188 



Prof. Forbes's Researches on Heat. 



jections, will give a just idea of the confidence due to the re- 

 sults. 



Sources of Heat. — (1.) The direct rays of the Locatelli 

 lamp. A slightly concave reflector was employed. (2.) The 

 same lamp, with a reflector having the form of a portion of a 

 sphere concentric with the wick ; the heat transmitted through 

 alum. (3.) Heat from the same source transmitted by isoin- 

 doiso glass '06 inch thick. (4-.) Heat from the same trans- 

 mitted by opake black glass (through which the disk of the 

 unclouded sun is just visible). (5.) Heat from the same trans- 

 mitted through dark coloured mica, by which direct sunlight 

 is absolutely stopped. This singular substance I long sought 

 for in vain, it is unknown to many practical mineralogists ; it 

 transmits green light at small thicknesses, when thicker its 

 colour is hair-brovii'n. By reflected light its colour is between 

 green and black. (6.) Heat from incandescent platinum. 

 (7.) The same sifted by rvindoKso-glass as above. (8.) The same 

 sifted by opake mica. (9.) Heat from dark brass about 700°. 

 This is obtained from a nearly cylindrical cover of smoked 

 brass placed over the flame of a spirit-lamp, so as entirely to 

 conceal it, and which gives remarkably good results, without 

 increasing considerably the angular breadth of the source 

 (which is greatly to be avoided when a lens is used). It is in 

 fact not much greater in size than the helical coil of platinum 

 wire used in (6). (10.) The same, sifted by clear mica '0044 

 inch thick. (11.) Heat from a crucible of mercury about 

 450°. The crucible was about 2 inches in the side, smoked 

 externally, and healed by a spirit-lamp. The temperature of 

 the mercury which it contained (covered with sand) was noted 

 at each observation by means of an inserted thermometer. 



The results were the following : 



Source of Heat. 



a h, or diagonal 



corresponding to the 



point of contrary flexure. 



/*, or Index of Refraction 

 for mean rays computed 

 by the Formula, page 183. 



Locatelli, direct 



-. with alum ... 



window-glass 



" opake glass 



. — — mica 



Incandescent platinum 



Ditto with glass 



opake mica 



Brass at 700° ..... 



Ditto with clear mica 

 Mercury at 450° 



Mean luminous rays... 



15-49 

 15-76 

 15-65 

 15-71 

 15-6] 

 1550 

 15-66 

 15-62 

 15-45 

 15-55 

 15-50 



15-8 



1-571 

 1-598 

 1-587 

 1-593 

 1-583 

 1-572 

 1-588 

 1-584 

 1-568 

 1-577 

 1-572 



1-602 



