Fourth Series.*— Powdered Surfaces. 119 



catelli lamp, heat transmitted through thick plate-glass, and 

 then modified by wire^gauze, was 22 0, 57 



Direct effect = 22*57 x -y- 75 -2 



Wire-gauze removed, and copper substituted 4 *15 



Ratio to direct 5*52 : 100. 

 In this way per-centages may be obtained with very nice ac- 

 curacy : another experiment gave in the same case 5*60 : 1 00. 



43. The table in art. 41 . demonstrates to my conviction 

 (strengthened by a careful examination of the very consistent 

 observations on which it is founded), (1.) That gold, silver, 

 and tin powders, instead of having the property which I was 

 disposed to assign to opake powders generally, do really 

 transmit more heat of high than of low temperature; that 

 is, act like glass, alum, and other transparent media in their 

 common state. (2.) With respect to copper, two series give 

 one result, and a third the opposite. Yet all of these were 

 made with great care, and contain internal evidence of their 

 accuracy. I am confident that the differences are not due to 

 errors of observation ; and I have observed other cases, in 

 which an increase of thickness of the obstructing medium, and 

 an increased intensity of the incident heat, gave altered re- 

 sults as to permeability, a result by no means paradoxical, 

 since intense heat may be sensibly transmitted through a 

 nearly opake substance, and thence acquire a new character, 

 which a feebler beam, transmitted through a less obstructing 

 medium, would not possess. At all events, I can offer no 

 further explanation at present. That copper possesses a pe- 

 culiar character, distinct from the other metals which I tried, 

 I am fully persuaded. 



44. The evidence which the experiments on metallic pow- 

 ders gave of the inadequacy of the mere powdery form to pro- 

 duce the effect of smoke, forced me to a more critical exami- 

 nation of other bodies in a similar state. 



45. I repeated my experiments with increased care on the 

 powders already employed. I tried a great number of new 

 ones, chosen amongst substances differing as widely in na- 

 ture as possible. Some of these substances were repeatedly 

 tried in different specimens, the powder more or less thickly 

 strewed, and at different times. 



46. One circumstance in particular raised a doubt as to the 

 result of my former conclusion, where it seemed most incon- 

 trovertible. I had argued, that if alum in powder arrested 

 equally all kinds of heat, the mechanical action of the powder 

 must have opposed and destroyed the specific action of the 

 alum (36.). I was gradually, however, led to admit, that, in 

 the state of powder, most diathermanous bodies are almost 



