Perhaps the strange doctrine taught by Galen, which prevailed for so many centuries afterwards, should be mentioned under this head as the first approach to a chemical theory. He considered all medicines to be hot, cold, moist, or dry. There were four degrees of each of these properties. In the Pharmacopæia Londinensis of 1702, translated by Dr. Salmon, it is stated of every herb that it possesses in a certain degree one or more of these qualities. It is amusing to find Dr. Salmon in great doubt as to whether Opium were hot or cold, as the Ancients said one thing, and the Moderns another. Galen supposed that diseases depended on similar qualities, and were to be counteracted by medicines; that, for example, we were to meet a hot disease by a cold remedy.
The next advance, if such it may be termed, was made by the Alchemists of the middle ages, who frequently turned their attention towards the healing art, and almost imagined that by their Philosophers' stone they could purify and rekindle the perishable base metal of the human body. One of their dreams was, that from Gold, the most durable of metals, or from Mercury, the most lively and volatile, they might by their magical arts be enabled to prepare a medicine that should render life perennial. A most impracticable formula for the preparation of this Elixir Vitæ was given, among others, by Carolus Musitanus. Basil Valentine, who flourished in the fifteenth century, did good service by adding to the Materia Medica the preparations of Antimony, as well as the Mineral Acids. In the sixteenth lived Paracelsus and Von Helmont, the latest and most enthusiastic of the medical Alchemists. They considered the chemical principles of medicines, by virtue of which they operated, to be three in number,—viz., Salt, Sulphur, and Mercury. And though the seventeenth century was illumined by the philosophy of Bacon, and the discoveries of Newton and Boyle, we find that this strange doctrine survived in full vigour at the commencement of the eighteenth. It is laid down as an axiom in Dr. Salmon's Pharmacopæia in 1702.[11]
About the middle of this century there arose a new sect of chemical philosophers, somewhat better informed than the last. They imagined that most diseases depended on the predominance in the blood of acid or alkalic humours, and that each of these conditions should be counteracted by a direct chemical antidote. They supposed also that the various secretions were the products of fermentations in the blood which took place in the neighbourhood of the glandular organs. (Vide Eliminatives.) In some of their ideas there was much that was reasonable; but it must be confessed that they were rather imaginative than argumentative, and, knowing really but little of the principles of that science on which their system was ostensibly based, they were ill-qualified to contend with their opponents of the mathematical school, who at least understood their own position. Foremost among these new chemical philosophers was Raymond Vieussens, who was severely censured by Dr. Pitcairn for having asserted that he had found an acid in human blood.[12] Vieussens was one of the earliest of the sect, which afterwards numbered many followers.
There is very little that is tangible to be discovered in these old chemical theories of the action of medicines; and it is not to be wondered at that most of them have faded away before the advance of science, and particularly before that wonderful development of the science of chemistry, which has distinguished the end of the last, and the first half of the present century.
We have seen that some of the early writers made great account of the affinities of acids and alkalies. So also a chemical explanation of the action of these remedies is generally adopted by writers at the present day. It is known that they have powerful tendencies to combine with each other, and it is supposed that these affinities are manifested even in the living blood.
Schultz attempts a further chemical explanation of their action in some diseases, particularly inflammations. He says that both affect the condition of the blood; but that acids tend to dissolve and destroy the corpuscles, wherefore he terms them Hæmatolytica Physoda; and alkalies prevent the coagulation of the fibrine of the plasma, for which reason he calls them Hæmatolytica Plasmatoda.
Some modern writers have tried to extend a chemical theory to the operation of medicines in general. This is an error to which those who have devoted themselves particularly to the study of chemical phenomena are especially prone.
Müller thinks that the agency of many remedies may be explained by their chemical affinities. He supposes that they may effect a change in the nutritive fluids, or that they may so disturb the state of combination in which the elements of an organ may be, that it becomes insensible to the action of morbid stimuli. Some chemists have accounted for the action of Alcohol by its chemical affinity for the brain substance. Liebig considers that the similarity of their composition to that of the brain may serve to explain the operation of such medicines as Quina and Morphia. Such ideas as these are at the best purely hypothetical, and even as theories they seem to me to be untenable,—for what reasons I shall have to show when I consider these remedies. Liebig has hazarded several other explanations of a similar kind, of which the following is an example:—"The frightful effects of Sulphuretted Hydrogen and Hydrocyanic Acid are explained by the well-known action of these compounds on those of Iron, when Alkalies are present, and free Alkali is never absent in the blood." (Organic Chemistry, p. 274.) Now in the first place it is not proved that the complete abstraction of iron from the blood would occasion sudden death, though doubtless it is a necessary constituent of that fluid. Further, Prussic Acid acts on the superficial nerves as an Anodyne when applied externally, which it can hardly do by displacing iron. Besides, by parity of reasoning, Ammonia, or Benzoic or Cinnamic Acid, should precipitate iron, if present in the blood in the soluble state, and Sulphuric or Nitric Acid should dissolve it, if in the state of peroxide; and yet none of these agents are frightful poisons. It is not to be imagined that chemical solutions and decompositions of every kind are allowed to take effect in the human system in the same way as in the laboratory of the chemist, for there are in the former many disturbing and controlling causes which suffice to hold them in check.