physics. 595 



the source has a temperature between 720° and 2000° ; those radiations 

 at comparatively low temperatures, 525° to 720°, alone penetrating the 

 selenium. A thin plate of selenium heated to 250° converts all radia- 

 tions into obscure oues. (C. B., xcv, 838; Am. J. Sci., December, 1883, 

 III, xxvi, 476.) 



Pringsheira has made a careful study of the radiometer, considering 

 particularly the influence of the glass-containing vessel, of the inclosed 

 gas and of the constitution of the vane. His apparatus consisted of a 

 single vane hung by a long bifilar suspension, and carrying a mirror 

 which reflected a beam of light on a scale. He concludes that a pressure 

 emanates from the heated side of the vessel, and that it increases with 

 the temperature and is independent of the nature of the material of 

 which the vessel is made. He finds that the absorption by the rarefied 

 medium is extremely small, and may be neglected. The action of the 

 vane is due to the rate of absorption and conduction on its two sides. 

 The theory of currents in the rarefied medium he considers untenable, 

 and believes that the kinetic theory of the radiometer is the most 

 reasonable one. The form of the vanes is without influence per se, 

 affecting the result only indirectly by their proximity to the sides of the 

 vessel. Hence his use of a single vane bifilarly suspended. ( Wied. 

 Ann., xxviii, 1 ; Phil. Mag., February, 1883, V, xv, 101 ; Am. J. Sci., 

 March, 1883, III, xxv, 229.) 



Eovelli has suggested various lecture experiments with the radiom- 

 eter. He places the instrument in the focus of a parabolic mirror, while 

 a mass of snow is put in the focus of a like mirror at a little distance, 

 facing the first. He puts the instrument under a bell jar containing 

 ether, on an air pump. On exhausting, the motion is reversed on ad- 

 mitting the air. He exposes the radiometer in the focus of a parabolic 

 mirror turned toward the weak light reflected from snow on a cloudy 

 day, and then turns the mirror away from the snow. He finds that 

 eight degrees of dark heat neutralizes the effect of the light emitted by 

 an ordinary candle at a distance of 45 centimeters. (Nature, March, 

 1883, xxvn, 144.) • 



4. Specific Heat. 



Cantoni and Gerosa have undertaken to determine the value of the 

 calory by measuring the rise of temperature in a mass of mercury 

 allowed to fall from a known height. The height selected was 2.225 

 meters, and the extreme values obtained in fifty-six experiments was 

 0.1 40° and 0.172°. The mean of the first series (twenty-two experiments) 

 was 0.1537°, of the second (twenty-two experiments), 0.1540°, and of 

 the third (twelve experiments), 0.1687°, the general mean being 0.1573°. 

 The authors found for the specific heat of mercury, 0.033375 as the mean 

 ;f three experiments; whence the mechanical equivalent of the calory 

 is 2.225 -h- (0.033375x0.1573°) = 423.82 kilogrammeters. The probable 

 error, however, is in the second significant number, the determinations 



