The second form of idiosyncrasy consists in the production of poisonous results by common articles of diet. Thus some people cannot partake of shell-fish or lobsters without having severe nettlerash. In rare instances the smallest amount of egg, or in other cases mutton, or pepper, or some other substance will produce severe indigestion or nettlerash. The third form consists in an inversion of the usual effects of certain substances, especially drugs. Thus opium in rare cases produces convulsions; while the aperient Epsom salts have been known to produce constipation. A fourth form, that of mental idiosyncrasies, may be added, as where there is a strange preference or aversion for objects usually regarded as indifferent. Many cases of mental peculiarity, short of actual insanity, will come under this head; as will instances of depraved appetite for food, etc. =Age and Sex.=—According to the period of life, danger arises from different sources. In _infancy_ and old age extreme changes of temperature are especially dangerous, and additional protection is required (see also page 271). Thousands of deaths occur in the first year of life, from substituting starchy foods for milk, the natural food for infancy and childhood (see page 303). In _childhood_ the danger from bad feeding is still present, and is evidenced by the frequency of rickets (page 28); infectious diseases claim their thousands; and the disorders associated with dentition are common. In _youth_ rapid growth is proceeding, and so the food must be abundant and nutritious. A proportionately larger amount is required than by an adult, as the functions of the body not only require to be carried on, but material is necessary to build up the growing tissues. _Manhood_ is the period of greatest stability of health. The health now depends on the use made of the previous periods of life, and on the habits acquired. With the onset of _old age_ come various degenerative diseases. The tendency is to death by gradual decay—a _euthanasia_ or easy death, which is too seldom seen. Commonly, bronchitis or apoplexy or kidney diseases bring the scene to a somewhat premature end. The mortality of man is greater than that of woman at all ages except 5—20. =Habits.=—The immense power of habits in the formation of character is perhaps duly appreciated; but their influence on physical health is not so well appreciated; though it would be difficult to exaggerate it. The laws of health are as inexorable and unaltering as all other laws of nature; and whether broken through carelessness or ignorance, the Nemesis of disease inevitably follows. Whatever a man sows he reaps, in health as in other matters. Habits are easily formed; but, when once formed, not so easily broken. They ought to be our servants; very commonly they become our masters.