20 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
radiation, and transmits 40 per cent, of light. Spectacles made of this glass are very 
pleasant. The glass is bluish with a tinge of green. In the tintometer the numbers 
are Yellow, 2'0 ; blue, 8‘0. 
Glass 221. 
Fused soda flux. 80'0 
Cerium nitrate, crystallised. 13 ‘4 
Uranoso-uranic oxide. 6'6 
ioo-o 
In this glass the action of uranium on the ultra-violet rays is added to that of 
cerium. The colour is faint yellow. It practically cuts off all the ultra-violet 
radiations, the limit being X 3685, and it also cuts off 39 per cent, of the heat rays. Its 
transparency is 60 per cent. In the tintometer the number is :—Yellow, 8'0. 
Glass 238. 
Raw soda flux. 77 '0 
Cerium nitrate, crystallised. 23'0 
ioo-o 
Class 238 is similar in composition to Glass 187, but it contains more cerium. Like 
187 it is practically opaque to ultra-violet light, the limit being about X3610, the 
injurious rays being beyond this wave-length. It cuts off 34 per cent, of the heat, and 
transmits 71 per cent, of the luminous rays. In the tintometer the numbers are 
Red, 0'375 ; yellow, 0"375 ; blue, 1"500. 
Glass 240. 
Further experiments with biotite showed that its special property was due to the 
iron protoxide it contained, and another glass was therefore made in which the iron 
could be introduced in the form of a definite salt of known composition. Ferrous 
oxalate was chosen, the proportions being as follows :—- 
Raw soda flux. 90 
Ferrous oxalate, FeC 2 0 4 2H 2 0. 10 
100 
The glass made from this material does not cut off quite so much of the heat rays as 
the one prepared from biotite, the rays obstructed being 88 as against 94 per cent. 
But the ultra-violet is entirely intercepted, the limit being X 3950. It transmits 36 per 
cent, of the luminous rays. It is of a smoky green colour. The tintometer numbers 
are Yellow, 4"0 ; blue, 4"0. 
