INTO THE LAWS OF THE CONDUCTION OF HEAT IN BARS, ETC. 



U5 



35. *' Final Result. — The projections of both the observed series of numbers 

 representing the conductivity in terms of the temperature projected in Diagram 



F 

 XIX., give a value of ^ pretty regularly decreasing from -0150 at 0° C. to -0092 



dx 



at 200°.' 



36. In order to reduce the unit in the last column of these tables to the absolute 

 unit K, which expresses the amount of heat necessary to raise one cubic foot of 

 water by 1° Cent., we must multiply the numbers in the final columns by the 

 specific heat of iron, and also by its specific gravity. These numbers are respec- 

 tively 0-114 (Regnault) and 7"79. Their product is 0888. Assuming the uni- 

 form decrease of the conductivities with increasing temperature, and making use 

 of loth the series, — as indicated at the close of the preceding paragraph, — and 

 adapting them to the water unit, we obtain the following approximate numbers. 

 The numbers in the second column refer to the following units — the English 

 foot, the minute, and the centigrade degree.* The third column has the ceiiti- 

 TMtre substituted for the foot.f 



Temperature, Centigrade. 



Conducting Power of Wrought Iron. 









Units : tlie Foot, Minute, and 



Units : the Centimetre, 





Centigrade Degree. 



Minute, and Cent. Degree.* 



0° 



•0133 



12^36 



25° 



•0127 



11^80 



50° 



•0120 



1115 



75° 



•0114 



10-59 



100° 



•0107 



9-94 



125° 



•0101 



9-38 



150° 



•0094 



8-73 



175° 



•0088 



818 



200° 



•0082 



7-62 



37. It will be recollected that these numbers are offered as approximate only. 



* Or (more fully) it expresses in centigrade degrees the temperature communicated to a cubic 

 foot of water in one minute, across a plate of iron one foot thick, whose surfaces are maintained 

 at a constant difference of temperature of one degree centigrade. 



f This is perhaps the most convenient unit of conductivity for general use. The original ther- 

 mal unit of Fourier (who first gave a correct definition of this quantity) was referred to the minute 

 and the metre as the units of time and length, to the interval from the freezing to the boiling point 

 of water as the unit of temperature and the unit of heat was the quantity required to melt one kilo- 

 gramme of ice (Theorie, Arts. 68, 69). It is plain from Art. 59, and others of the same work, that 

 Fourier had no idea that the conductivity varied with the actual temperature — an admission which 

 must be held to leave the Newtonian law true inform only, since the flux is proportional in any one 



substance not to -^ only, but is also a direct function of v. 



dt 

 VOL. XXin. PART I. 



2r 



