We often notice that on days when there is no sunshine especially when at the same time it is cold and damp, we feel depressed in mind. In such an atmosphere there is a diminution of the respiratory and transpiratory functions of the skin, and, in consequence, a retention of toxic products. On the other hand, on sunny and dry days these functions are facilitated, and there is less work to be thrown upon the kidneys. Such a rest does good to an organ that is almost constantly at work, and is a wise economy for the days of old age. It is astonishing how sunshine can influence our mental condition. We feel better able to work, and also to take outdoor exercise, on sunny days. Particularly in old age is sunshine precious, and we see our old house dog and our cat lying in the sunshine and taking a sun bath. In the same way, instinct tells old people that the sun is good for them, and thus they eagerly watch for it to shine, and like to sit in it, especially on cold days. It has been shown by exact researches that the sun’s rays can kill bacteria, and statistics have shown that in sunny weather there are fewer infectious diseases, like influenza, than in dull weather. In sunshine there are two elements which possess antitoxic and healing properties: light and warmth. Its light is made use of in the treatment of certain infectious diseases, like lupus (Finsen treatment). Both these properties can be used to great advantage in the sunbath treatment. Lying in the sun for a certain length of time causes a dilatation of the blood-vessels and, later on, abundant perspiration. The ancient Romans made use of the therapeutic action of sunshine, and many of them took sun baths on the roofs of their houses. There were also public sun-baths, much visited by the population. The ancient physicians recognized the value of sunshine in the treatment of various ailments. According to Hippocrates, the sun-bath augments transpiration and makes us more resistant against disease, giving us more strength. Celsus also advised its use in nervous diseases. Certain people have undertaken scientific experiments on animals to prove the great effects of light. Moleschott, the great physiologist, found that the embryos of frogs gave off far more carbonic acid under the influence of light than when they were in the dark. The stronger the light the more carbonic acid was given off. Edwards found that such embryos could not develop at all in the dark. Very important was the discovery of Arloing,[241] and also of Duclaux,[242] that the growth of bacteria when exposed to the sun is checked, and that later on they will even be killed. This is mainly due to the action of blue and ultra-violet rays. Footnote 241: Arloing: C. R. de l’Academie des Sciences, p. 378 et 511, Paris, 1885. Footnote 242: Duclaux: Revue Scient., 1887.