320 : 
Physical assumed a new arrangement. By increasi 
Properties. 
Dr Brews- 
ter’s experi- 
ments on 
glass tears. 
the tem- 
perature, the cavities E, F, disappeared : ‘the lower 
side of the drop, upon which it rested, was indented by 
the bottom of the crucible ; but it had in no other res 
s lost its external shape, the appearance of the 
cleavage in Fig. 2, remaining unaltered. In this state 
Mr Jardine measured the specific gravity of the drop, 
and found it to be 3.278, which is almost exactly the 
same as that of the annealed drop. 
In order to observe the manner in which the cavities 
disappeared, I snapenieg one of the drops by a wire, 
and viewed it With a telescopic microscope when ex- 
posed to a strong heat. Soon after the drop became 
red hot, the cavities gradually contracted, and at last 
vanished, the centre of the cavity being the part that 
was last filled up. The drop had begun to melt at its 
smaller extremity, but the lines represented in Fig. 2, 
were still visible, the heat probably not having been 
sufficiently intense to affect its superficial structure. 
As the specific gravity of the crystallized drop is 
nearly the same as that of the annealed drop, the cavi- 
ties must be produced by the contraction which the in- 
ternal part experiences in cooling, for the sudden indu- 
ration of the outer layer prevents the contraction from 
taking place in any other way. The manner, too, in 
be | e cavities disappear, is a complete proof that 
they contain no air, and hence we may consider their 
magnitude, which increases with the size of the drop, 
as a measure of the contraction which the glass un- 
dergoes in its transition from the temperature at 
which it melts, to the ordinary temperature of the at- 
mosphere. See Exeanston, p. 255. 
I am informed by Dr Hope, that he has obtained un- 
annealed drops of crown glass, in which there were no 
vacuities, ne that they ail burst spontaneously in the 
course of a few months.” Phil. Trans. 1815, p.1.— 
Unannealed The unannealed glass cups, which we have mention- 
glass cups. ed under ANNEALING, are represented in Fig. 5. Plate 
Prate = CCLXXIV. The lower end B is made very thick, and 
CCLXXIV. the bodies, such as a musket ball or a fragment of flint, 
Fig. 5. are dropped into the mouth of it at A. The stroke of 
the ball upon the thick bottom will produce no effect, 
while the blow of the small fragment of flint will burst 
the cup with great violence. The following are the 
dimensions of the cup represented in Fig. 5:—Length 
43 inches; width at top 1; width at bottom 14; 
thickness of glass at top +; greatest thickness of glass 
at bottom +4. 
The bursting of these cups is effected, when they 
are even three inches thick at the bottom. In an 
experiment made by Dr Littleton upon a cup of this 
magnitude it resisted a blow from a musket ball let fall 
from a height of nearly three feet, while it was instant- 
ly broken by a shiver of flint weighing only two grains. 
An account of numerous experiments made with 
these cups will be found in the Philosophical Transac- 
tions for 1745, vol. xliii, No. 477, p. 505. 
The Right Hon. Sir Robert Moray discovered that 
hollow balls made of unannealed glass, with a smail 
hole in them, would be burst in pieces by the heat of 
the hand alone, by stopping up merely the small hole 
with the finger. This obviously arose from the pres- 
sure of the expanded air on the interior of the ball. 
About the year 1740, when Mr C. Orme, of Ashby 
de la Zouch, was drying the glass tubes for his diago- 
nal barometers, he observed that they had not only .a 
rotatory motion about their axis, but also a progressive 
motion towards the fire. These tubes were about*four 
feet long, and half an inch thick, and when placed 
about 6 or’8 inches from the fire, they moved “ not only 
fnannealed 
hollow 
balls. 
Rotatory 
motion of 
glass tubes. 
GLASS. i 
i 
it 
he 
: 
z 
8 
i 
z 
: 
their weight.” The Rev. Granville Wheler, : 
Mr Orme shewed these experiments, repeated. 
with great care, and found that the im: 
ceeded best with a moderate fire, poss pts sae the 
were about 20 or.22 inches and 
of an inch in diameter, when they had in each 
pretty strong pin fixed in a cork as an axi 
they were supported on other glass tubes of nearl 
same diameter. When the ive moti 
tubes was st by an Fas gh We 
about their axis. When the tubes were 
zontally on a large fragment of lags, instead. of 
vancin, ‘ceniatoa firea as Pats np ae 
fire, and about their axis in a direction : 
they had done before. In this case, as xd 7 
receded from the fire, even when the plate of-glass was 
a little inclined. Mr Wheler very ingeniously explains © 
these phenomena by the expansion DE. the of He 
the tube nearest the fire, which, by sbengaee wee f 
at a greater distance from the centre of motion, de- tab 
stroys its equilibrium. The heavy side of the tube there- 
fore descends, and a fresh, of it being e to 
hn fire, expands and descen aahonnonlss A writer ina 
modern dictionary . i lanation. on. the 
ground. that « the fandamental oe e on which it 
porate is false, for though fire, in will make 
ies expand, it does not increase in wei 
therefore the sides of the tube, though one 
expanded by the fire, must still remain in equilibrio, 
hence we must conclude that the causes of these 
nomena remain yet to be discovered.’”? In this. 
ordinary reasoning, the author has overlooked the 
damental Paap in seoshentice, that the force with which 
any quantity of matter tends to turn round a fulcrum, 
ba ional to the sum of all the products of each 
particle of matter multiplied by its distance from the 
centre of motion. In the case of the glass tube, the num- 
ber of particles remains the same, and the distance of all 
hgeteanen > reer of motion is increased. Hence _ 
the sum of the ucts is increased, and consequen: mn 
the equilibrium destroyed. ‘ 7 7 
luminous body, Cur: 
When we look at the sun, or any 
through the common coloured glasses, the transmitted tic 
light, tho nate with one colour, nevertheless ry 0 
transmits rays of all the other colours, as may be pro. “! 
ved by decomposing it with a prism. It was ; 
however, by M. Monge, and the observation has been __ 
subsequently confirmed by M. Hassenfratz and M. 
Arago, that the glass of old churches which has been — 
stained either red or green by the oxide of copper, — 
has the. surprising property of transmitting a 4 
homogeneous red 
EF 
3 Qe 
rilise 
is 
it 
a 
but the homogeneous en or the 
rays. This property will be of the greatest use in so= 
lar observations, as it will remove completely the im- 
ections of telescopes arising from their different res 
rangibilities. A telescope should be constructed with — 
a compound object glass, to destroy as much as pos< 
sible the aberration of sphericity. The red or en 
glass will remove all the het eous rays, aati 
most perfect image of the sun will thus be obtained. 
A series of new. riments have recently been Ne 
made upon glass by Dr Brewster, and the results n 
which he has obtained are of such a singular nature, 8! 
as to lay the foundation of a new science, in 
ty and 
u 
7 
3 
2 
its general character to the sciences of 
magnetism. He has shewn that when radiant heat 
i : 
oe 
