -0.033937.

Sodium. For t between 0° and 9504,

X=32-54-0.1172t +0.0001271; fort between 96° 1 and 120°,

=23-38-0-07222t. Potassium melted between the temperatures 46°.8 and 56°, and sodium between 9504 and 96.1.

The curves (fig. 7) show the variation of the conducting power for different temperatures as calculated with these formulas. The

* See Regnault, Poggendorff's Annalen, vol. xcviii. p. 411; Phil. Mag. vol. xii. p. 489.

following Table gives the differences of the observed values of a and those calculated from the above formulas :

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The contraction of the metals on solidifying in the tube caused the conducting powers to appear too small, the diameter being considerably more reduced in some places than in others on account of small vacuums forming. With sodium this inequality was so great that the continuity of the wire was completely broken; in these cases the tube was heated in a bath above the fusing-point and then rapidly cooled, in order to make the vacuums as small as possible. With tubes of smaller bore the vacuums caused by the contraction were smaller, as the adhesion of the metal to the glass counteracted this tendency in some degree. The diameter of the tubes used for the experiments was 0.446 millim. for potassium, and 0.368 for sodium. The vacuums were here small, but still visible to the naked

The influence they had on the determination of the conducting power may be estimated by the comparison of the results obtained with the pressed wires with those found by this method for the same temperature. By the first we found for potassium, 90 On the Electric Conducting Power of the Metals of the Alkalies. error introduced into these results by these vacuums may be rectified by multiplying the expressions before given by the factors

eye.

a=20-85 if t=20:4; for sodium,

X=37.43 if t=21.7. By the second for the same temperatures,

1= 18.56 for potassium,

a=30.06 for sodium. If the vacuums be considered constant for potassium at all temperatures under 460.8, and for sodium at all under 95°-4, the * Ann. de Chim. et de Phys. 3rd series, vol. xliii. p. 472.

28.85

=1:1233,

18:56 and

37.43

=12451;

30.06 by which we obtain for potassium,

X=22-62-0.0920t + 0.000263t; and for sodium,

i=40.52-0.1459t +0.00015812. We may take it for granted that above the point of fusion these vacuums entirely disappear, and hence that from this point upwards the given expressions are correct. With potassium, between the temperatures 46°.8 and 56°8, there is certainly some change in the nature of them; but as we know no law according to which this takes place, we are not in a position to estimate their influence for this interval of temperature.

Matteucci* has found, that with bismuth the conducting power increases at the point of fusion, and I am able to corroborate his results by experiments performed in a similar way to those just described : there is probably some connexion between this and the contraction of this metal at the same point.

For the sake of another example, Rose's metal was experimented with; the curve obtained was similar to that for potassium. An interesting fact was observed with the pressed wires which was not found with those cast in tubes ; namely, if a pressed wire of Rose's metal be gradually heated and then cooled and the resistances at certain temperatures noted, these are found the same either with increase or decrease of temperature if the wire has not been heated above 40° C.; if, however, it has been heated above 40° and cooled, greater values are found than before, which become gradually smaller, at first quickly, and afterwards slowly, till they attain their original magnitudė. A wire which had been heated to 80° showed a change in its resistance even after an interval of six weeks.

The possibility of obtaining wires of the metals of the alkalies and alkaline earths, affords the means of ascertaining their order in the thermo-electric series, on which subject I am at present occupied, and shall shortly, if possible, make my results known.

The foregoing experiments were performed in the Heidelberg Physical Laboratory, under the direction of Professor Kirchhoff

, to whom I may be allowed in this place to express my sincere thanks for the valuable aid and advice I received from him.

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