on the terrestrial Rays that occasion Heat. 479 



122^ Experiment. 



I took now the piece of talc used in the 30th experiment, 

 and, placing it over the transmitting hole, I had the following 

 result. But, as the unexpected event of a calcination, which took 

 place, was attended with circumstances that ought to be noticed, 

 I shall, instead of the usual abridgment of the experiments, give 

 this at full length. 





Fire. 



Talc. 



, 





Therm. D. 



Therm. C. 





o' 



65 



65 





1 



72 



6 7 ... 



. . 7 : 2 = ,289 



2 



77 



&J- 



.12 : s| =,281 



3 



8o± 



?# |T 



■ 15| : 4i = > 2 90 



4 



83 



70.. 



. 18 \"s = ,278 



5 



85 



7°i- 



• 20 : 3J = ,287 



iubstan 



ce stops 71Q ra 



ys of fire-heat 





I am now to point out the singularity of this experiment; 

 which consists, as we may see by the above register of it, in the 

 apparently regular continuance of its power of transmitting heat, 

 while its capacity of transmitting light was totally destroyed. 

 For, when I placed this piece of talc over the hole in the screen, 

 it was extremely transparent, as this substance is generally 

 known to be ; and yet, when the experiment was over, it ap- 

 peared of a beautiful white colour ; and its power of transmitting 

 light was so totally destroyed, that even the sun in the meridian 

 could not be perceived through it. Now, had the power of trans- 

 mitting heat through this substance been really uniform during 

 all the five minutes, it would have been quite a new phenome- 

 non ; as all my experiments are attended with a regular increase 



32 s 



