[ 69 ] 



XIV. Researches on Heat. — Fourth Series. On the Ef- 

 fect 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 University of Edinburgh. f 



Arts. 1 — 12, Laminated and Smoked Sw faces. 13 — 29, 

 Rough Surfaces. 30 — 34, Metallic and other Gratings. 

 35 — 53, Powdered Sutfaces. 54 — 65, Conclusions. 



1. CX& the 2nd September 1839, M. Arago communicated 

 V^ to the Academy of Sciences of Paris a letter by M. 

 Melloni, containing some very interesting experiments 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 facility) acquires, by being 



f From the Transactions of the Royal Society of Edinburgh, vol. xv. 

 Part 1. 



* 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 of 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 digest- 

 ing, until now, the results of these experiments, and from stating the grounds 

 of the conclusions which 1 formerly 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 re- 

 ferred to, reprinted from the Proceedings of the Royal Society of Edin- 

 burgh : — 



" On the Effect of the Mechanical Texture of Screens en the immediate 

 transmission of Radiant Heat. By Professor Forbes. — On the 2nd of 

 September 1839, M. Arago communicated to the Academy of Sciences a 

 letter by M. Melloni, containing some very interesting experiments on the 

 transmission 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 tempe- 

 rature, 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, show that this is 

 equivalent to saying, that substances in general allow only the more re- 

 frangible rays to pass; and as M. Melloni had been led by his previous ex- 

 periments to the same conclusion, his statement amounts to this, that, 

 whilst rock-salt presents the analogy of white glass, by transmitting all rays 

 in equal proportions, every substance hitherto examined acts on the calo- 

 rific rays as violet or blue glass does on light, absorbing the rays of least re- 

 frangibility, and transmitting only the others. 



" M. Melloni believes, that the first exception to this rule, or the first 

 analogue of red glass, is rock-salt previously smoked. I desire, however, 

 first to call attention to the fact, that, in a paper published in May 1838 

 (Researches on Heat, Third Series), I described a substance having similar 

 properties, namely, mica split by heat to extreme thinness, such as I employ 



