universe of simplest matter and definitely operating energy, which
forms our hypothetical starting point, may not itself be a product of
evolution from a universe of such matter, in which the manifestations
of energy were not definite--in which, for example, our laws of motion
held good for some units and not for others, or for the same units at
one time and not at another--and which would therefore be a real
epicurean chance-world?

For myself, I must confess that I find the air of this region of
speculation too rarefied for my constitution, and I am disposed to
take refuge in 'ignoramus et ignorabimus.'

[Sidenote: Other achievements in physical science.]

The execution of my further task, the indication of the most important
achievements in the several branches of physical science during the
last fifty years, is embarrassed by the abundance of the objects of
choice; and by the difficulty which everyone, but a specialist in each
department, must find in drawing a due distinction between discoveries
which strike the imagination by their novelty, or by their practical
influence, and those unobtrusive but pregnant observations and
experiments in which the germs of the great things of the future
really lie. Moreover, my limits restrict me to little more than a bare
chronicle of the events which I have to notice.

[Sidenote: Physics and chemistry.]

In physics and chemistry, the old boundaries of which sciences are
rapidly becoming effaced, one can hardly go wrong in ascribing a
primary value to the investigations into the relation between the