PHYSICS. 257 



Mouton, in an elaborate memoir, lias given the results of his experi- 

 ments to determine the wave-length of dark heat rays by a method 

 depending- on the position of interference bands in polarized light. In 

 the formula the wave-length appears as a function of the thickness of 

 the interfering film, of the difference of the indices for the two rays of 

 this wave length, and of the number of wave-lengths concerned. By 

 the measurement of these values the wave-length was easily calculated. 

 The result obtained is noteworthy, since it gives us a wave-length 

 actually measured of 0.002140'"™, or 2.14 lu If this be compared 

 with the wave-length of the ultraviolet line R, measured by Mas- 

 cart, 0.00031775, it will be seen that the wave length of this ultra-red 

 line is more than six and a half times greater. The entire ether gamut 

 known is more than two and a quarter octaves, while the total chemical 

 and luminous radiation covers scarcely a single one. — {Ann. Chim. Pliys., 

 V, xviii, 145, 1870 5 J. Phijs., viii, 393, December, 1879 ; ix, 1 13, April, 

 1880.) 



Eosetti has given a series of measurements upon the temperature of 

 the voltaic arc, using a method which consists in receiving upon the 

 face of a thermopile placed at a convenient distance, rays emitted from 

 a surface of known area, and measuring the thermic effect on a sensitive 

 galvanometer. The following are his conclusions: 1st. The carbon pos- 

 itive pole from the moment of the production of the electric light has 

 always a higher temperature than the negative. 2d. These tempera- 

 tures vary according to the variation in the strength of the current. 

 3d. They are as much higher as the radiating surface is smaller, pro- 

 vided, obviously, that the end of the point be included. 4th. For the 

 negative pole the minimum temperature was found to be 1910° C, the 

 radiating surface being large and not very brilliant in some places. The 

 maximum temperature observed was 2532° C, the radiating surface being 

 one-half the preceding. 5th. For the positive pole the minimum temper- 

 ature found was 2312*^ C, the carbon being very large and the radiating 

 surface extended. For the maximum temperature, 3200° C, where the 

 carbon was thin and the radiating surface nearly one-fourth of that cor- 

 responding to the minimum temi^erature. Gth. The extreme T^oint of 

 the negative carbon may be considered at 2500° C. at least; that of the 

 positive carbon not less than 3200°. — {Ann. Chim. Phys., V, xviii, 457, 

 December, 1879 ; J. PJiys., viii, 257, August, 1879.) 



Eosettihas published also an extended memoiron the temperature of the 

 sun, giving the experimental methods employed and the results obtained. 

 For the purpose of determining the law of radiation as a function of the 

 temperature, the thermopile, which was connected with a Wiedemann re- 

 flection galvanometer, received the heat radiated from a Leslie cube, con- 

 taining water for the lower and mercury for the higher temperatures, 

 up to 300° C. Accurate thermometers gave the temperatures of the air 

 outside and of the liquid inside the cube. From the data thus obtained 

 an empirical formula was derived, which the author thinks rei)resents 

 S. Mis. 31 17 



