864 Notices respecting New Boohs. 



weighing *018 gr. per square cm. caused a drop of *401 in 

 the logarithm (to base 10) o£ the primary rays, and only of 

 •447 in the case of the emergence secondary rays from 

 copper, of *645 in the case of platinum rays, and *805 

 of iron rays. But when four filter-papers weighing '02 gr. 

 per square cm. were used as screen, the drop in the case of 

 the primary rays was "010 — only one-fortieth of the drop 

 caused by a copper screen of nearly equal weight. In the 

 case of the secondary rays, however, the same screen caused 

 a drop in the case of copper rays of *100, platinum rays *053, 

 and iron rays of *188 — that is to say, for these soft rays the 

 filter-papers are much more nearly on an equality with 

 copper, weight for weight, than they were for hard rays. 

 It is interesting to bear this in mind when considering the 

 very large quantities of secondary ionization which some 

 substances seem to give. The ionization is always measured 

 in air, which of course consists of atoms not very different 

 in weight from those contained in filter-papers. Con- 

 sequently primary rays, and secondary rays which differ 

 very little from the primary, are very penetrating to air, 

 and cause relatively small ionizations therein. But secondary 

 rays from Cu and Fe are softened so much as to bring them 

 within reach, so to speak, of air, which rapidly converts 

 them into cathode rays, so that there is a very large 

 ionization. For the cathode rays produced from these 

 secondary rays have probably but little less energy than 

 those produced from the primary ; the speed of the cathode 

 ray does not differ very greatly with the penetration of the 

 primary X-ray, so far as experiments have shown. The 

 very large secondary radiations, which some substances 

 appear to give, therefore, owe their magnitude largely to the 

 fact that the air in which they are measured is sometimes 

 ten to twenty times as favourable to them as to the primary 

 rays which produced them. In this way we may account 

 to some extent for the startling results obtained by Crowther 

 in the case of arsenic and bromine (Phil. Mag. Nov. 1907). 



L XXXII. Notices respecting New Books. 



Lehrhuch der Thermodynamic. Based on Lectures of Dr. J. D. 

 v. d. "Waals by Dr. Ph. Kohnstamm. First part. Leipzig 

 and Amsterdam : Maas & van Suchtelen. 



THE present volume deals in general with the theoretic side of 

 thermodynamics from the point of view of the physical chemist, 

 and most especially with the portion concerned with the phenomena 



