TRANSACTIONS. 



Researches on Heat. FOURTH SERIES. On the Effect of the Mechanical Texture 

 of Screens on the immediate transmission of Radiant Heat.* By JAMES D. 

 FORBES, Esq., F* R. SS. L. fy E., Professor of Natural Philosophy in the Uni- 

 versity of Edinburgh. 



Arts. 1-12, Laminated and Smoked Surfaces. 13-29, Rough Surfaces. 

 30-34, Metallic and other Gratings. 35-53, Powdered Surfaces. 54-65. 

 Conclusions. 



1. ON the 2d September 1839, M. ARAGO communicated to the Academy of 

 Sciences of Paris a letter by M. MELLONI, containing some very interesting expe- 

 riments on the transmission of Radiant Heat. M. MELLONI finds that rock-salt 

 (which is well known to transmit rays of heat from all sources yet tried with equal 



* The substance of the present paper was communicated to the Royal Society of Edinburgh on 

 the 16th December 1839, in the words of the memorandum which forms part of this Note. The 

 memorandum itself was read, with some verbal explanation and citation of additional facts, on the 

 6th January. Every experiment to which reference is made in the present paper, was performed 

 between the 12th November 1839 and the 4th March 1840. Since that time, I have not made a 

 single experiment on the subject. Occupation of other kinds has prevented me from digesting, until 

 now, the results of these experiments, and from stating the grounds of the conclusions which I for- 

 merly announced. The present paper, as it stands, having been submitted to the Council on the 

 15th May 1840, is printed by their authority. The following is the memorandum just referred to, 

 reprinted from the Proceedings of the Royal Society of Edinburgh: 



" On the Effect of the Mechanical Texture of Screens on the immediate Transmission of Radiant 

 Heat. By Professor Forbes On the 2d September 1839, M. ARAGO communicated to the Academy 

 of Sciences a letter by M. MELLONI, containing some very interesting experiments on the transmis>- 

 sion of Radiant Heat. M. MELLONI finds, that rock-salt (which is well known to transmit rays from 

 every source with equal facility) acquires, by being smoked, the power of transmitting most easily 

 heat of low temperature, or that kind of heat stopped in greatest proportion by glass, alum, and 

 (according to M. MELLONI) every other substance. The experiments contained in the Third Series 

 of my Researches on Heat, shew that this is equivalent to saying, that substances in general allow 



VOL. XV. PART I. A 



