1888.] Mr J. T, Bottomley on Glass Globes with Cavities. 109 
became visible in the middle of the mass a few very minute bright 
points — little specs it seemed — and these quickly grew, and were 
then perceived to be hollow spaces in the midst of the glass, and 
finally they assumed the appearance of the large bubbles now to 
be seen. 
The conclusion of the operation was the cutting across of the 
neck of glass which supported the globe, and allowing the globe to 
drop off, and the passing of it through the leer or annealing furnace 
in the ordinary way. The place where the neck was broken ofi was 
finally ground flat by the polishers. 
The explanation of the cavities is obvious. The sudden cooling 
of the outside of the glass globe caused the outer layer to become 
rigid, while the interior mass was still extremely hot and molten. 
But when the cooling reached the interior, that portion underwent 
great contraction of volume, and as the outer skin, which had 
enormous strength, on account of its shape, refused to be drawn in, 
the interior was forced to part somewhere, and these cavities were 
formed. 
To follow a little further this interesting phenomenon, my friend 
Mr Griffin was good enough to make for me a globe without cavities, 
by allowing the whole mass to cool more uniformly, and letting the 
skin fall in towards the centre along with the interior mass. Thus 
he produced the globe marked A. 
The process, which, of course, is the ordinary one for producing 
large glass paper-weights, and articles of that kind where flaws or 
bubbles are considered as blemishes, consisted in very frequently 
putting the cooling globe into the mouth of the glass-pot, and thus 
warming up the outer skin sufficiently to keep it from becoming 
suddenly rigid. Thus the cooling was gradually carried on with 
frequent partial reheating of the surface skin. 
On examining the globes marked B^, &c., it will be seen that the 
distribution of the smaller cavities is very curious in appearance. 
It will easily be noticed that these little cavities are distributed 
over concentric spherical surfaces (to speak roughly). The cause of 
this was not difficult to trace. 
The glass worker, in gathering a ball at the end of his iron, is in 
the habit of taking from the glass-pot a small quantity of glass to 
begin with. He then draws out the point of the iron into the air 
