[Illustration: _Seal Rocks near San Francisco, California, showing
slight effect of waves where there is no beach._]

No sooner was the spectroscope invented than astronomers hastened by
its aid to explore the chemical constitution of the sun. These studies
have made it plain that the light of our solar centre comes forth from
an atmosphere composed of highly heated substances, all of which are
known among the materials forming the earth. Although for various
reasons we have not been able to recognise in the sun all the elements
which are found in our sphere, it is certain that in general the two
bodies are alike in composition. An extension of the same method of
inquiry to the fixed stars was gradually though with difficulty
attained, and we now know that many of the elements common to the sun
and earth exist in those distant spheres. Still further, this method
of inquiry has shown us, in a way which it is not worth while here to
describe, that among these remoter suns there are many aggregations of
matter which are not consolidated as are the spheres of our own solar
system, but remain in the gaseous state, receiving the name of nebulæ.

Along with the growth of observational astronomy which has taken place
since the discoveries of Galileo, there has been developed a view
concerning the physical history of the stellar world, known as the
nebular hypothesis, which, though not yet fully proved, is believed by
most astronomers and physicists to give us a tolerably correct notion
as to the way in which the heavenly spheres were formed from an
earlier condition of matter. This majestic conception was first
advanced, in modern times at least, by the German philosopher Immanuel
Kant. It was developed by the French astronomer Laplace, and is often
known by his name. The essence of this view rests upon the fact