Fourth Series. — Bough Surfaces, 1 1 1 



any cause of specific action becomes immediately apparent. 

 Not so with any other substance, which, exercising already a 

 specific action in virtue of its nature, is to have that specific 

 action modified by a modification of surface. At least the 

 question is, whether or not this modification will occur ? An 

 example will best illustrate how this modification may be dis- 

 covered and expressed. I took a plate of mica with its na- 

 tural bright surfaces, and so thin as to transmit in consider- 

 able abundance heat from different sources. The per-centages 

 in this state were determined as follows : — 



Locatelli, with Glass. Locatelli. Dark Heat. 



Mica with bright surfaces ... 83'5 74? 37 



Both sides of the mica were depolished with emery-paper, and 

 the experiment repeated (27th November, 1839) : 



Locatelli, with Glass. Locatelli. Dark Heat. 



Mica with rough surfaces, ... 45*5 51 31'5 



Denoting the original transmissions by 100, the diminished 

 effect due to the roughness of the surface will be represented by 



54- 69 85 



demonstrating as clearly as possible that the stoppage is pro- 

 portioned to the temperature of the source of heat; thus, 

 whilst 46 per cent, of the first kind was arrested by the rough- 

 ness of the surface, only 15 per cent, of dark heat was stopped. 

 29. (3.) With regard to the third question, the action of a 

 comparatively small number of scratches on a polished sur- 

 face, instead of a general diminution of its polish, I proceeded 

 thus : — I caused a series of extremely minute lines to be drawn 

 mechanically with a diamond point, on a well -polished surface 

 of rock-salt, so as to divide it into squares having one-hun- 

 dredth of an inch for their side. A similar plate was scored 

 by fine lines in the same manner, parallel to one another, and 

 one two-hundredth of an inch apart. A portion of this second 

 plate was crossed rectangularly, by lines drawn at the same 

 distance, so as to divide the surface into squares four times 

 smaller than in the first instance. These three media gave 

 the following results with two very different kinds of heat 

 (December 6-11, 1839) :— 



