22 



PROFESSOR FORBES'S EXPERIMENTS ON HEAT. 



49. If this be the case, if the differences be so trifling for a reticulation 

 of regularly-formed, transparent, and polished threads of glass, much more must 

 it hold with impalpable crystalline or other powders, presenting (no doubt) mi- 

 nute surfaces at every angle, and minute fissures in every direction. 



50. The following Table contains the results of a large number of experi- 

 ments on powders of various kinds, many of them repeated under various cir- 

 cumstances. The investigation is, as in the case of the metallic powders, con- 

 fessedly imperfect ; but since the broad simple principle which I at first tried to 

 establish respecting the diathermanous quality of opaque powders does not ap- 

 pear to hold universally, I stopped this series of experiments, which were trouble- 

 some and laborious, after establishing a few general facts, which I will presently 

 lay down, without attempting to exhaust a subject of which, by and by, we 

 shall no doubt know more, but which at present it would be perhaps a waste of 

 time to pursue into its insulated details. These powders were in all cases dusted 

 between polished salt-plates, united at the edges, and then attached to dia- 

 phragms of card, so arranged as to transmit the heat in every case through the 

 same parts of the surface. 



Per-centage of Transmission of Heat, from different sources, through Non-Me- 

 tallic 1 Powders. 



51. On the preceding table, I would observe, (1.) That the pulverized crys- 



1 By non-metallic is meant, not in the state of a. pure or uncombined metal. 



