264 PHYSICS. .■ 



volume of the outer cylinder is 400 times that of the inner. In making " 



an experiment the 200 grams of mercury, weighed with care, is heated 

 in an iron vessel, and poured into the central cup. The thermometer is 

 at once immersed in it, and its temperature noted by means of a tele- 

 scope a meter distant. The time of passage of the column from five 

 to five, or from ten to ten divisions is noted on a chronograph. The 

 pressures experimented with were from 700 to 0,400™™; and the general 

 result obtained was that the law of Dulong and Petit ceased to be ap- 

 plicable at about 1,520™™, the pressure exponent passing from 0.45 to 

 0.05 ; though at 3,040™™ it Avas 0.54, and at 0,080™™, 0.44. Studying the 

 phenomena more carefully between 800 and 2,000™™, Witz found that 

 Dulong's law represented the facts from 800 to 1,200™™ ; and that from 

 1,200 to 1,000™™ there was a very rapid change, the exponent rising to 

 0.85, the curve of velocities presenting a point of inflection near this 

 I)ressure. This the author shows in a graphic curve. — {Ann. Cliim. Phys., 

 y, xviii, 208, October, 1879.) 



LIGHT. 



1. Reflection and Refraction. 



Schwendler has proposed a new light-unit founded upon the ignition 

 of a strip of platinum, as originally suggested in 1844 by Draper and in 

 1859 by Zollner, and put into practical use for electrical illumination by 

 Farmer also in 1859. This unit of luminous intensity is given by a cur- 

 rent of 0.15 webers passing through a platinum strip 2™™ wide, 3G.28™™ 

 long, and 0.017™™ thick, weighing 0.0204 gram, having a calculated re- 

 sistance of 0.109 mercury-unit, and an actual resistance (including that 

 at the two contacts) of 0.143 mercury-unit at the temperature of 00° F. 

 This strip should be of pure metal, cut into tlie U-form, and connected 

 by large conductors with eight Grove cells of low resistance. The cur- 

 rent is maintained constant by means of a mercury rheostat, in the form 

 of a U-shaped groove, a millimeter in cross-section and a meter long, 

 cut in a board and filled with mercury. A copper bridge movable along 

 this groove enables the current to be kept constant, as indicated on a gal- 

 vanometer. The P. L. S. (platinum light standard) is equal to nearly 

 0.7 standard candle. The economy of this light in comparison with that 

 of the voltaic arc is over 70 times in favor of the arc. — [Phil. 2Iag., Y, 

 viii, 392, November, 1879; J. Phys., ix, 135, April, 1880.) 



Eder has suggested a chemical photometer. The solutions are pre- 

 pared by dissolving separately, each in one liter of water, 40 grams of 

 ammonium oxalate and 50 grams of mercuric chloride (corrosive sub- 

 limate). When used, two volumes of the former solution are mixed 

 with one volume of the latter, and exposed to the light. The liquid be- 

 comes turbid, and a black precipitate is thrown down. The weight of 

 this precipitate per square centimeter of surface exposed to the light 

 gives the measure of its intensity.— (i3cr. Ale. Wien, 1879, 240; J. Phys., 

 ix, 110, March, 1880.)- 



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