Vol,. 8, 1922 
PHYSICS: DUANE AND MAZUMDER 
45 
Where CO.NH groupings functionate in such constructions it should be 
possible to replace hydrogen in such amide groupings with methyl by ac- 
tion of this reagent. The hydrolysis of the products of alkylation should 
lead to the formation of methyl derivatives of known constitution, thereby 
contributing new knowledge regarding the constitution of the parent sub- 
stances. The final results of our researches will be published in the 
Journal of the American Chemical Society. 
ABSORPTION OF SHORT X-RA YS BY ALUMINIUM AND COPPER 
By Wiujam Duank and K. C. Mazumder 
Jeffkrson Physical Laboratory, Harvard Unive;rsity 
Communicated, February 1, 1922 
1. In 1914 one of us presented to the American Roentgen Ray Society 
and to the American Physical Society descriptions of methods of measuring 
the average or "effective" wave-length of a beam of X-rays, without the 
use of an X-ray spectrometer. Mr. Hunt and one of us^ have shown 
that even if a constant voltage (from a storage battery, for instance) 
drives a current through an X-ray tube the resulting X-rays are not mono- 
chromatic but contain X-rays covering a considerable range in the spec- 
trum, that depends upon the absorption of the matter through which the 
rays pass, as well as upon the voltage applied. We define the term ''effec- 
tive" wave-length to mean the wave-length that has the same absorp- 
tion in a given case as that of the whole beam. For many purposes this 
"effective" wave-length appears to be an important wave-length to know. 
The methods of estimating it described in 1914 were based upon a measure- 
ment of the absorption of the beam of X-rays in aluminium or copper 
and the determination of the wave-length from a curve giving the rela- 
tion between the absorption of monochromatic X-rays and their wave- 
lengths. Curves were drawn from all the data available at the time rep- 
resenting this relation. It was proved that over a very wide range of 
wave-lengths, namely, from about 3 angstroms down to .35 of an ang- 
strom, the relation between the mass coefficients of absorption and the 
wave-lengths could be reasonably well represented by the equation 
^ = 14.9 X^ (1) 
P 
where - is the total mass coefficient of absorption and X is the wave-length 
P 
in angstroms. 
Since 1914 Hull and Rice^ and Richtmyer^ have measured the co- 
efficients of absorption in aluminium and copper for wave-lengths lying be- 
tween .75 and .135. They find that the relation between the mass co- 
