Mabch 6, 1896.] 



SCIENCE. 



359 



board slide of the plateholder. Bits of sheet 

 aluminum (J mm. thick) and of sheet lead (J 

 mm. thick) were laid upon the cardboard slide 

 of the plateholder. Two to five hours' expos- 

 ure to a 900 Watt arc at a distance of 25 cm. 

 produced no perceptible effect. 



The bits of sheet metal were then for conven- 

 ience placed next to the gelatine film and the 

 plates, masked with two thickness of black 

 cardboard, were exposed to the arc for three 

 hours at a distance of about 12cm. The plates be- 

 come quite hot, about 80°C. after development 

 the action was found to be quite strong where 

 the plate was not screened by the bits of metal. 

 The bits of metal, each several square centi- 

 meters in area, screened the plates about 

 equally. The portions of the films under the 

 bits of metal showed veiy faintly the texture 

 of metal surface, as if by reflection. 



The plates were then arranged so as to ob- 

 viate excessive heating by ventilation, and 

 masked with two thicknesses of black card- 

 board and two to four thicknesses of mask 

 paper, the bits of sheet metal being placed out- 

 side the cardboard slide of the plate holder as 

 at first. Three hours' exposure at a distance of 

 15 cm. from the arc produced no perceptible 

 effect. 



The arc was then arranged to play between 

 zinc and carbon, taking about ten amperes at 

 thirty-five volts. The plates arranged as de- 

 scribed in the previous paragraph were exposed 

 to this zinc arc for two hours at a distance 

 of about ten centimeters. The zinc rod was 

 cathode for about one hour and anode for about 

 one hour. No perceptible effect was produced. 



It seems justifiable to conclude from these 

 experiments that Eontgen rays- are not given 

 off in any abundance by the electric arc, and 

 that they are not of the same nature as the 

 ultra-violet of the spectrum, or at least that they 

 are not of the same nature as the ultra-violet, 

 which is present in any abundance in the light 

 emitted by the electric arc between carbon 

 electrodes or between zinc electrodes. 



In demonstrating the presence of Rbntgen 

 rays it is necessary in every case to exercise 

 the greatest care in the rigid exclusion of every 

 other agent capable of affecting the sensitive 

 plate, such as ordinary and ultra-violet light, 



electric charge acting directly upon the film, 

 mechanical pressure, high temperature, etc. 

 These rays and the cathode rays are distin- 

 guished among all other actinic radiations by 

 the facility with which they pass through 

 metals and from each other by their different 

 behavior in the magnetic field, as appears from 

 Rontgen's paper. W. S. Feanklin. 



Ames, Iowa. 



SCIENTIFIC LITERATURE. 

 CrTundzuge der Marinen Tiergeographie. Anleit- 

 ung zur Untersuchung der geographischen 

 Verbreitung Mariner Tiere mit besonderer 

 Beriicksichtigung der Dekapodenkrebse. Von 

 De. Aenold E. Oetmann, in Princeton, N. 

 J., U.S. A. Mit 1 Karte. Jena, Verlag von 

 Gustav Fischer. 1896. Pp. 96. M. 2. 50. 

 This is an excellent contribution to zoogeogra- 

 phy, which ought to be in the hands of every- 

 body interested in the fascinating questions of 

 animal distribution. A great number of highly 

 interesting points are also discussed, important 

 for the biologist and geologist. 



The principal aim of this work, the author 

 states in the introduction, is to call the atten- 

 tion of the scientific world to the highly inter- 

 esting study of the distribution of marine ani- 

 mals hitherto greatly neglected. Before all, the 

 principles had to be established, according to 

 which the distribution of marine animals has to 

 be examined; in doing so it was necessary to 

 discuss the general principles of animal distri- 

 bution. Since the question of the distribution 

 of species is most intimately connected with that 

 of their origin the latter had to be examined, 

 and the result is reached that the principle of 

 separation or isolation is one of the most im- 

 portant factors. As an example of distribution 

 Dr. Ortmann selected the group of decapod 

 crustaceans, of which he has made special 

 studies. He finishes his introductory remarks 

 with the very pertinent sentence that without 

 extensive and critical systematic preliminary 

 work fruitful geographical studies are absolutely 

 impossible. 



The work is divided into seven chapters. 

 The first chapter gives an historical review of 

 the development of zoogeographical science. 

 He distinguishes three periods. 



