386 TRANSACTIONS OF SECTION A. 
may be made. If we proceed on the old mechanics we obtain the law of equipar- 
tition (Rayleigh’s Law) as the final result, true for all wave-lengths; unless 
we assume also that the observed state is not one of true thermodynamical 
equilibrium.! If we make this additional assumption, a definite law can be 
obtained, but it is open to two fatal objections. First, since the electron orbits 
are different in different substances, the resulting law for black-body radiation 
is found to be different for different substances, in opposition to experiment ; 
secondly, as a result of numerical calculation, it is found that to account for 
the observed law of radiation, the electron orbits and free-paths would have to 
be of a size which is quite incompatible with what we know as to molecular 
dimensions. ? 
With regard to Professor Lorentz’s remarks on the possibility of making a 
distinction between ‘matter’ and ‘vibrators,’ surely the work of Debye has 
shown quite convincingly that any such distinction is purely fictitious. For it 
now appears that the oscillations of the ‘vibrators’ required by the theory of 
Planck are simply the elastic vibrations of the mass of matter as a whole. 
Whether we regard these as oscillations of vibrators or of matter is for us to 
choose, but it seems to be clearly proved that there is only one set of oscilla- 
tions, and the bodies which oscillate are Professor Lorentz’s ‘ vibrators’ and 
‘matter’ rolled into one. 
DEPARTMENT OF GENERAL PHYSICS. 
The following Papers were read :— 
1. Crystals and X-Rays. By Professor W. H. Braaa, F.R.S. 
We know from recent experiment that X rays of wave-length A can be reflected 
by a set of parallel crystal planes when the relation nA=2d sin @ is fulfilled. 
In this formula d is the distance between the consecutive planes, A is the wave- 
length, @ is the angle between the planes and the incident or reflected ray, and n is a 
whole number. ‘This principle leads to two distinct lines of research. The first is 
based on the original experiment of Laue. A pencil of heterogeneous rays is passed 
through a crystal slip and the various crystal planes reflect pencils from the primary 
beam, which pencils are allowed to fall upon a photographic plate. Hach pencil 
consists of homogeneous rays of wave-length determined by the above relation. 
Since the various sets of planes have different values of d, heterogeneous rays are 
wanted in order that each set may be able to reflect. A bulb with a Pt anticathode 
has been generally used and is very suitable. By examining the distribution of the 
energy in the various spots on the photographic plate information can be obtained as 
to the position of the reflecting planes and their ‘ richness’ in atoms. 
The second method is based on the use of homogeneous rays. If the angle be- 
tween the primary pencil and the crystal is gradually increased and the strength of 
the reflective pencil is measured by the ionisation it effects in a suitable chamber, 
any homogeneous pencil in the primary rays is reflected only at angles given by the 
formula. In this way a ‘spectrum’ of the primary rays is mapped out. Platinum 
anticathodes emit several lines in addition to the heterogeneous radiation, and are not 
so suitable for these experiments as rhodium or palladium ; osmium and indium are 
still less suitable. The spectrum of the rhodium has one strong pencil of wave-length 
0-607 x 10-%, and a weaker pencil of wave-length 533 x 10-*. Palladium has an exactly 
similar spectrum, but the wave-lengths are nearly six per cent. smaller. There is 
very little radiation of other wave-lengths from either bulb. When the spectrum of 
rhodium or palladium is mapped for various sets of crystal planes of various crystals 
we obtain measures of the spacing of the planes and so obtain data which help in the 
determination of crystal structure. 4 
The diamond may be taken as an example. The spectrum reflected by the three 
important sets of planes show that the (100) planes are closest together of the three, 
that the (110) planes are more openly spaced, in the proportion of 2 to 1, and the 
(111) planes in the proportion 73tol. These latter show a further important feature 
1 Phil. Mag. June 1909. 2 Ibid. July 1909. 
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