72 Dr Stark on the Itifliience of Colour on Heat, 



to the same conclusion, I may here notice a series of experiments 

 made by Sir Everard Home at a much later period (18^0), 

 to ascertain why the rete mucosum of the Negro formed a de- 

 fence against the scorching effects of the sun's rays. He ex- 

 posed the back of his hands to the rays of the sun, one hand be- 

 ing covered with black cloth, the other naked. A thermometer 

 was placed upon each. After ten minutes the degree of heat 

 of each thermometer was marked, and the appearance of the 

 skin examined. This was repeated three different times with the 

 following results. 



\st, Thermometer under black cloth 91° the other 85° 



2d, 940 91° 



3</, 106° 98* 



The skin where uncovered was scorched ; the other hand had 

 not suffered in the slightest degree. In another experiment, the 

 back of a Negro's hand was exposed to the sun with a thermo- 

 meter upon it which stood at 100°; at the end of ten minutes 

 the skin had not suffered in the least. 



An experiment with the rays concentrated by a lens for fifteen 

 minutes singed the black kerseymere with which Sir Everard's 

 arm was covered, but made no impression on the skin ; while 

 with white kerseymere in the same circumstances a blister was 

 formed. Even the rays concentrated on the back of the hand 

 of a Negro for fifteen minutes produced no sensible effect.* 



Nearly about the period when the last experiments of Count 

 Rumford were made public, the celebrated Essay on Heat of 

 Sir John Leslie appeared. This work was published in 1804, 

 and included a series of experiments which have formed the 

 basis of what has since been written on the subject of heat. 

 Though the results of most of Sir John's experiments seem to 

 have been powerfully influenced by the black colouring of part 

 of the apparatus employed ; and though the differential ther- 

 mometer and photometer are constructed on the absorbent power 

 of a black-coloured ball, yet he expresses himself doubtful of 

 the influence of colour in producing these results. " The 

 last quality," says he, " which may perhaps have some influ- 

 ence in modifying the power of a substance to emit or absorb 

 heat is its colour. On colour the disposition to imbibe the 

 rays of light principally depend. Blacl<: is most absorbed of 

 * Phil. Trans. 1821, 3-5. 



