CUAP. in.] ON THE CONDUCTIHILITY OF HEAT. 361 



latter have a double superiority, i.e., both as regards the fabric or 

 as regards the material. 



Daily experience confirms these inductive results, woollen garments 

 are the best protectors against the cold, because they oppose the 

 passage of heat from the body. These, too, provided the stuff be 

 somewhat light, we should also prefer in summer for stopping the rays 

 of heat, and preventing them from penetrating to our body. Besides 

 this, we know that colour has also an influence, that black or sombre- 

 coloured clothes give out the he'at with greater facility than those 

 which have a bright colour or are white. So that in winter the 

 latter are preferable to the others, since they are less favourable to the 

 loss of heat from the body. In summer, the heat without is less 

 easily absorbed by white clothes than by dark, and they, in this case, 

 are preferable to the others. 



It is not the weight of the stuff that makes a garment hot, it is 

 the divided structure of the tissue, an eiderdown quilt filled with fine 

 and light down, is warmer than a heavy and thick counterpane. 



We see from these examples, how necessary it is to take into 

 account the various properties of bodies, their conductivity, their 

 radiating, absorbing, and emissive power, in their ordinary applica- 

 tions to heating, to the construction of houses arid to clothes ; but 

 we must also take -health into consideration, which has to do not 

 with physics but with physiology. 



We seek for warmth in winter, for coolness in summer, but we 

 must take care how far we go in this, in order that our health, which 

 is the equilibrium of the functions of our bodies, may be kept in a 

 good state. 



III. MINERS' SAFETY LAMPS. 



We saw in the Forces of Nature, that a metallic gauze placed 

 above a gas flame prevents the combustion from being propagated 

 above the gauze. The latter absorbs so much heat, that the tem- 

 perature of the gas after passing through the little opening of the 

 mesh work is not sufficient for ignition. 



An illustrious English physicist, Davy, has made use of this 

 important property to prevent accidents in mines, happening from the 



