Fourth Series. — Laminated and Smoked Surfaces. 71 



tempted to demonstrate, directly and numerically, that the 

 rays of heat which have passed through alum, glass, and in- 

 deed every substance which I tried, have a mean refrangibility 

 superior to that of the rays before such transmission ; and as 

 M. Melloni had been led in a general way by his previous 

 experiments to a similar conclusion, he inferred, and justly, 

 that most diathermanous bodies absorb the less refrangible 

 rays in excess, and therefore are to heat what green, blue, or 

 violet diaphanous media are to light. Rock-salt alone (so far 

 as we know) possesses the property of indifferent diathermancy > 

 and is the single analogue of white transparent glass. 



3. The generalization of this principle is a matter of much 

 importance, and especially as it carries our knowledge a step 

 higher in the scale of truth, by teaching us to refer to the 

 quality of refrangibility certain properties of heat, which be- 

 fore were connected only with certain vague characters of the 

 nature of the source whence it was derived. Amongst other 

 things we find, what was long suspected, but what M. Melloni 

 first conclusively proved, that the presence or absence of light 

 is, to a great extent, immaterial ; no doubt a concomitant, but 



lysed by transmission through another plate of smoked salt, through lami- 

 nated mica, and through roughened salt, the per-centage is raised from 36 

 to 41 in the tvro former cases, and to 40J in the latter, proving incontest- 

 ably the specific action of these transmissions in arresting the more refran- 

 gible rays. 



" I next considered, that as a moderate number of scratches appeared to 

 produce this modification, it might be practicable to obtain the effect by 

 transmitting heat simply through fine wire gauze. I could not obtain it 

 finer than sixty wires to the inch, and in this case, I could obtain no indi- 

 cations of differences in the transmitted ratios of one or other kind of heat. 

 The proportion transmitted to the direct effect, was, in every case, almost 

 exactly that of the area of the interstices of the gauze to its entire sur- 

 face. 



" When fine gratings (used for Fraunhofer's interference fringes) made 

 of cotton-thread were used, even in this case no difference was perceived ; 

 here, however, the thread, having probably a certain degree of permeability, 

 might mask the effect. 



"When fine powders were strewed between salt plates, leaving minute 

 interstices, the easier transmission of heat of low temperature was again 

 apparent. 



" Having procured delicate lines to be drawn with a diamond point on 

 a polished salt surface, first dividing it into squares l-lOOcJth inch in the side, 

 then into parallel stripes l-200dth inch apart, and finally into squares of the 

 latter dimension, in every case the effect resembled that of random scratches, 

 and was more apparent as the surface was more furrowed. 



" I have finally to observe, that the mere process of natural tarnishing 

 by the exposure of salt to the air, produces a similar effect. 



"These facts evidently point to phaenomena in heat, resembling diffrac- 

 tion and periodic colours in light. I cannot doubt that the simple trans- 

 mission through fine metallic gratings would produce effects similar to those 

 of the striated surfaces of rock-salt. — December 16, 1839." 



