Emission and Transmission of Heat 31 



i St. The cooling of a body results from its radiation and 

 from the contact of the surrounding gas. 



2d. The rate of cooling due to radiation is independent of 

 the substance of which the body is composed ; but its absolute 

 value varies with the nature of the surface of the body. 



It is represented by the formula, 



v=maQ (a* i) 



in which m is a number depending on the nature of the surface, 

 a the number 1.0007, ^ the temperature of the surroundings and 

 / the excess of the temperature of the body over that of the sur- 

 roundings in degrees Centigrade. 



3d. The rate of cooling due to the contact of the surround- 

 ing gas is also independent of the substance of which the body is 

 composed, but its absolute value is independent of the nature of 

 the surface ; it depends solely on the form of the body and the 

 excess of its temperature over that of the surroundings. 



This rate for air at 760 mm. pressure is given by the formula 



v = nt 1 - 2 



in which n is a number varying with the form and the extent of 

 the surface of the body, and t the excess of temperature of the 

 body above that of the surrounding air in degrees Centigrade. 



779. New Experiments. While admitting the exactness of 

 these laws, the formulas which represent them are of no service 

 as long as the coefficients m and n are unknown for surfaces of 

 different natures and for bodies of different forms. I may add 

 that Laprevotaye and Desains have found, in certain cases, results 

 that do not at all agree with the above formulas. 



I have therefore thought it best to take up the question 

 again, but limiting it to the study of the cooling of a body in air 

 under ordinary pressure and in a chamber with dull walls ; for 

 the cooling of a body in different gases, under different pressures 

 and in a chamber with gilded walls, is a purely speculative ques- 

 tion which never occurs in practical applications. 



780. There will be found at the end of this book the details 

 of the apparatus and the methods of calculation employed in the 

 experiments ; I shall here confine myself to a general description 

 and to the setting forth of the results arrived at. 



781. Experiments, for the purpose of finding the absolute 



