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.