162 Scientific Intelligence. 



warm, the other to the cold season. During the summer there is 

 a pronounced minimum in the hot hours of the day, which is 

 more pronounced the nearer the point of observation is to the 

 surface of the earth, and the more it is free from the influence of 

 trees or buildings. There is another minimum in the night, the 

 importance of which varies in an inverse direction ; the existence, 

 however, of a double daily oscillation is well marked. During 

 the winter the afternoon minimum disappears more or less com- 

 pletely, while that of the night increases; there is thus a single 

 oscillation with a maximum by day and a minimum at night; the 

 hour of the latter corresponds to that of the second minimum of 

 summer, about 4 p. m. Observations at other localities show 

 very much the same phenomena. 



It is concluded, in the second place, that the diurnal variation 

 at the summit of the Eiffel Tower during the summer differs 

 entirely from that observed simultaneously at the Bureau central 

 a few hundred meters distant, but offers a close analogy in its 

 single oscillation to the variation observed in winter. Similar 

 results are obtained at the summit of a wooden pylon used to 

 support anemometers at the observatory at Trappes. 



The final conclusions reached are to this effect : that the influ- 

 ence of the soil and the maximum in summer (due, according to the 

 suggestion of Peltier, to the water vapor emanating from the sur- 

 face, and negative like that) is the cause of the perturbation in 

 the diurnal variation ; that the general law of this variation is 

 very simple, having a maximum in the day and a minimum 

 between 3:30 and 4:30 a. m. — Seances Soc. Fran$. de Physique, 

 1899, p. 91. 



3. Lehrbuch der Photochromie {Photographic der natihrlichen 

 Farben) von Wilhelm Zenkeb. Neu herausgegeben von Prof. 

 Dr. B. Schwalbe. Mit einem Bildniss des Verlassers und einer 

 Spectraltafel. Pages 15V. Braunschweig, 1900 (Fr. Vieweg u. 

 Sohn. — This volume is for the most part a re-publication of a work 

 by Zenker (1829-1899) issued by the author, in 1868, but thus far 

 little known. The sketch of the author's life by G. Krech, with 

 which the volume opens, shows that he was a remarkable man of 

 original talents, whose contributions to science covered a wide 

 range of subjects. Early interested in the work of E. Becquerel 

 (1848) on color photography, he devoted much time and study 

 to the subject, and his results are given in the volume before us. 

 The chief point which he regarded as being established was this : 

 that silver chloride, particularly the violet silver sub-chloride, 

 takes the same colors as the light rays which act upon it, and 

 shows them both by reflected and transmitted light. This result 

 was explaiued physically as due to the formation of stationary 

 light-waves which caused the separation of particles of metallic 

 silver in the silver chloride. It will be recognized at once that 

 this is the line in which such brilliant work has since been done 

 by Lippmann, and the relation of the experiments and theoretical 

 discussion of Zenker to the labors of those who have followed 

 him is given in the closing pages of the volume (133-157) by E. 

 Tonn. 



