Take a glass vessel with a long narrow neck, which, being filled with water, will hold exactly a quart; then put into this vessel a pint of water, and a pint of acid of vitriol, and you will presently perceive, that the mixture will not fill the vessel, as it did when a quart of water only was put into it. The acid of vitriol must be put in gradually, by little and little at a time, mixing each portion with the water before you add more, by shaking the bottle, and leaving its mouth open, otherwise the bottle will burst. The mixture in this case also possesses a considerable degree of heat, though the two ingredients of themselves are perfectly cold; and this phenomenon is not to be accounted for, by supposing that the acid of vitriol is received into the pores of the water, for then a small portion of it might be absorbed by the water, without augmenting its bulk, which is known not to be the case; but the very form of the bodies in this experiment is changed, there being, as Dr. Hooke, who first noticed the fact, observes, an actual penetration of dimensions. Chemistry also furnishes a number of other instances, which show that two bodies, when mixed together, possess less space than when they are separate.