Water hot or cold, externally applied, has three main functions: (i.) to cleanse, (ii.) to promote or equalise circulation, (iii.) to invigorate and harden. It is mainly to its excellent effects with regard to the third of these functions that the complete cold plunge owes its popularity; for nothing--_given that the heart is strong, and the liver not prone to sluggishness_--is more delightfully exhilarating than the sudden thrill of cold water when one is either warm from hours in bed, or hot from violent exercise. But it must be remembered that it is distinctly not given to everyone to be sufficiently robust to be able to indulge in it. There are some people who are clearly benefited--as far as we can see--by it; there are many who are not injured by it, and can continue to take it for the sake of the pleasurable sensation it produces; but there are also many who are injured by it, who run a risk anyhow in its use. For the shock and contrast of it is a shock: a sudden drag is put on the racing heart, a sudden check is given to the hot open-pored skin. It is exactly that which is pleasant: it is that also which may be dangerous. Luckily the test of its desirability, or at any rate, harmlessness in any special case, is easy to apply, and if anyone feels chilled after cold bathing, or finds that after it the feet and hands are a little numb and cold, with perhaps a loss of colour under the nails, it is fairly certain that he or she has not the circulation which can be benefited by a cold bath; but may be hurt by it. Or if anyone finds it necessary to make a very vigorous use of the rough towel afterwards in order to get warm again, the cold bath probably does not suit him. Again, in some it produces a certain torpidity of the liver, equally easily indicated; here, also, we have one of Nature’s clear danger-flags waving us off. The cold bath suits and probably benefits most of those who do not feel these dispiriting after-effects, and the reason for it is obvious. The cold water checks the circulation momentarily; it is literally a cold sponge on the heart, and it momentarily drives away the blood from the skin. But then, if the bather has good natural reaction, this momentary chill will be succeeded by a fresh assertion of his vigour; the heart momentarily checked will, combatively, reassert itself, the blood will rush back to the skin vessels from which it has been momentarily driven, and the exhilaration of the cold shock will be succeeded by a fresh exhilaration of tingling vigour. But in any case a cold bath should not be taken by anyone already cold or chilled; it is because the body is warm, the heart beating fully, and the circulation vigorous that the subsequent reaction is vigorous. But to take a cold bath when one is cold is to demand an effort from something which should be not checked but encouraged.