130 



SCIENCE 



[N. S. Vol. XLII. No. 1073 



wide selection of familiar figures, which have 

 served, quite as much as the text which accom- 

 panied them, to advance the science of em- 

 bryology. 



Chapter 1 begins with the following interest- 

 ing statement : 



In the seventeentli and eighteenth centuries, the 

 most confused ideas of the nature of the process 

 of animal development still prevailed. Influenced 

 involuntarily by the religious dogmas of their time, 

 the greatest anatomists and physiologists, with few 

 exceptions, were of the opinion that the germ was 

 merely a much reduced miniature of the later fully 

 developed condition. 



Was not the idea of preformation a direct 

 result of observation and reflection upon nat- 

 ural phenomena, quite apart from " religious 

 dogmas " ? This indeed appears to be true, and 

 the comment of a distinguished theologian 

 upon Professor Hertwig's statement is as 

 follows : 



Let the men of science assume the parentage of 

 their own homunculi ! I certainly know of no 

 dogma that the germ was a miniature of the man 

 that was to be, nor even a doctrine which could be 

 understood or misunderstood in that sense. 



Thus it appears that this introduction needs 

 explanation or revision. The entire work 

 might profitably be expanded at many points, 

 notably so as to include some account of the 

 development of the lymphatic system. But 

 the title of the book disarms such criticism; 

 the elements are admirably presented in a text 

 which is simple, direct and substantial 

 throughout. 



Frederic T. Lewis 



SPECIAL ASTICLES 



ELECTRICAL DENSITY AND ABSORPTION OP |8-RAYS 



There have been a number of attempts to 

 relate the absorption coefficients of various 

 bodies for the j8-radiation to some physical 

 properties of the absorbing substance. In 

 1895, Lenard determined the absorption co- 

 efficients for cathode rays of a number of 

 bodies, and concluded that the absorption 

 varied approximately as the density, though 



his values of /x/D differed by more than one 

 hundred per cent. Similar results were ob- 

 tained for the absorption of /S-rays from 

 radium and uranium by Strutt and by Ruther- 

 ford. 



The first to determine the absorption coeffi- 

 cient of a considerable number of elements 

 for the j3-Tajs was Crowther, in 1906.^ 

 Crowther found the ratio of the absorption 

 coefficient to the density of the elements to in- 

 crease with the atomic weight of the absorb- 

 ing element, but apparently not according to 

 any regular law. Crowther, however, plotted 

 the ratio of the absorption coefficient to the 

 density of 31 elements against their respective 

 atomic weights, and obtained a number of 

 points \7hich he divided into groups having 

 no apparent physical or chemical relationships, 

 and showed that the elements in each of these 

 groups could be joined by curves having some 

 resemblance to one another. 



It is the purpose of this paper to show that 

 the absorption coefficient of the elements for 

 j8-rays is dependent rather upon the electrical 

 density of the absorbing agent than upon its 

 mass density. 



It has been shown in a number of papers 

 by the present writer how the electrical charges 

 of the dissociated ions in an electrolytic solu- 

 tion may be calculated from their masses and 

 their migration velocities in an electric field. 

 Knowing these charges and the volume occu- 

 pied by a gram-atom of an element in its 

 solid state, we may calculate the electric den- 

 sity of the element by dividing its atomic 

 charge by its atomic volume. It is the elec- 

 tric density calculated in this way which seems 

 to be an important determining factor in the 

 absorption of the /^-radiation. 



Unfortunately, only a small number of 

 atoms have had their charges calculated in this 

 way, but eleven of these are included in the 

 list of thirty-one elements whose absorption 

 coefficients for the /?-rays of uranium were 

 determined by Crowther. 



In the table below column ii contains the 

 values of A/p for these eleven elements taken 

 iPMl. Mag., 12, p. 379 (1906). 



