-1859] WRITINGS OF JOSEPH HENRY. 251 



1. Lampblack 100 



2. Grasses 103 



3. Silicious sand 103 



4. Leaves of the elm and poplar 101 



5. Poplar sawdust 99 



6. Varnish 97 



7. Glass _.. 93 



8. Vegetable earth 92 



Polished metals are of all substances the worst radiators; 

 they reflect the rays of heat as they do those of light, and it 

 would appear that the escape of heat from the substance of 

 the metal is prevented by internal reflection. In order that 

 the surface of a body should cool down to the lowest degree 

 it is necessary that it should be a good radiator and a bad 

 conductor, particularly if it be in a large mass and un-in- 

 sulated. Thus the surface of a mass of metal coated with 

 lamp black, though it radiates heat freely, will not be as 

 much cooled under a clear sky as a surface of glass, since 

 the heat lost at the surface is almost immediately supplied 

 by conduction from within. If however a very small quan- 

 tity of metal such as gold leaf be suspended by fine threads, 

 the dew will be deposited, because the heat which is radiated 

 is not supplied by conduction from anj^ other source, and 

 hence the temperature will sink to a low degree. 



M. Melloni has within a few years past repeated the experi- 

 ment of Wells, and established the correctness of his conclu- 

 sions ; and has also added some particulars of interest. He 

 found that the apparent temperature of the grass, which in 

 some cases was 8°or 10° lower than that of the airat the height 

 of 3 or 4 feet, was not entirely due to the actual cooling of the 

 air to that degree, but to the radiation and cooling of the 

 thermometer itself, the glass bulb of which is a powerful radi- 

 ator. To obviate this source of error in estimating the tem- 

 perature he placed the bulbs of his thermometer in a small 

 conical envelope of polished metal of about the size of an 

 ordinary sewing thimble. This prevented a radiation and 

 by contact with the air indicated its true temperature. He 

 found with thermometers thus guarded that the solid body 

 was in no case cooled down more than 2° below the temper- 



