HEAT OR CALORIC. 



71 



PRACTICAL QUESTIONS. 



(cc.J Why are black* clothes hotter in the summer and in the sun, 

 than in the winter and in the shade ? In order to settle this ques- 

 tion, it is necessary to ask another, that is, in what circumstances will 

 the absorption exceed the radiation of heat ? This will plainly be in 

 the summer, and the reverse will be true in the winter. 



(dd.) Why do black people endure heat better, and cold worse, than 

 white people / The answer depends on the same cause, taking into 

 view the average animal temperature. 



(ee.) Why should steam, which we wish not to condense, be con- 

 veyed in bright tubes, and vice versa ? Because such surfaces radi- 

 ate heat badly. 



(ff.) Why does a common rolled iron stove pipe diffuse heat bet- 

 ter than a bright tinned one ? Because its surface is rough, and 

 therefore radiates heat powerfully, f 



(&&) Why does water keep hot longer in a bright polished vessel 

 than in a dark and rough one ? J The answer is the same as in ee. 



(M.) Why does water become heated rapidly in a rough iron 



kettle, and slowly or not at all in one of bright copper ? The 



answer is the same as in jf, reception being substituted for radiation. 



(ii.) Why would an earthern ware tube, when gilded, preserve 

 steam longer uncondensed, than the same tube with its natural sur- 

 face, or than bright tinned iron ? Because the substance is a bad 

 conductor, and the surface a bad radiator. 



(./}) Why does snow melt rapidly where the dirt is thrown upon 

 or mixed with it, as in the travelled path, and slowly, or not at all, 



* Quere, (communicated )" Are black clothes, when worn in the shade during 

 summer, warmer or cooler than white clothes in the same circumstances ?" The 

 answer will depend on the radiating and receiving power of the surfaces, and on the 

 temperature of the air, compared with that of the body. 



i In neither of these cases, is the final cause assigned ; it is unknown. 



t Experiment in Yale College Laboratory, Nov. 10th. 1826. A blackened and a 

 polished canister of plated tin of the same form and size, being filled with water at 

 200 their times of cooling were as follows. 



Blackened Canister cooled 

 in the 1st 12 minutes, 



2d 12 



3d 12 



4th 12 



5th 12 



6th 12 



7th 12 



8th 12 



96 min. 



In one hour and thirty six minutes, the blackened canister cooled 61, during which 

 time the polished one cooled but 35. (At two hours Trona the completion of the 

 above experiment*, viz. three hour^ and thirty six minutes from the commencement, 

 the water in the polished canister was still 20 warmer than in the blackened one.) 



