Fourth Series. — Powdered Surfaces. 



121 



nute surfaces at every angle, and minute fissures in every di- 

 rection. 



50. The following table contains the results of a large 

 number of experiments on powders of various kinds, many of 

 them repeated under various circumstances. The investiga- 

 tion is, as in the case of the metallic powders, confessedly 

 imperfect ; but since the broad simple principle which I at 

 first tried to establish respecting the diathermanous quality 

 of opake powders does not appear to hold universally, I 

 stopped this series of experiments, which were troublesome 

 and laborious, after establishing a few general facts, which I 

 will presently lay down, without attempting to exhaust a sub- 

 ject, 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 at- 

 tached to diaphragms 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-metallic* Powders. 



Powder of 



Alum, No. 1 



No. 2 



Citric acid, No. 1 



No. 2 



Rock-salt, No. 1 



No. 2 



Sulphur 



Red-lead 



Galena 



Charcoal A 



• No. 1. 1st Series §... 



■2nd Series^... 



Source of Heat. 



Locatelli Lamp 



Through 

 Glass. 



No. 2. 



Chalk, No. 1 



No. 2 



No. 3 ... 



Carbonate of magnesia 



170 

 15-2* 

 29 



12-9* 

 12-8* 13 4 

 31-5: 

 500 

 30-2 

 263 



5|| 

 11-4 

 15-1 



3-2* 



305 



15-5* 15-6 



27*5 



8-3 



Through 

 Smoked Salt. 



30 



ii-8 



22-4 

 13-9 



18-4 



i*2-6 



Dark 

 Hot Brass. 



171 

 130* 

 33 + 



8-7* 

 11-3 

 29-2J 

 44-7 

 34-0 



* II 



160 

 3-5* 

 34-5 

 17-9 

 320 



Hot 

 Water. 



31-5 



17 



* The observations marked thus were made with a powerful beam 

 of heat in the way described in art. 42. f Not directly comparable 



with the other two observations on the same line, and probably 3 or 4 

 per cent, too high. \ Extremely good observations. 



|| The intensities very feeble. § The circumstances in these 



two series varied, so as to make the one not directly comparable with 

 the other ; but each is perfectly good. 



* By non-metallic is meant, not in the state of apure or uncombined metal. 



