20 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



stances will presently occur. With a view to determine the quality of thickly 

 strewed surfaces yielding a very feeble per-centage of transmitted' heat, it was 

 desirable to use an intense incident beam. In order, however, to keep the com- 

 parison within the range of galvanometer degrees, whose numerical values have 

 been tested (Second Series, arts. 7-8), the observations in the preceding table 

 marked thus * were made in the following manner. The direct effect of the 

 incident heat on the pile was never observed, but only that part of it which 

 penetrated the wire-gauze, No. 3 of art. 33, which transmits almost exactly 

 30 per cent, of every kind of heat. The direct effect was estimated at ^ of the 

 degrees of deviation corresponding to this transmission, and then the wire-gauze 

 being removed, and the medium to be examined substituted, the effect was com- 

 pared to the computed direct effect. For example, with the copper powder, No. 2, 

 the effect of the LOCATELLI lamp, heat transmitted through thick plate-glass, and 

 then modified by wire-gauze, was 22.57 



Direct effect = 22.57 x 1? 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 accuracy : Another ex- 

 periment gave in the same case 5.60 : 100. 



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 opaque 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 se- 

 ries 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 results as to permeabi- 

 lity, a result by no means paradoxical, since intense heat may be sensibly trans- 

 mitted through a nearly opaque 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 farther explanation at present. That cop- 

 per possesses a peculiar character, distinct from the other metals which I tried, I 

 am fully persuaded. 



44. The evidence which the experiments on metallic powders gave of the 

 inadequacy of the mere powdery form to produce the effect of smoke, forced me 

 to a more critical examination 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 dif- 

 fering as widely in nature as possible. Some of these substances were repeatedly 



