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V. An Experiment in Search of a Directive Action of one Quartz Crystal on another. 
By J. H. PoYNTiNG, Sc.D., F.R.S., and P, L. Gray, B.Sc. 
Received September 27,—Read November 17, 1898. 
Since so many of the physical properties of crystals differ along the different axes, 
our ignorance of the nature and origin of gravitation allows us to imagine that the 
gravitative field of crystals may also differ along those axes. Dr. A. S. Mackenzie 
(‘ Phys. Rev.,’ vol. 2, 1895, p. 321) has described an experiment in which he failed 
to find any such difference. Using Boys’s form of the Cavendish apparatus, he 
showed that the attraction of calc-spar crystals on lead and on other calc-spar 
crystals was independent of the orientation of the cryslalline axes within the limits 
of experimental error—about one-half per cent, of the total attraction. He further 
showed that the inverse-scpiare law holds in the neighbourhood of a crystal, the 
attractions at distances 3714 centims., 5‘5G5 centims., and 7'421 centims. agreeing 
with law to one-fifth per cent. 
One of the authors of this paper had already pointed out (‘ The Mean Density of 
the Earth,’ 1894, p. 7) that if the attraction between two crystal spheres were 
different for a given distance, according as their like axes were parallel or crossed, 
such difference should show itself by a directive action on one sphere in the field of 
the other, d’his directive action is suggested by the growtli of a crystal from solu¬ 
tion, where the successive parts are laid down in parallel arrangement—a fiict which 
which we might perhaps interpret on the molecular hypothesis as showing that, 
within molecular range at least, there is directive action. 
The experiment now to be described is a modification of one indicated in the work 
above referred to, carried out for two quartz spheres, and we may say at once that 
we have certainly not succeeded in proving the existence of a directive action of the 
kind sought for. 
To bring out the principle of the method, let us suppose that the law of the attrac¬ 
tion between two spheres with their like axes parallel, as in fig. 1 (a), is GMMVr^, 
where M, M' are the masses, r the distance between the centres, and G a constant 
for this arrangement. Let ns further su})pose that the law of attraction when the 
axes are crossed, as in fig. Ih, is G'MM'/r", where G' is a constant for this arrange¬ 
ment, and different from G. 
Let us start with the spheres r apart, as in fig. 1 (a). The work done in removing 
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