Vol. 7, 1921 
PHYSICS: C. BARUS 
47 
than figure 1 for measuring the relative strengths of the components. 
In figure 1 there appears to have been a drop in intensity just before 24 
was reached, in the measurement from high to low atomic weights. The 
curve is of interest as still containing 28 faintly and so serving to accurately 
locate the weights which otherwise would have been uncertain to a fraction 
of a unit. 
Figure 2 is one of several later curves taken under steadier conditions. 
These all have very closely the same appearance. The components 25 
and 26 are present very nearly in equal amounts; in some measurements 
25 was found about nine- tenths the intensity of 26. The component at 
24 is approximately 6 times as strong as the one at 26. The ratio of 
1:1:6 gives an average atomic weight 24.375, which is in as good agree- 
ment with the accepted atomic weight for magnesium as could be ex- 
pected with the wide slits used in these first experiments. 
THE ENERGY CONTENT OF THE DIAPASON 
By Carl Barus 
Department of Physics, Brown University 
Communicated January 10, 1921 
In Science, 52, 1920 (586-8), I indicated a method by which the in- 
tensely luminous achromatic fringes could be used, without further mech- 
anism, to determine the compression in the interior of a sounding organ 
pipe. 
Meanwhile Profs. A. T. Jones, of Smith College, and H. F. Stimson, 
of the Bureau of Standards, have called my attention to papers of Boltz- 
mann (Pogg. Ann., 141, p. 321) and Raps (Wied. Ann., 50, p. 193) and 
to some work of Stimson himself, which I had overlooked. These re- 
searches make most of my work superfluous. I will, therefore, confine 
myself to a few special features, as the interferometer which I set up, 
admitting of any separation of the interfering beams, longitudinally or 
laterally, is better adapted for work of this character than the Fresnellian 
fringes or the Jamin interferometer used heretofore. Two opposed 
nodes may be examined simultaneously. Moreover the ease with which 
fringes of any size or inclination are producible is a further advantage. 
As the transformations are adiabatic, the density increment Ap at 
any time is of the form Ap = C.n\/IR, if n fringes of wave-length X are 
displaced when the ray passes through a pipe of length /. R is the gas 
constant and 
C = £2 = 10 7 X 1.27 
*o 0*o-l) 
the optic constant when p Q and & Q are standard pressure and absolute 
temperature, and fi 0 the corresponding index of refraction of air. Thus 
the mean mechanical energy per cm. 3 is pAp/p for the length I surveyed, 
