‘arts and the sciences has been so much indebted 
+ ag glass ; and there is none which has contributed more 
plendor and the comfort of civilized society. 
, We do not to enter at 
re ‘into any de- 
- tailed account of the electrical, the ehemical, or the 
ical properties of glass, which the reader will find 
fly discussed under our treatises on Curmistry, Evrc- 
griciry, and oe We intend cart to — 
rate some of the physical properties which either dis- 
be omit canoe bedien, or which could not 
with propriety be noticed under other heads. 
Glass possesses the remarkable pro of suffer- 
ing no change by the application of the most intense 
heat. The effect of great heats is only to melt the glass, 
or to dissipate it in vapour; but as long as any of the 
_ glass remains, it still preserves its transparency, and 
- other distinguishing properties. The conversion of glass 
into porcelain by long continued cementation with other 
materials, happens only to that particular kind which is 
_ made of alkaline salt and sand. 
- Of all the solid substances whose expansibility has 
ty been accurately examined, glass sses the proper- 
ty of being the least affected by heat and cold. Its 
expansion, according to General Roy, with an increase 
of heat equal to 180° of Fahrenheit’s thermometer, is 
only 0.000776, while that of platina is 0.000856, and 
that of hammered zinc 0.003011. On account of this 
property, glass is peculiarly fitted for containing fluids 
whose expansions are under examination, as its own 
change of form may in ordinary cases be neglected. 
For the same reason, it is better than any other sub- 
stance for the simple pendulum of a clock. See Expan- 
‘ston, p. 254, 255. 
‘lity of The great ductility of glass is one of its most re- 
-markable properties. When heated to a sufficient de- 
gree, it may be moulded into any possible form with 
the utmost facility, and it can be drawn out into 
the finest fibres. The method of spinning glass is 
‘very simple. The operator holds a piece of glass over 
the flame of a lamp with one hand; he then fixes 
a hook to the melted mass, and withdrawing it, 
he obtains a thread of glass attached to the hook. 
‘The hook is then fixed in the circumference of a 
cylindrical drum, which can be turned round by the 
hand; and a rotatory motion being given to the 
drum, the glass is drawn in the finest threads from 
the fluid mass, and coiled round the cylindrical cir- 
cumference. M. Reaumur supposed, with great proba- 
bility, that the flexibility of glass increased’ with the 
fineness of the threads, and he therefore conjectured, 
that if they were drawn to a sufficient degree of fine- 
ness, they might be used in the fabrication of stuffs. 
He succeeded in making them as fine as a spider’s web, 
but he was never able to obtain them of a sufficient 
length when their diameter was so much reduced. The 
cireumference of these threads is generally a flat oval, 
about three or four times as broad as it is thick. By 
using opake and t glass of different colours, 
artists have been enabled to produce the most beautiful 
ornaments. p 
, . When glass has been annealed or cooled slowly, it is 
able to resist very considerable force without being 
broken ; but when it has been cooled suddenly, either 
re in the open air, or by immersion in water, 
it exhibits very remarkable rties. These proper- 
ties are shewn in what are called 1 Prince Rupert's drops, 
* and glass cups. 
— The phenomena and the formation of Prince Rupert’s 
and the theory of their explosion, have already 
been explained in our article ANNEALING. The earliest 
GLASS. 
319 
experiments upon glass tears were made in 16.56, both 
in London and Paris; but it is not certain in what 
Physical 
country they were invented. ‘They were first brought p51 Ru. 
to England by Prince Rupert, third son of the Elec por.’s drops, 
tor Palatine 
rederick V. and the Princess Elizabeth, or glass 
daughter of James I. and experiments were made up- tears. 
on them by the Right Hon. Sir Robert Moray, in 
1661, by the command of his Majesty. _An account of 
these experiments is to be found in the Registers of the 
Royal Society, of which he was one of the founders. 
he following experiments have been recently made 
on these drops by Dr Brewster, and published in the 
Phil. Trans. for 1814, Part I], and 1815, page 1. 
«Having ascertained that glass melted ana sudden! Dr Brews-~ 
cooled, 
possessed all the optical properties of crystal- ter’s experi- 
lized bodies, I was anxious to determine if it exhibited ™ents on 
any other marks of a crystalline structure. 
the bulb of an unannealed drop 
tears. 
~ & 
AB, Plate PEATE 
4 CCLXXIV. 
ro | 
CCLXXIV. Fig. 1. by holding it between the eye and j;,, 1, 2, 
a sheet of white paper, I observed a number of lines 
converging to the vertex a, as represented in Fig. 2. 
This structure was more or less apparent in every bulb 
which I examined, but never appeared ‘in annealed 
drops. It exhibited itself even on the surface, and 
seemed to be owing to-an imperfect‘ crystalline form, 
yet it was riot marked with sufficient distinctness toen- 
title me to consider it as the effect of crystallization. 
In one specimen, however, where the bulb AB remain- 
ed unshattered, while all the rest of the drop was burst 
in pieces, the lines diverging from a were most dis- 
tinctly marked, and the bulb was actually cleft in the 
direction of these lines, so.as to produce a real disloca- 
tion at the surface of the drop. We may therefore con- 
sider the drop as possessing that crystalline structure 
which gives cleavages in the direction of lines diver- 
ging from its apex. By examining the fragments of 
the drop after it is burst, another cleavage is distinctly 
perceptible: it is parallel to the outer surface, and pro~ 
duces a concentric structure like that of an onion. A 
third cleavage is visible in the direction of lines inclined 
to the axis of the drop, as represented in Fig. 3; but 
it is not so distinct as the two first. These cleavages are 
represented in section in Fig. 4. 
As it appeared probable that the glass drops pos- 
Figs. %. 43. 
sessed a less degree of density than if they had been _ 
annealed, I attempted to ascertain this point by mea- 
suring their specific gravities in these two different 
states. The unannealed drops, however, had always 
one or more vacuities, such as E, F, Fig. 1. so that I 
was able to obtain only approximate results by estima~ 
ting the magnitude of these cavities. 
The following specific gravities were measured bf. 
my friend Mr Jardine, with his usual correctness. 
Unannealed flint glass drop, Fig. 1. . . . 3.20405 
Annealed flint glass from the same pot . 3.2763 . 
In order to correct the first of these measures, TI 
moulded a piece of bees’ wax into the size and form of. 
the cavities E, F, Fig. 1. by examining them under a 
fluid of the same refractive power as the glass. I then 
formed the two pieces of wax into a sphere, and thus 
ascertained, with tolerable accuracy, the weight of a 
quantity of water of the same magnitude as the cavi- 
ties. By this means, I’ obtained the corrected specific 
gravity of the unannealed drop 3,264. 
With the view of obtaining some farther insight into- 
the structure of the crystallized: drop, I brought the 
one, represented in Fig. 1, nearly to a red heat. Its 
shape suffered no change at this temperature, and the 
vacuities E, F, still remained ; but it had now lost the - 
faculty of depolarisation, and the particles had therefore: 
