ideas derived great support from the principles inculcated by Dr.Herman Boerhaave, the learned physician of Leyden. He likewise supposed that many discases of the solid parts were to be attributed to a weakness or laxity of the animal fibres, and were to be cured by external or internal agents, which should act mechanically on those fibres so as to increase their tenacity. Also, that disorders of the fluids often depended on their being too viscid, and that this condition might be improved by agents which should attenuate this viscidity. Dr. Archibald Pitcairn, a Scotchman, the immediate predecessor and contemporary of Boerhaave, was elected to the Chair of Physic in Leyden in 1691, and was also an able exponent of the mathematical theories. But he applied to physiology those ideas which were employed by the other to throw light upon physic; if that may be called light which was at least an improvement on the ignorance which preceded it.* He explained the digestive process by the mechanical trituration to which the food was subjected in the stomach; and accounted for secretion by supposing the existence in glands of vascular pores of different sizes, which intercepted certain particles of the blood; actually giving for the process a mathematical formula. He was a vehement opponent of those who based their theories on the then youthful science of chemistry, who, having scarce yet shaken off from them the dust of alchemy, only substituting Acids, Alkalies, and Fermentations, for Salt, Sulphur, and Mercury, fell easy victims to his satire.

Dr. Charles Perry, in 1741, propounded a mechanical view of the action of Mercury and Arsenic. He thought that the particles of the former, being round and heavy, were able,

He was one of the first who, leaving the old conjectural method of physical writers, struck into a new and more solid way of reasoning, grounded upon observations and mathematical principles."—Preface to Dr. Pitcairn's Works: 1715.

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when shaken about in the vessels, to break up and to annihilate those crude acrid humours which were the causes of disease: and that Arsenic acted as an irritant by the sharp and pointed nature of its atoms. He attributed the occasional poisonous effects of Mercury to the presence of Arsenic or some such substance as an impurity.*

Dr. Mead, in 1751, states that the administration of mercury is dangerous in cases where there is carious bone, as there is a fear that its ponderous particles may break the weak lamellæ.† He was the Court Physician in the reign of George II. He accounted for the poisonous nature of the venom of serpents by asserting that it consisted of pointed particles, which pierced and destroyed the globules of the blood.

Dr. Perry conceived that some medicines, such as Steel and Antimony, did not act by their mere bodily presence, but by certain subtle vapours which emanated from them, and affected the vital spirits. This was a very misty notion. He stated that he borrowed this idea from a great philosopher of the German nation. This was probably Boerhaave, who lectured at Leyden in 1707.

Among those who regarded with favour the mechanical hypothesis, were Fourcroy and Hecquet in France, Van Swieten and Huygens in Holland, and Bellini in Italy. Excepting perhaps the case of external irritants, these explanations of the action of medicines have been universally condemned by scientific men at the present day. Doubtless these old authors were in the wrong, both in applying one hypothesis to the action of all remedial agents alike, and still more, in carrying their theories into such minute details, where it is impos

*"A Treatise of Diseases in General, wherein the true causes, natures, and essences of all the principal diseases incident to the human body, are mechanically accounted for and explained, and their respective intentions of cure assigned upon the same principles," vol. ii. p. 313, 320. "Medical Precepts and Cautions," page 294.

sible that they should be verified. And yet we may go too far in our condemnation of all such ideas. It does not seem to me to be so impossible, or even improbable, that the operation of some medicinal agents, particularly those which act on the nerves, may depend in some way on the shapes of the atoms of these substances, as related to those of the tissues which they influence. At least, there is no other more plausible explanation of the power of such substances. We know that the nerves are very much under the influence of mechanical impressions, upon which depend the phenomena of two at least out of the five senses, those of hearing and touch, as probably also of the other three, if we understood them better. We know also that if we accept the Atomic theory, by which so many chemical phenomena are cleared up and explained, we must admit a certain definite and peculiar arrangement and shape to the ultimate particles of every compound body. These considerations render it possible that the ultimate particles of a stimulant medicine may be of such a nature as to irritate, or to refuse to coincide with, the ultimate molecules of the sensitive nerve with which they come in contact; and those of a sedative may, on the other hand, be so shaped and arranged as to dovetail with those particles, and by extinguishing, as it were, their salient points, to cloak their vital sensibility. This is obviously a mere conjecture, and the only value which can attach to it is, that it appears in some sort to explain a thing which without it is inexplicable.

Some modern writers have attempted to clear up the actions of certain medicines by their supposed influence on the physical process of endosmosis, as carried on through the coats. of the stomach and intestines. Poisseuille and Matteucci have attempted to prove that the action of saline liquids in causing purging, and that of a solution of morphia in preventing the same, may be explained by the endosmotic properties of these liquids, as ascertained by experiment. It

does not seem to me that these ideas can be successfully maintained. (Vide Prop. II.)

2. Several attempts have been made to explain the general action of medicines on chemical principles.

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 Pharmacopoeia 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 Vita 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 Van 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 Pharmacopoeia in 1702.*

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.† 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 won

"If there be any bodies-suppose gold and silver-out of which all these principles cannot be drawn, let not the artist think that it is be. cause they are not therein contained. No! it is because that God has so firmly united them against the possibility of our separation.”—p. 809. Dr. Pitcairn's Works, 1715, p. 219.