PHYSICS. 351 



partition. In the latter case the velocity changes sign and the conden- 

 sation does not ; in the former the opposite is the fact. (C. R., Decem- 

 ber, 1886, c III, 1255.) 



HEAT. 



1. Production of heat. — Thermometry. 



The values of the solar constant which have been given bj' Pouillet, 

 Hagen, Crova, Violle, and Langley have been examined by Maurer. 

 In his opinion the considerably higher values of Violle and Langley 

 arise from the assigning by these experimenters of too high a value for 

 the amount of solar radiation on the earth's surface. Actimometric meas- 

 urements have recently been- made with a new apparatus of Weber's, 

 under remarkably good atmospheric conditions. Six of these were 

 made on the terrace of the Polytechnicum at Zurich, two on the top of 

 the St.Gothard Pass (2,100 meters), aud one on the Pizzo Centrale (3,000 

 meters). According to these, the maximum heat from the sun at mid- 

 day which a surface of one square centimeter receives in a minute un- 

 der perpendicular radiation is, in Zurich, from 1.10 to 1.32 thermal unit; 

 on the St. Gothard 1.38 to 1.4L unit; and on the Pizzo Oentrale, 1.52 

 unit. (Beibl. Phys., X, 82 ; Phil. Mag., Sept., 1886, V, xxii, 312.) 



Keller has sought to measure the increase of temperature produced 

 by a water-fall. In his experiments at Terni he obtained measurements 

 varying between 0.08° and 0.73°, while the calcuhited value was 0.37°. 

 Hence, while these results seem to prove the transformation of kinetic 

 energy into heat under these conditions, it is evident that they are 

 powerfully affected by the sources of error which he discussed. (Beibl. 

 Phys., X, 333 ; Phil. Mag., Sept., 1886, V, xxii, 312.) 



The use of gas for heating and motor purposes has rendered the de- 

 termination of its heating power of great interest. Witz has made 

 careful experiments in this direction, using a nickel-plated steel explo- 

 sion cylinder 2.36 inches in internal diameter and 3.54 inches high, the 

 metal being 0.079 inch thick. The top and bottom covers were screwed 

 on air-tight. Through the top cover a wire passed, and in the bottom 

 •was a valve for filling or emj)tying the cylinder. In use it was placed 

 in a vessel 4 inches in diameter and 8 inches high, which acted as a 

 calorimeter, and held about 1.76 pints of water. The mixture of air 

 and gas was placed in the cylinder over mercury and fired by an elec- 

 tric current. As the result of a large number of experiments it appears 

 that the gas used in the experiments gives in burning about 5,200 calo- 

 rics (kilogram-degrees Centigrade) per cubic meter, equivalent to 5S4 

 pound-degrees Fahrenheit per cubic foot. This accords fairly with the 

 values obtained by Dugald Clerk, which are 489,268 and 504,888 foot- 

 pounds per cubic foot as the mechanical value of London and Manchester 

 gas, corresponding to 5,372 and 5,640 calorics. The heating power of gas 

 may be increased 77 percent, by carburation. (Ann. Chim. Phys., 1885, 

 VI, VI, 256 ; Science, May, 1886, vii, 467.) 



