234 THE HUMAN MOTOR 



Take, as an example, a man wearing a woollen garment (c = 

 0-0000686) with a total surface of 19,000 square centimetres, a 

 thickness of 0-75 cms., and let the difference of temperature 

 between inside and outside be 10, what will be the amount of 

 heat lost by conduction in 24 hours ? 



We have 



0-0000686 x 19,000 x 3,600 X 24 x 10 



0-75 x 1,000 

 It has been divided by 1,000 so as to express Q in large calories. 



171. In regard to the emissive powers of various materials 

 we have only some results obtained by Peclet.f 1 ) He agreed with 

 Melloni that the colour of the tissues is immaterial, except for 

 the fact, demonstrated by all authorities, that black is the most 

 radiant. 



A table of known emissive powers is given later ( 250 of 

 Technics). But Peclet's measurements are not beyond criticism ; 

 they are also rather old (1841). Thus Peclet finds, in opposition 

 to more modern experimenters : 



Smoke black Kj = 0-00011 c., instead of 0-00016c. found 

 by Stefan and Christiansen. For the skin, we will take 

 K x =0-00015 c. For the air, recent determinations ( 2 ) have given 

 K! = 0-000040c, Glogner,( 3 ) using a water calorimeter, studied 

 the radiation of the skin of Europeans and of negroes (Malays). 

 Black skin was found to be more radiant. Thus per hour and 

 per square metre the former gave 17-40 Cal. and the latter 21 

 Calories for the fore-arm, a ratio : 



In the same way in an air calorimeter, the rise of temperature 

 of a given volume of air in a given time was 1-21, more for black 

 than white skin (the thigh). 



This very distinct difference explains also the resistance of the 

 black races to the effects of the tropical climates which they 

 inhabit. 



An estimate of the radiated heat is easily made by Stefan's law, 

 so often verified, ( 4 ) which is this : 



The radiation from a surface is proportional to the 4th power 

 of its absolute temperature. 



(*) Peclet, TraitS de la Chaleur, 3rd Edn., 1860, vol. i., p. 373 (reviewed by 

 L. Ser). 



( 2 ) Average, according to Wellanmann and Exner (Acad. Sciences de 

 Vienna, 19th Jan., 1911). 



( 8 ) Glogner (Virchow's Arch., vol. cxvi., p. 540, 1889). 



(*) For instance, recently Wamsler (Zeitzch. d. Vev. Deutsch. Ing., No. 15 

 and 22, 1911). 



