Two highly experienced and cautious physicists, Professors Landolt and Heydweiller, have not hesitated to announce the sensational result that in certain circumstances the weight is no longer the same after as before the reaction.

In particular, the weight of a solution of salts of copper in water is not the exact sum of the joint weights of the salt and the water.

Such experiments are evidently very delicate; they have been disputed, and they cannot be considered as sufficient for conviction.
It follows nevertheless that it is no longer forbidden to regard the law of Lavoisier as only an approximate law; according to Sandford and Ray, this approximation would be about 1/2,400,000.

This is also the result reached by Professor Poynting in experiments regarding the possible action of temperature on the weight of a body; and if this be really so, we may reassure ourselves, and from the point of view of practical application may continue to look upon matter as indestructible.
The principles of physics, by imposing certain conditions on phenomena, limit after a fashion the field of the possible.

Among these principles is one which, notwithstanding its importance when compared with that of universally known principles, is less familiar to some people.