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CHAP. IX.] of linen is 50, of silk 40, and of buckskin 1; but a practical comparison of the heat values of these materials shows that flannel feels decidedly the warmest when worn. Of course it may be said that it feels warmer because it is a better non-conductor, but I believe another cause for this effect may be found in its higher ventilating power. I said in a previous chapter that the skin breathes as well as the lungs, though in a less degree, and if the air is permitted to reach the skin it not only removes waste and injurious substances from the body, but it also gives oxygen to the body. This oxygen combines with the carbon in the small blood-vessels, which in countless multitudes underlie the skin, and heat is given off.

Speaking of these little blood-vessels leads me to mention another point about clothing, namely, that if clothes fit too tightly they are not so warm as those of looser make, and the reason of this is twofold. First, tight clothes press upon the little blood-vessels in the skin, and thus mechanically interfere with the circulation of the blood in them, and that hot fluid, the blood, not being permitted to flow to the skin, that organ feels the loss of its heat supply. Secondly, tight garments, permitting but little air to lie between them and the skin, do not so freely permit the interchange of those good offices of which I have spoken, between it and the air, as would looser garments.

The value of woollen clothing for occupations or sports which bring about copious perspiration is generally acknowledged, and the reason of this is