may be carefully arranged exhibitions of some phenomenon which occurs
only under peculiar conditions. They all, however, agree in this,
that their aim is to present some phenomenon to the senses of the
student in such a way that he may associate with it the appropriate
scientific idea. When he has grasped this idea, the experiment which
illustrates it has served its purpose.

In an experiment of research, on the other hand, this is not the
principal aim. It is true that an experiment, in which the principal
aim is to see what happens under certain conditions, may be regarded
as an experiment of research by those who are not yet familiar with
the result, but in experimental researches, strictly so called, the
ultimate object is to measure something which we have already seen--to
obtain a numerical estimate of some magnitude.

Experiments of this class--those in which measurement of some kind is
involved, are the proper work of a Physical Laboratory. In every
experiment we have first to make our senses familiar with the
phenomenon, but we must not stop here, we must find out which of its
features are capable of measurement, and what measurements are
required in order to make a complete specification of the phenomenon.
We must then make these measurements, and deduce from them the result
which we require to find.

This characteristic of modern experiments--that they consist
principally of measurements,--is so prominent, that the opinion seems
to have got abroad, that in a few years all the great physical
constants will have been approximately estimated, and that the only
occupation which will then be left to men of science will be to carry
on these measurements to another place of decimals.