144 W. Ferret — Weber's Law of Thermal Radiation. 



quired than if the loss of heat in the wire depended upon 

 radiation only. 



12. That there is such a decrease of the rate of losing heat 

 with increase of temperature, where there is only an exceed- 

 ingly small air tension left in the tube, is shown by the 

 asymptotic experiments of Mr. Bottomly.* Taking his values 

 of C 2 R/Js for the temperatures of the wire at 408° and 505° C. 

 respectively, the temperature of the inclosure or water jacket 

 being 16°, and coordinating them graphically with the corres- 

 ponding pressures, and smoothing off a little the irregularities^ 

 as usual, in drawing a curve through the different points deter- 

 mined by the coordinates, the values of C a R/Js in the follow- 

 ing table, corresponding to the several tensions in the first 

 column, are found : 





C 2 R/Js at 





Air-tensioD. 





Ratio. 









408° 



505° 





0-10 mm 



0-125 



0-172 



1-38 



•09 



•117 



•162 



1-39 



•08 



•109 



•153 



1-40 



•07 



•102 



•144 



1-41 



•06 



•095 



•135 



1-42 



•05 



•088 



•127 



1-44 



•04 



•080 



•118 



1-47 



•03 



•071 



•108 



1-52 



•02 



•061 



•096 



1-57 



•01 



•050 



•084 



1-68 



•00 



•037 



•072 



1-92 



From this table it is seen that with an air-tension of only 

 0'l mm in the tube the rate of loss of heat in an increase of the 

 temperature of the wire from 408° to 5u5° is as 1 to 1*38, 

 while for a vacuum, with the numbers here given, it is as 1 to 

 1*92. Hence in the former case Weber's formula, in order to 

 represent these results, would require a much smaller value of 

 a than in the case of a perfect vacuum. And for an air- 

 tension of only O'Ol 111111 the rate of increase is as 1 to 1*68, which 

 is still much less than in the case of a perfect vacuum, and the 

 value of a in Weber's formula would still have to be much less. 

 It should be here stated that the numbers in the last line of 

 the table, for a perfect vacuum, are uncertain, on account of 

 the uncertainties in the measurements of so low tensions in 

 approximating to a vacuum. 



18. The effect of a very small air-tension upon the rate of 

 heating or cooling of a thermometer bulb in a glass globe of 

 1*5 inches in diameter, is shown by the experiments of Mr. 

 Crookes.f The temperature of the inclosure being kept at 65°, 



* Phil. Trans., vol. clxxviii, p. 129, 1888. ' 

 \ Nature, vol. xxiii, 1880-1881, p. 234. 



