MATHEMATICAL PHYSICS. 103 
respect to one another; and the comets also with respect to 
the sun; we must, in conformity with this rule, declare that 
all bodies are heavy with respect to one another.” 
It may be supposed by some that the method of science 
has changed since Newton’s day, and that, while the 17th 
century was content with hypotheses, the 19th and 20th 
centuries go deeper, and inquire into “ causes.” How- 
ever, there is really nothing in the suggestion. We may 
be able to see further by standing on the shoulders of 
giants; but our method of advancing is just the same as 
that of our forefathers. Look at any guiding principle of 
modern science, e.g., the ether theory, whose development 
has been the most striking feature in the advancement of 
physical science within the last century. The idea of an 
all-pervading medium filling space was lictle more than a 
wild speculation among the ancients. Among moderns, 
Descartes was the first to employ the idea in physical science, 
in connection with his theory of vortices. We all know that 
Newton toyed with the notion to help him over the gulf of 
space with his gravitating forces. _ Huyghens, however, was 
the first (in 1678) to expound what we now regard as cor 
rect views on the general nature of the elasticity of the 
ether, which he did in explaining his undulatory theory of 
light. The success of the ether idea in the field of optics 
gave it the entrée to other departments of science. ‘‘ At 
one time,” says: Maxwell, ‘those who speculated as to the 
causes of physical phenomena were in the habit of account- 
ing for each kind of action at a distance by means of a 
special ethereal fluid, whose function and property it was 
te produce these actions. They filled space three and four 
times over with zthers of different kinds, the properties of 
which were invented merely to save appearances.’ 
Gradually, however, the old method of scientific progress 
revealed icself in the working down from many ethers to 
one; the same for light, heat-radiation, electricity, and mag- 
netism. In 1800 Young assures us that the existence of an 
electric ether has been demonstrated, and adds “ whether 
the electric ether is to be considered as the same with the 
‘luminous’ ether, if such a fluid exists, may perhaps at 
some future time be discovered by experiment.’’ In 1806 
Sir Humphrey Davy advanced the hypothesis that chemical 
and electrical attractions were produced by the same 
“cause.” A little later Faraday did more than anyone else 
to advance the idea of an ether as the “explanation” of 
electric and magnetic phenomena; and in 1845 he made the 
very important discovery that a field of magnetic force 
