FOURTH SERIES. LAMINATED AND SMOKED SURFACES. 3 



2. In the Third Series of these Reseaches, 3, I have attempted to demon- 

 strate, directly and numerically, that the rays of heat which have passed through 

 alum, glass, and indeed 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 refran- 

 gible rays in excess, and therefore are to heat what green, blue, or violet diapha- 

 nous media are to light. Rock-salt alone (so far as we know) possesses the pro- 

 perty 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 

 before 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 



ceptible as the character of the heat is more removed from that which these media transmit most 

 readily, that is, as the temperature of the source is higher. Thus, heat derived from a lamp, has 

 36 per cent, transmitted by a certain smoked plate of rock-salt. But if the heat transmitted by the 

 smoked salt has previously been sifted or analyzed by transmission through another plate of 

 smoked salt, through laminated mica, and through roughened salt, the per-centage is raised from 

 36 to 44 in the two former cases, and to 40^ in the latter, proving incontestably the specific action 

 of these transmissions in arresting the more refrangible rays. 



" I next considered, that as a moderate number of scratches appeared to produce this modifi- 

 cation, 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 

 indications 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 surface. 



" 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-100th inch in the side, then into parallel stripes l-200th 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 phenomena in heat, resembling diffraction and periodic colours 

 in light. I cannot doubt that the simple transmission through fine metallic gratings would pro- 

 duce effects similar to those of the striated surfaces of rock-salt December 16. 1839." 



