424 Scientific Intelligence. 



natural colors by photography. He pointed out that the Lipp- 

 mann process requires great care in the preparation of the bromide 

 of silver emulsion in order to obtain the layer of finely divided 

 silver which is essential to the success of the Lippmann process. 

 Moreover there is no possibility of the multiplication of copies of 

 the photograph. Each copy requires an exposure and a separate 

 development. A resume is then given of the endeavors of various 

 workers to combine three negatives taken through red, yellow 

 and blue glass. Later investigators employed (in 1869), red, 

 green and violet. Suitable plates of the required sensibility, 

 however, could not be obtained. In 1873 Vogel prepared ortho- 

 chromatic plates of greater sensibility and an advance was made 

 in the representation of the values of colors. Further improve- 

 ment has been made in orthochromatic plates and in the method 

 of using suitable color screens. What formerly could be accom- 

 plished by chromo-lithography after a month's labor by the 

 employment of twenty stones can now be done in eight days with 

 only three plates. — Ann. der Physik und Chemie, No. 7, 1892, 

 pp. 521-527. J. T. 



10. Electrical Resistance of Allotropic Silver. — A. Overbeck 

 has examined the resistance of the new forms of silver discovered 

 by M. Carey Lea, of Philadelphia. The latter has shown that 

 allotropic silver can be converted into the ordinary silver in the 

 following ways : 



1. By heating. 



2. Mechanical means (pressure). 



3. By intense light. 



4. By electrical discharges. 



5. By treatment with different acids and solutions of salt. 

 Overbeck finds that all operations which tend to approximate 



allotropic silver to ordinary silver lessen its electrical resistance. 

 The electrical resistance proves to be a very sensitive reagent, far 

 more sensitive than any chemical process, to show differences of 

 molecular state. — Ann. der Physik und Chemie, No. 6, 1892, pp. 

 265-280. j. T. 



IT. " On the Simultaneity of Magnetic Variations at different 

 places on occasions of Magnetic Disturbance, and on the relation 

 between Magnetic and Earth Current Phenomena y" by William 

 Elles. (Abstract.) — In this paper the author refers to the ordi- 

 nary variations of the magnetic elements as observed at Green- 

 wich ; the annual progressive change ; the diurnal variation — 

 large in summer, small in winter, and also larger when sun spots 

 are numerous and smaller when sun spots are few; the irregular 

 magnetic disturbances and magnetic storms, and the accompany- 

 ing earth currents ; phenomena which are generally similar at 

 other places. 



He then invites attention more particularly to magnetic disturb- 

 ances. Those at Greenwich may, after a calm period, arise 

 gradually, or commence with great suddenness. When sudden, 

 the movement is simultaneous in all elements. The first indication 



