756 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1800. 



5j=.90 9; 



which shows that 9 1 rays of heat were stopped. In the table before 

 referred to, we find that this glass stops 34 rays of light. 



Before proceeding it will be necessary to adopt a method of reducing the detail of 

 the experiments into a narrower compass. It will be sufficient to say, that they 

 have all been made on the same plan as the 2 which have been given. The obser- 

 vations were always continued for at least 5 minutes ; and by examining the ratios 

 of the numbers given by the thermometers in all that time, it may be seen that, 

 setting aside little irregularities, there is a greater stoppage at first than towards the 

 end ; but as it would not be safe to take a shorter exposure than 5 minutes, on ac- 

 count of the small quantity of heat transmitted by some glasses, I have fixed on 

 that interval as sufficiently accurate for giving a true comparative view. The ex- 

 periments therefore may now stand abridged as follows. 



Exper. Q6. I took a piece of highly polished crown glass, 



of a greenish colour, and, cutting it into several parts, ex- Min. Sun. 



amined the .transmitting power of one of them, reserving 66% 



the other pieces for some experiments that will be men- 5 73 

 tioned hereafter. 



This glass therefore stops 259 rays of heat, and 203 of light. 



Exper. 27. I cut likewise a piece of coach glass into Min. Sun. 



several parts, and tried one of them, reserving also the 68f 



other pieces for future experiments. 5 75% 



It stops 214 rays of heat, and l6"8 of light. 



Exper. 28. I examined a piece of Iceland crystal, of Min. Sun. Iceland crystal, 



nearly -^ of an inch in thickness. 67 67 



It stops 244 rays of heat, and 150 of light. 5 72* 71$ . . • 5f 



Greenish crown glass. 

 66% 

 71i...6|:5 = .741 



Coach glass. 

 ..7 



fj8f 



74'-. 



5£ = .786 



Min. Sun. Talc. 

 Exper. 29. 67\ 67h 



5 72 7l|...4j:8f= .861 



Min. Sun. An easily calculable talc. 



Exper. 30. 50 50 



5 54| 73|4|:3J = .816 



44 = .756 

 It stops 139 rays of heat, and 90 of light. 



It stops 184 rays of heat, and 288 of 

 light. 



Transmission of solar Heat through Glasses of the prismatic Colours. 



Exper. 31. 



Exper. 32. 



Exper. 33. 



Exper. 34. 



Exper. 35. 



Exper. 36. 



Min. 

 

 5 



Min. 







5 

 Min. 







5 



Min. 







5 

 Min. 







5 

 Min. 







5 



Sun 



73 



79k 



Sun. 



68f 



72i 



Sun. 



671 



74| 



Sun. 



70k 



744 



Sun. 



70£ 



7H 



Sun. 



67k 



74i 



Very dark red glass. 

 73 

 744 ... 64 : 14 = 



This glass stops 800 rays of heat, and 

 9999, out of 10000 rays of light j which 

 .200. amounts nearly to a total separation of 

 light from heat. 



Dark- red glass. This red glass stops only 606 rays of 



684 heat, and above 4999, out of 5000 



70 . . . 4\ : 1* - = .394. rays of light. 



Orange glass. This orange-coloured glass stops 604 



67^ rays of heat, which is nearly as much as 



70|- . . . 6% : 2£ = .396. is stopped by the last red one ; but it 



stops only 779 rays of light. 



Yellow glass. 

 704 



73 ... 3| : ty = .667. 

 Pale-green glass. 

 70£ 



71£...3|: 1| = .367. 

 Dark-green glass. 

 67 h 

 68| ...6* -: 1 = .151. 



It stops 333 rays of heat, and 319 of 

 light. 



It stops 633 rays of heat, and only 535 



of light. 



This glass stops 849 rays of heat, 



and 949 of light. This accounts for 



its great use as a darkening glass for 



telescopes. 



