1874.] Atomic Matter and Luminiferous Ether. 183 
Gravitation is called attraction—‘‘ The sun attracts the 
earth.” Attraction, as the name of the phenomenon, is 
free from objeCtion ; but if attraction be a quality, it is the 
same in kind as suction, and suction we know is the name 
—and not the explanation—of phenomena. That one atom 
or mass of atoms can resist and push, and so impart motion 
to another, is comprehensible; but that one atom or mass 
should at a distance attract or suck another towards it is 
incomprehensible. The fact of the approach is seen, and 
is called attraction, but the reasons for it have to be sought. 
Prof. Guthrie, in 1869, communicated to the Royal 
Society some experimental results, which assist the com- 
prehension of attraction. He found that a sounding tuning- 
fork attracted such things as the smoke of a candle and 
delicately suspended pieces of paper. Due precautions 
were taken to show this attraction was the result only of 
the vibration, and not of electricity or currents of air, &c. 
A tambourine strongly vibrated, and other vibratory sur- 
faces, give more powerful manifestations. 
In 1870 Sir W. Thomson pointed out that these results 
were in accordance with general mathematical laws, appli- 
cable to all elastic fluids whose particles were put in motion 
by immersed bodies, either vibrating to and fro, or whirling 
about in any way. In Guthrie’s experiments the vibrations 
were those of sound, which ac¢t to and fro in the line of 
progression. 
In the sunbeam there are both visible and invisible rays, 
but all those rays obey strictly similar laws, as to speed, 
reflection, refraction, and polarisation, and hence must be 
‘motions of the same kind, and, like hght, are vibrations 
across the line of progression. 
‘The sun attracts the earth,” and gravity acts when the 
sun’s light is absent ; hence the lines or rays of the force 
of gravity are independent of the sunbeam, and probably 
follow different laws. 
So far as attraction is concerned, Guthrie has shown that 
to-and-fro vibratory motion suffices, and, as all known 
effects of gravity can be shown to follow from attraction, 
gravity itself may be the effect of to-and-fro vibrations. It 
is necessary, moreover, that these vibrations be such as the 
vibrations of light, &c., do not affect. 
Mechanically, all motions that are at right angles to each 
other proceed without what is technically known as “ inter- 
ference,”’.or, so to say, proceed uninterruptedly. A cannon, 
whose axis is parallel to the horizon, may project balls with 
very small and with very great velocities, but each ball will 
