The _quantity_ or _dose_ is the most important of these; many substances which are deadly in large doses being exceedingly useful as remedies in small quantities; such are prussic acid, opium, digitalis, arsenic, &c. Then again, the _mechanical_ and _chemical_ state of aggregation are all-important; a solid being usually much less active than a fluid or a gas, and a pure substance much more active than one mixed with insoluble materials. Even more important is the chemical constitution of the poisonous agent; as already pointed out, poisonous effects result from absorption of the poisoning body and absorption implies solution; the more soluble, therefore, the compound is, the more speedy are its effects, whilst compounds insoluble in water or any of the juices of the body are inert. It is not, however, enough that the substance be insoluble in water; it must be so also in the gastric juice, or it may give rise to characteristic symptoms. Thus, calomel is insoluble in water, yet it is a powerful medicine; orpiment is insoluble in water, yet when swallowed, it may give rise to symptoms of arsenical poisoning, and so on. As already pointed out, the mode in which the poison is introduced into the body is of great consequence in estimating its effects. Then again the _mental_ and _bodily condition_ of the recipient must be taken into account. Thus, in excited maniacs doses of medicines may be given without producing any effect which in ordinary individuals might give rise to serious consequences. The bodily condition, especially as influenced by habit, is still more important. It may be broadly stated, that by gradually increasing the dose of a substance ordinarily poisonous, in course of time enormous quantities may be borne without producing immediate ill effects. This is especially seen in the practice of opium eating and smoking, and in a less degree in arsenic eating, as practised in Styria. The latter instance is, however, contrary to the usual rule; for whereas with vegetable substances, such as opium the dose requires to be constantly increased to keep up the effects, with minerals, the contrary seems to be the case, especially with antimony and mercury, which cannot be long given without danger to the recipient. CHAPTER II. DIAGNOSIS OF POISONING—DUTIES OF THE PRACTITIONER.