Chemistry and Physics. 403 



equation. A few data are also given for the absorption by 

 aluminum of the end radiation after passing through iron. 



The observations on the absorption of X-rays in aluminum and 

 lead throw important light on the difficult question of the prob- 

 able wave-lengths of the penetrating gamma rays from radio- 

 active substances. The line of argument may be suggested by 

 the following brief account. The numerical data for X-rays 

 obtained by Rutherford and by Hull and Rice are collected 

 in the first six horizontal lines of a table. The first and 

 second columns contain respectively the voltages (84,000 to 

 3^96,000) and ^ the corresponding shortest wave-lengths (0.147 

 A to 0.063 A), conformably to the equation ^ = /iv. The 

 values of the mass-absorption coefficient (ft/p) for aluminum 

 and lead are entered in the third and fourth columns. The 

 seventh line pertains to the penetrating gamma rays from radium 

 C. In this line the first and second spaces are to be filled in, 

 while the third and fourth contain the values of /x/p given by 

 Ishino. A discussion of all the data in the last two columns 

 leads, by extrapolation, to a general estimate of the voltage 

 required to excite the gamma rays, and from this datum together 

 with E =^hv the order of magnitude of the wave-length of these 

 rays is obtained. The following quotation contains in concise 

 form the final conclusions. 



''In our present ignorance of the law of variation of /x/p with 

 frequency in this region of the spectrum, it is only possible to 

 estimate the actual wave-length of the most penetrating gamma 

 rays. It is clear, however, tha^- the waves are at least three 

 times and may be ten times shorter than those which correspond 

 to 200,000 volts, I. e., they correspond to waves generated by 

 voltages between 600,000 and 2,000,000 volts, and thus lie between 

 .02 and .007 A.U. It is thus clear that the gamma rays from 

 radium C consist mainly of waves of about j^-q the wave-length 

 of the soft gamma rays from radium B, and are of considerably 

 shorter wave-length than any so far observed in an X-ray tube, 

 with the highest voltages at our disposal." 



The last part of the paper deals with the f3 rays from radium 

 B and radium C and their probable relation to the associated y 

 rays. ''The results as a whole suggest that the groups of ^ 

 rays are due to the transformation of the gamma rays in single 

 and not multiple quanta, according to the relation E ^hv.'' 

 "If the single quantum relation should prove to hold generally 

 for the conversion of y rays into (3 rays, the magnetic spectrum 

 of p rays should afford a reliable method of extending the investi- 

 gation of X-ray spectra into the region of very short waves where 

 the crystal method either breaks down or is practically ineffec- 

 tive, and thus places in our hands a new and powerful method 

 of analysing waves of the highest obtainable frequency." — Phil. 

 Mag., xxxiv, p. 153, September, 1917. h. s. u. 



Am. Jour. Sci. — Fourth Series, Vol. XLIV, No. 263.— November, 1917. 



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