16 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
brown, and with the highest proportion it is yellowish brown. The opacity for ultra¬ 
violet light increases as the glass is richer in metal, the one with about 4 per cent, 
uranium being opaque to the indigo and violet down to the blue. The heat absorbed 
at most is about 55 per cent. These results show that uranium is a metal likely 
to be useful in combination. 
Composition of Classes Specially Selected for Practical Use. 
Whilst bearing in mind that the chief object of this research is to find a glass that 
will cut off as much as possible of the heat radiation, I have also attacked the problem 
from the ultra-violet and the transparency points of view. Taking each of these 
desiderata by itself I have succeeded in preparing glasses which cut off over 90 per 
cent, of heat radiation, which are opaque to the invisible ultra-violet rays, and are 
sufficiently free from colour to be scarcely noticeable when used as spectacles. But I 
have not been able to combine in one specimen of glass these three desiderata in the 
highest degree. The ideal glass which will transmit all the colours of the spectrum 
cutting off the invisible rays at each end, is still to be discovered. 
As far as transparency, however, is concerned it will not be an unmixed advantage 
for the sought-for glass to be quite clear and colourless. The glare of a strong light 
on white cliffs, expanses of snow, electric light, &c., is known to be injurious to the 
eye, and therefore a tinted glass combining good obstruction to the heat radiation and 
ultra-violet rays is the best to aim for. 
Grey or neutral tints are the most pleasant to wear. They do not appreciably alter 
the natural colours of objects, and are a great relief to the eye. Many glasses are 
met with in commerce of different colours which are found by experience to suit the 
public demand for tinted spectacles. They are of various tints of yellow, green, blue, 
and neutral, and therefore I do not think it will be wrong to select the tints of my 
glasses, suitable in other respects, no darker than those which appear to suit the public 
taste. 
As a basis for the preparation of the coloured glasses, Mr. Powell prepared for me 
a quantity of hard soda-flux mixture of the following composition :— 
Sand. 61'00 
Sodium carbonate, anhydrous. 25‘50 
Sodium nitrate, re crystallised. 5 '00 
Calcium carbonate, precipitated .... 7‘20 
Borax. 0’75 
Arsenic trioxide. 0'55 
lOO'OO 
This mixture whilst melting loses about 25 per cent, in weight. When first melted, 
it is colourless ; on second melting it acquires the usual faint greenish tint of soda- 
