A word of caution, however, is required because of the tremendous power of the sun and its powerful chemical effect when sun bathing is carried too far. Those of very fair skins particularly need to be careful. Brunettes, with considerable pigment of the skin can stand a great deal of sunlight without harm, but light-skinned persons, while needing a certain amount of sunlight, should not expose themselves for too long a time to the midday sun in summer, or at least not until they have gradually become sufficiently tanned to do so. Everyone knows the painful character of a sunburn. This only illustrates the powerful chemical effect of the sun's rays. In taking sun baths one should very gradually accustom himself to the sunshine until he is so tanned that the pigment in his skin will protect him. The short or chemical rays of the sun are actually destructive to white men in the tropics. In May, June and July they have a pronounced chemical effect even in our own latitude. They are stimulating up to a certain point, but beyond that point one should be careful. I may say, therefore, that brunettes in summer may take sun baths even at noon, but blondes should take them preferably before nine or ten o'clock in the morning or after three o'clock in the afternoon. In winter, however, when the sun's rays are more slanting, the sun baths can be taken even by the blondes at any time. And because of the more limited amount of sunlight in winter, special attention should be given to sun bathing during that season. Everyone needs a certain amount of sunlight, and if you cannot take a sun bath regularly every day you should at least wear clothing of a character that will permit the light-rays of the sun to penetrate. I will refer to this again, however, in the chapter on the subject of clothing. After all that we can say in regard to these various methods of stimulating the skin there is really nothing so effective as active exercise for those who are strong enough to take a sufficient amount of it. Exercise, so far as function of the skin is concerned, is valuable because of the copious perspiration which is induced when one gets enough of it. In these days great numbers of people no longer "earn their bread by the sweat of their brow," and their health suffers in consequence. If you do not have to perform such an amount of physical labor as will promote free perspiration, then for the sake of acquiring the very purest quality of blood your special exercise should be sufficiently active and continuous to bring about free perspiration. There is really nothing so effective as a good old-fashioned "sweat" for rapidly purifying the blood. Anyone who perspires each and every day as a result of physical activity, and whose habits are fairly satisfactory in other respects, can depend upon enjoying absolutely pure blood, or a condition which is not far from it.