Chemistry and Physics. 419 



And the authors find that nickel thus purified has an atomic mass 

 of 58*58, agreeing well with that of Russell, 58*74. — J. Chem. 

 Soc, lvii, 749, August, 1890. G. F. b. 



5. Waves in air produced by Projectiles. — Mach and Wentzel 

 employed photography to study the waves in air produced by 

 the motion of projectiles. In passing through the focus of a 

 photographic lens the projectile caused a discharge from a Ley- 

 den jar placed in the axis of the lens, at a distance greater than 

 the point of crossing of the projectile. The illumination produced 

 by the spark served to take an instantaneous photograph of the 

 passage of the projectile. The photograph showed a wave of 

 condensation before the projectile provided that its velocity was 

 more than that of sound. When the velocity was sufficient 

 the wave which preceded the ball had the form of an hy- 

 perboloid, of which the summit was in advance of the ball, and 

 the axis of which corresponded to the direction of the ball. 

 There were also traces of conical waves, of which the axes were 

 also in the line of fire and which arose at the base of the ball. 

 Some traces of less distant waves were seen upon denser points 

 of the surface of the . ball. All these waves made a less angle 

 with the axis of the projectile than the wave in front. When 

 the velocity was augmented, the angles made by the waves 

 with the line of fire were diminished. When the greatest veloci- 

 ties were attained, the space behind the projectile was filled 

 immediately with little clouds, there was no trace of a vacuum 

 behind the ball even when the velocity w r as 900 meters per 

 second. The waves produced in the air by the projectile at 

 higher velocities than that of sound progress more rapidly than 

 those due to feeble velocities, so that the compression in front of 

 the projectile is not sufficient to be depicted upon the photograph 

 under the form of waves. — Revue Scientifique, Sept. 13, 1890, p. 

 338. 



6. E. Mach and P. Salcher have extended the method of ob- 

 servation employed by Mach and Wentzel to the study of 

 streams of air blown from various orifices. — Ann. der Physik 

 und Chemie, xli, p. 144, 1890. 



E. Mach and L. Mach have also employed the method for 

 studying the interference of sound waves of great excursion. — ■ 

 Ann. der Physik, xli, p. 141, 1890. j. t. 



7. Re-determination of the Ohm. — Prof. J. V. Jones read a 

 paper on this subject at the late meeting of the British Associa- 

 tion at Leeds. He reviewed the method employed by Lorenz and 

 by Lord Rayleigh, and suggested a direct determination of the 

 mercury unit by this method, instead of the employment of solid 

 conductors by the shunt method and afterwards a comparison 

 with a mercury unit. He points out " if the artificial B. A. unit 

 can be dropped out of one's experiments as well as out of the 

 results, and the measurements made directly on mercury, the 

 simplicity would seem to be a recommendation, and the argument 

 is perhaps enforced by the consideration that there is very nearly 

 as much divergence in the results of different observers for the 



