4 



18 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



obtained a similar result for Charcoal powder (whose affinity with smoke suggest- 

 ed its use), and yet it does not appear that the general conclusion which I intended 

 is entirely warranted. 



37. It is well known that Sir ISAAC NEWTON overlooked the variable disper- 

 sive power of bodies for light, in consequence of having compared two, in which 

 the dispersion happened to be proportional to the mean refraction. A similar 

 haste to generalize would have led to error on the present occasion, had not a 

 simultaneous investigation led me to re-consider the subject of powders. Whilst 

 waiting for the arrival of fine wire-gauze from Paris, it occurred to me to try the 

 effect of metals in a state of extreme division. It seemed, however, first desirable 

 to ascertain whether the metals are as incapable of transmitting heat as is com- 

 monly supposed. 



38. For this purpose, I stretched a piece of the thinnest gold-leaf across a 

 wide diaphragm of pasteboard, and suffered an intense parallel beam of heat from 

 LOCATELLI'S lamp to fall directly upon the pile. A screen of glass was interpo- 

 sed, which, by experiment, was known to stop 43 per cent, of this sort of heat. 

 The needle of the galvanometer deviated 31.2, the glass being interposed ; the 



1 f\r\ 



equivalent direct effect would have been 31.2 x ^ 72. When the glass was 



removed, and the gold-leaf put in its place, on the brass screen being alternately 

 introduced and removed, not the faintest motion was perceptible in the needle ; 

 had it amounted to ^ of a degree, that is, had ^g of the incident heat been trans- 

 mitted by the gold-leaf, I considered that the effect would have been perceptible. 

 Yet this gold-leaf was so thin that the features of a landscape could be distinctly 

 seen through it, of the usual bluish-green tint. No more convincing proof cer- 

 tainly can be desired, that conduction plays no sensible part in these experiments, 

 since it did not sensibly act on a film of one of the best known conductors of heat, 

 and perhaps not more than smooth f an mcn thick. I thought it worth while 

 to repeat the experiment with dark-heat, and with the same results. The analogy 

 of the action of split mica on light to metallic reflection led me to suspect, that if 

 any kind of heat were transmitted by metallic leaves, it would be that of low 

 temperature. 



39. The imperviousness to heat of gold-leaf, the thinnest continuous film of 

 metal which we can obtain, satisfied me of the importance of obtaining the metals 

 in a condition to verify my experiments with the powder of other substances. 

 When the hope diminished of obtaining wire-gauze of a degree of fineness (I mean 

 fineness in the mire, not closeness of texture, for that was comparatively imma- 

 terial), which might vie with the diamond scratches on the salt surface, which 

 presented, under the microscope, an irregular furrow, probably nearly g^, inch 

 in mean breadth, I recurred to the project of using the metals in powder. It 

 was evident from the experiments on depolished and scored surfaces, that the 

 irregularity of these streaks had nothing whatever to do with the phenomenon 

 of checking rays of high refrangibility and admitting others. Sand-paper scratches, 



