6 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
from this device, however, the glass must be kept perfectly quiet and at the highest 
temperature for a longer time than in my case was always practicable. 
When the crucible has cooled slowly for about twelve hours it is removed from the 
furnace, and the solid cone of glass removed by breaking the crucible by a few judicious 
blows with a light hammer. The lump of glass is now taken to the lapidary’s table, 
and slit across the middle and a plate cut from it, which is ground and polished to a 
determined thickness, usually 2 mm. 
Testing Synthetic Glasses for Opacity to Ultra-violet Light. 
The plates are now tested for the opacity of the glass to the rays of the ultra-violet 
end of the spectrum. The spectrograph with complete quartz train, already referred 
to, was used. B)" superposing on the radiation from a Nernst lamp the light from a 
high-tension electric discharge between poles of pure metallic uranium it is possible to 
produce a practically continuous beam extending from X 2000 to X 8000, and the 
absorption of such a beam of radiation, produced by flat plates 2 mm. thick of all my 
experimental glasses, has been thereby recorded. 
To ascertain the amount of heat obstructed by the plates of glass I first used a 
Melloni’s thermopile as described in his papers,^ but I soon found that modification 
was needed as the pile responds to the orange and red rays as well as to the infra-red. 
Plates of dark smoky quartz 2 mm. thick were tested with the thermopile, and it 
was found that they transmitted nearly all the heat whilst cutting off 80 or 90 per 
cent, of the light. Biottte (black mica) exerts a similar effect, and is more easily 
experimented with than smoky quartz. For many years I have used biotite for cutting 
off light and transmitting heat ; I accordingly investigated the properties of many 
specimens of black and dark brown mica to find out which would be best for this 
special purpose. 
Selection of Black Mica ( Biotite) for Diathermancy. 
Samples of dark brown and black mica vary considerably in their power of obstructing 
light and transmitting the infra-red rays. By the kindness of friends connected with 
the mining and importation of mica, I have been able to examine a large number of 
samples from different parts of the world. 
Some very fine pieces of black biotite were sent by Messrs. Attwater and Sons, who 
tell me they were mined by them in the extreme north of Norway from a cleft in a 
mountain at about 2000 feet. This mica is extremely regular in the thickness of the 
flakes which can be split from it, and the colour of thin pieces is uniform. A piece 
* “On the Free Transmission of Radiant Heat through Different Solid and Liquid Bodies,” ‘Ann. de 
Chim. et de Phys.,’ vol. liii., p. 1; Taylor’s ‘Scientific Memoirs,’ vol. 1, p. 1; “New Researches Relative 
to the Immediate Transmission of Radiant Heat through Different Solid and Liquid Bodies,” ‘ Ann. de 
Chim. et de Phys.,’ vol. lv., p. 337 ; Taylor’s ‘Scientific Memoirs,’ vol. i., p. 39. 
