14 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
was decidedly pink, and it required a further addition of nickel solution to restore the 
neutrality of colour at the boiling-point, raising the proportion of nickel to cobalt 3‘5 
to 1 at 100° C. against 2'5 to 1 at 16° C. As the hot solution cooled the neutral tint 
gradually changed until it became decidedly green. 
The same mixed solution, neutral tinted in the cold, was acidified with sulphuric 
acid. It immediately assumed a very faint tinge of pink, but not so decided a tint as 
the same neutral coloured solution took when heated to the boiling-point. 
A solution was prepared containing nickel and cobalt in the proportion of 2 "5 to 1 ; 
it was evaporated to dryness and ignited. The mixed oxides were then added to the 
hard soda flux, and the whole melted together at a high temperature ; the glass 
resulting was cut and polished into a plate 2 mm. thick. It was decidedly blue, 
although the metals were in the proportion to give neutrality at the ordinary 
temperature when in aqueous solution. More nickel was added in small proportions 
at a time, and it was not until the proportion in the glass was 1 cobalt to 5 nickel 
that a neutral grey glass was obtained. 
As a colouring agent cobalt is stronger than nickel. It is not easy to get an exact 
proportion, as the neutral point is difficult to hit with accuracy. If 4 of nickel 
instead of 5 are used with I of cobalt the glass is of a decided bluish tint, while if 6 
of nickel are added the colour is brown. 
Copper. 
Copper by itself as a constituent of glass is not of much advantage. It colours the 
glass blue, has not much action on the ultra-violet rays, but cuts off three-fourths of 
the heat rays. It forms a useful addition to other colouring agents in tending to 
neutralise those of the orange-yellow colour. 
Iron (FeO). 
Iron is introduced into the glass as ferrous sulphate, care being taken to avoid all 
oxidising agents. The fusfed mass is stirred with a carbon rod, and a little powdered 
charcoal added to the melt in the crucible. In quantities from 1 to 2‘5 per cent, the 
obstruction to heat radiation is great, and increases with the quantity of metal 
present. One per cent, of iron cuts off about 65 per cent, of the heat, and 2'3 per 
cent, cuts off about 89 per cent. The action on the ultra-violet end is but slight, the 
photographs extending to A 3467 with the lowest amount of iron, and to A 3560 with 
the largest amount experimented with. The light transmitted by the 2 mm. plate is 
71 per cent, with the least amount of iron and 50 per cent, with the largest 
amount. The colour of the glass is greenish blue. Iron in the ferrous state, there¬ 
fore, will prove useful on account of its communicating adiathermic property to the 
glass. 
