Just as every crystallizable chemical substance assumes a definite and constant crystalline form which cannot be accounted for otherwise than by regarding it as one of the properties of the substance, so every living organism assumes a characteristic form which is the outcome of the properties of its protoplasm.
But whereas the crystalline form of a chemical substance is stable and fixed, the organized form of a living organism is unstable and subject to change.
The influence of the chemical substance is either that of attraction or repulsion, the one being known as positive, the other as negative chemiotaxis.
The same idea pervades old medical treatises; for a drug was not a chemical substance taking effect naturally on the human system, but something into which a supernatural virtue had been magically introduced, in order the more easily and efficaciously to be brought to bear upon the patient.
Kirchhoff in announcing that the lines of the spectrum were characteristic of the chemical substance which emitted them, and in indicating the value of this discovery in chemical analysis.
