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ANNIVERSARY ADDRESS 
those which best enclose a layer of air, and it is this layer of 
entangled and scarcely moving air which gives to them their use¬ 
fulness. It is the air within the blanket or cloth of wool, not 
the wool itself, which arrests the passage of heat. So it is with 
cotton or with linen. Place the hand on a layer of unspun cotton, 
commonly termed cotton-wool. The feeling is that of warmth. 
Spin the stuff into a thread and weave the thread into sheeting and 
place the hand on the sheeting. The feeling is that of coldness. 
Obviously there has been no change in the material itself, therefore 
it is the change in the condition of the material that has resulted 
in the change of effect from warmth to coldness. Either cotton 
itself or linen itself, that is, flax, is not by any means a had 
conductor of heat; a sheet of cotton or of linen feels cold enough 
to the hand or foot; in other words it rapidly conducts heat away 
from the skin. Bring that same cotton or that same flax in the 
unspun or woolly condition against the skin and it feels warm. 
The fact is that, in the latter cases, very little of the layer of 
actual cotton or of flax, but a great deal of the entangled layer of 
air, is brought against the skin; and the layer of air, being a 
good non-conductor of heat, takes so little heat away that relatively 
we regard the materials as being warm. Wool or similar hairs of 
various kinds, silk, flax, cotton, are all useful as materials for 
clothes, the porous condition of the material rather than the 
material itself being the main consideration. Even skin, whether 
thick for foot-covering in the form of boots, or thin for hand¬ 
covering in the form of gloves, has its value for health purposes; 
but any extended use of such a slightly porous material as skin is 
inconsistent with the laws of health. The non-porous india-rubber 
waterproof clothing must only be worn when one desires to avoid. 
the lesser of two evils, namely, the lesser disadvantage of over¬ 
action of the pores of the skin as against the greater dangers 
attending the wearing of wet clothes. These “ waterproofs ” must 
be taken off at the earliest possible moment. 
The laws of heat govern the effect of the colours or pigments 
with which clothing is dyed as well as the effect of the material 
itself. A given fabric will reflect more heat if it is white than if 
it is black, and, generally, will have a somewhat varying power as 
a conductor or as a non-conductor of heat according to its colour. 
This influence of colour is of course greater in fabrics that enclose 
but little air than in those which enclose much air. The material 
of which a given pigment or dye is formed has some influence; 
different black dyes, for example, differing in their power of 
absorbing heat. 
