Intelligence and Miscellaneous Articles. 11 



water below 100 metres do not obey the law of variation of trans- 

 parency established by M. Forel for the more superficial layers. 



Compared with the series of plates exposed in the Lake, the 

 series which we have brought back from the Mediterranean show 

 a more slow and regular gradation, which leads us to think that, 

 whilst in the Lake the light would be quickly intercepted by the 

 more or less turbid deep layers, in the Mediterranean the absorption 

 due to the pure water alone would be the principal, if not the only 

 factor in the arrest of the luminous rays. — Gomptes Rendus, April 

 13, 1885. 



ON THE ELECTRICAL CONDUCTIVITY OF SOLID MERCURY AND OF 

 PURE METALS AT LOW TEMPERATURES. BY MM. CAILLETET 

 AND BOUTY. 



The electrical resistance of pure metals increases with the tem- 

 perature. Prom the experiments of Matthiessen * and those of 

 M. Benoitt, the mean coefficient of increase in the resistance for 

 one degree between 0° and 100° differs little in various metals, and 

 is only slightly removed from ^\-^ that is, the coefficient of the 

 expansion of gases. If the same law held at low temperatures, the 

 resistance of a metal, varying like the pressure of a perfect gas 

 under constant pressure, would furnish a measure of the absolute 

 temperature, and would cease to exist at absolute zero. 



Our experiments have been made with mercury and various 

 other pure metals. The mercury was contaiued in a spiral capillary 

 glass tube terminating in two large reservoirs in which dipped two 

 thick electrodes of amalgamated copper. The reservoir of a 

 hydrogen -thermometer J was in the interior of the spiral ; and the 

 whole being immersed in ice, or in a bath of methylic chloride or 

 of ethylene cooled by a current of air, according to the method 

 which one of us has giveu. To work with another metal, copper 

 for instance, the wire is coiled in a spiral form on an ebonite tube 

 in which are loDg slits, so as to be certain that the liquid was pro- 

 perly mixed, and that the bath and the resistance were measuring 

 at the same temperature. 



We have only made relative measurements. The resistance in- 

 vestigated was compared with that of a column of mercury at 0°, 

 by means of a Wheatstone's bridge and a very sensitive renecting- 

 galvanometer. The following are the results which we have 

 obtained : — 



1. Mercury. — The empirical formula given by MM. Mascart, 

 De JSTerville, and Benoit, for the apparent resistance of mercury 

 in glass above 0°, also holds for the freezing-point. "When it 

 solidifies, its conductivity suddenly increases in a ratio, which at 

 — 40° is equal to 4*08. The resistance of solid mercury decreases, 



* Proceedings of the Royal Society, vol. xi. p. 516. 

 t Gomptes Rendus, vol. lxxvi. p. 342 (1873). 



\ Hvdrogen-thermometer of constant volume, in which the pressure at 

 0° was" 509-3 millim. 



