When successive equal quantities of a salt are dissolved in a constant quantity of water, the successive increments of displacement of the solution, so produced, are generally unequal. They are usually the greater, the greater is the amount of salt which has already been dissolved.

In order usefully to discuss the change produced by any physical action, it is advisable to compare it with that which would be produced if it acted in accordance with some law which can be specified with precision. If the results observed agree with those calculated in terms of the law postulated, it is good evidence that the particular physical action takes place under the law. If no such agreement appears, then the observed results must be compared with those calculated in terms of the specification of some other law.