March i6, 1905] 



NATURE 



477 



escencp is revived wlien the substance is heated, while the 

 colour fades or disappears. In quartz, gflass, and calcspar 

 it is easy to locate the seat of phosphorescence within the 

 layers which have been penetrated and coloured by the rays. 

 This penetration may take place to the depth of several 

 millimetres, and in materials like quartz, glass, or calcspar 

 it is certain that whatever changes occur in these layers 

 must be chemically self-contained and quite removed from 

 atmospheric influences. The view, therefore, that coloration 

 is due to the reduction of one of the elements of the sub- 

 stance, e.g. potassium in glass, affords only a partial ex- 

 planation of the phenomena. It is necessary to suppose that 

 the separation and retention of the metal ions must equally 

 involve the separation and retention of the ions of the acid 

 radicle with which the metal had been combined. P'orther. 

 in order that the different ions may be kept apart, the un- 

 altered molecules must act as barriers or insulators to prevent 

 their re-combi nation. But the molecules are not always 

 immovable barriers, for, as the temperature is raised, their 

 mobility is increased, and their insulating power is corre- 

 spondingly diminished. E,\periments were made on the 

 storage of latent phosphorescing power at all temperatures 

 between — ioo° and -1-300°. While for each substance there 

 is a range of temperature over which its storage capacity 

 is at a maximum, yet the range over which storage can 

 take place is sometimes very wide. In calcspar, storage 

 occurs over the whole range investigated, while in crystal- 

 lised platinocyanide of barium it was only observed between 

 — 100° and —50°. 



February 16. — " Polarised Rontgen Radiation." By Dr. 

 Charles G. Barkla. Communicated by Prof. J. J. Thom- 

 son, F.R.S. 



Experiments on secondary radiation from gases and light 

 solids subject to X-rays led to the theory that during the 

 passage of Rontgen radiation through such substances each 

 electron has its motion accelerated by the intense electric 

 fields in the primary pulses, and consequently is the origin 

 of a secondary radiation which is most intense in the 

 direction perpendicular to that of acceleration of the electron, 

 and vanishes in the direction of that acceleration. The 

 direction of electric intensity at a point in a secondary pulse 

 is perpendicular to the line joining that point and the 

 origin of the pulse, and is in the plane passing through the 

 direction of acceleration of the electron. 



A secondary beam the direction of propagation of which 

 is perpendicular to that of the primary will, according to this 

 theorv, be plane polarised, the direction of electric intensity 

 being parallel to the pulse front in the primary beam. If 

 the primary beam be plane polarised, the secondary radiation 

 from the electrons has a maximum intensity in a direction 

 perpendicular to that of electric displacement in the primary 

 beam, and zero intensity in the direction of electric displace- 

 ment. 



In these experiments the secondary radiation from light 

 substances was too feeble to allow accurate measurement of 

 the intensity of the tertiary radiation. 



A consideration of the method of production of primary 

 Rontgen rays in an X-rav tube, however, leads one to expect 

 partial polarisation of the primary beam proceeding from 

 the antikathode in a direction perpendicular to that of pro- 

 pagation of the impinging kathode rays, for there is 

 probably at the antikathode a greater acceleration along the 

 line of propagation of the kathode rays than in a direction 

 at right angles ; consequently, in a beam of X-rays proceed- 

 ing in a direction perpendicular to that of the kathode 

 stream, there should be greater electric intensity parallel to 

 the stream than in a direction at right angles. 



Using such a beam as the primary radiation, and a light 

 substance, as air, paper, or aluminium, as the radiator, the 

 intensity of a secondary beam as indicated by an electroscope 

 was found to reach a maximum when the direction of the 

 kathode stream was perpendicular to that of propagation of 

 the secondary beam, and a minimum when these two were 

 parallel. 



.\ number of experiments made this evidence of partial 

 polarisation of the primary radiation conclusive. 



When heavier metals, such as copper, tin, and lead, which 

 emit a secondary radiation differing considerably in character 

 from the primary producing it, were used as the radiators, 

 no variation in intensity of secondary radiation was observed 



NO. 1846, VOL. 71] 



as the bulb was rotated, though experiments were made 

 with primary radiations varying considerably in penetrating 

 power. 



Geological Society, February 17. — Dr. J. E. Marr, 

 F.R.S., president, in the chair. — .\nnual general meeting. — 

 In his anniversary address, the president directed attention 

 to the classification of the sedimentary rocks, pointing out 

 that the arrangement of the events which, taken together, 

 constitute earth-history, according to their proper sequence 

 in time must ever remain the territory of the geologist in 

 which he will pursue his labours by e.xclusively-geological 

 methods. He pointed out that, since the time of William 

 Smith, and mainly by the adoption of his principles, the 

 classification of the strata had progressed towards perfection 

 by the method of successive approximations. He directed 

 attention to the many similarities between the records of the 

 geologicaf column and the records preserved in the " meteorc- 

 grams " of meteorologists. In each case the records were 

 impressed as zigzag and broken lines, though an additional 

 difficulty occurred in the case of the geological records owing 

 to their frequently-blurred nature. Further, the meteoro- 

 logist had his chronometer, whereas the geologist must con- 

 struct his time-scale from the records on what might, for 

 purposes of comparison, be referred to as the " geograms," 

 or strips of the geological sediments. In some cases the 

 lines of the geograms closely coincided with time-lines, in 

 other cases they departed therefrom more or less widely, and 

 it was one of the tasks of the geologists, from study of the 

 geograms, to attempt to draw in the time-lines. It was 

 to be remembered, however, that however closely the time- 

 lines and lines of the records coincided, they were not the 

 same lines. The principal variations in the records of the 

 geograms are due to alternate formation and cessation cf 

 deposit ; to the differences in character of the deposits owing 

 to various local conditions; to accumulation of contempor- 

 aneous volcanic material ; to variations in the nature of the 

 earth-movements ; to changes in the nature of the included 

 organisms ; and lastly to climatic changes, and proceeded to 

 consider the significance of these records as bearing upon 

 the classification of the sediments. The president advocated 

 the adoption of a triple classification, such as had been 

 already tacitly adopted in the case of some of the sediments, 

 as, for instance, those of Jurassic age, where divisions were 

 made according to (i) lithological change, (2) organic 

 change, and (3) time ; and pointed out how such a classifica- 

 tion could be adopted without any violent changes in an 

 existing nomenclature or in the rules of priority. He illus- 

 trated the suggested changes by a more detailed discussion 

 of the classification of the Ordovician strata, and pointed 

 out that we had names which might be used with chrono- 

 logical significance in the case of the divisions of the rocks 

 of most of the great systems; and maintained that, as our 

 knowledge increased, we could refer beds of new areas to 

 their places among the different series, marking periods of 

 time with a confidence similar to that with which we have 

 long assigned strata of remote regions to one or other of the 

 great systems. 



February 22. — Dr. J. E. Marr, F.R.S., president, in the 

 chair. — Exhibition of a series of Danish rocks illustrating 

 (i) the share that Echinoderms may take in rock-building; 

 (2) the transition from the Secondary to the Tertiary Era in 

 the Baltic basin near Denmark ; (3) the special conditions at 

 the close of the Glacial Period, in the limited area where 

 alone these rocks are now found as erratic blocks : Dr. F. A. 

 Bather. — On the order of succession of the Manx slates in 

 their northern half, and its bearing on the origin of the 

 schistose breccia associated therewith : Rev. J. F. Blake. — 

 On the wash-outs in the Middle Coal-measures of south 

 Yorkshire : F. E. Middleton. The opinion of the author 

 is that the wash-outs occupy the sites of winding streams, 

 meandering through the alluvial tracts in which the coal- 

 seams were being formed. 



Zoological Societ> , February 21 Mr. Howard Saunders, 

 vice-president, in the chair. — .\ contribution to our know- 

 ledge of the varieties of Laccrta miiralis in western Europe 

 and North Africa : G. A. Boulenger.— The Nigerian giraffe 

 {Giraffa camelopardalis peralta) and the Kilimanjaro Giraffe 

 (G. camelopardalis lippelskirchi) : R. Lydekker.— Dolphins 

 from Travancore : R. Lydekker. In this paper the author 

 made special reference to two specimens of the genus 



