534 BURNING - G LASS.- • 
An emerald melts into a fubffance like a torquois done; 
a diamond weighing 4 grains .lofes 7-8ths of its weight : 
the albedos vitrifies, as all other bodies will do if kept long 
enough in the focus; but, when once vitrified, the mirror 
can go no farther with them. Phil. Tranf. vol. iv. p. 198. 
T fichirnhaufen’s refleHing mirrors produced equally (ur- 
prifmg efiedts ; as they may be feen defcribed in the A6ta 
Erudit. for 1687, p. 52. And other perfcns have made 
very good ones of wood, ffnuv, paper, ice, and other fub- 
iiances capable of taking a proper form and polilh. Every 
kns, whether convex, plano-convex, or convexo-convex, 
collects the Inn’s rays, difperfed over its convexity, into a 
pomt by refraction ; and it is therefore a but ning-glafs. 
I he molt conliderable.of this kind is that made by Tfchint- 
haufen, and defcribed in the fame Adta Erudit. The dia¬ 
meters ot his lenfes are from 3 to 4 feet, having the focus 
at the difkmce of 1 2 feet, and its diameter an inch and a 
half. To make the focus more vivid, the rays are collect¬ 
ed a fecond time, by a fecond lens parallel to the fir II, and 
placed at fitch a diffance that the diameter of the cone of 
rays formed by the firft lens is equal to the diameter of 
the fecond, fo that it receives them all ; and the focus is 
reduced from an inch and a half to half the quantity, and 
ccnfequently its force is quadrupled. This glafs vitrifies 
tiles, Hates, pumice-ffones, &c. in a moment. It melts 
fulphur, pitch, and all rofins, under water; the allies of 
vegetables, woods, and other matters, are tranfmuted into 
glafs; and every thing applied to its focus is either melted, 
changed into a calx, or into fumes. The author obferves, 
that it fucceeds bell when the matter applied is laid on a 
hard charcoal well burnt. But, though the force of the 
iolar rays be thus found fo furprifing, yet the rays of the 
full moon, collected by the fame burning-glafs, do not 
lhew the lead increafe of heat. 
Sir ifaac Newton prefented a burning-glafs to the Royal 
Society, confiding of 7 concave glades, fo placed that all 
their foci join in one phyiical point. Each glafs. is about 
1 1 a inches diameter ; fix of them are placed contiguous, 
to, and round, the feventh, forming a kind of fpherical 
fegment, whofe fubtenfe is. about 344 inches : the com¬ 
mon focus is about 22^ inches didant, and about an inch 
in diameter. This glafs vitrifies brick or tile in one fe¬ 
cond, and melts gold in 30 feconds. M. BufFon alfo made 
a variety of very powerful burning-glades, both as mir¬ 
rors and as lenfes; but at length concluded with one which 
is probably of the fame nature with that of Archimedes, 
and confided of 400 mirrors reflecting their rays all to one 
point, and with which he could melt lead and tin at the 
didance of 140 feet ; and with others he confirmed fub- 
ftances at the didance of 210 feet. See Phil. Tranf. vol. 
xliv. or Buffon’s Hidoire Naturelle, Suppl. vol. i. or 
Montucla’s Hidoire des Math. vol. i. p. 246. 
It would feem there is no fubdance capable of refiding 
the efficacy of modern burning-glades, though water, &c. 
are not affected by them at all. Thus, Meliks. Macquer 
and Battme have fucceeded in melting fntal'l portions of 
platina by means of a concave glafs, 22 incites diameter, 
. and 28 incites focus; though this metal is not fufibl'e by 
the drongeft fires that can.be excited in furnaces, or fuf-. 
tained by any chemical apparatus. Yet it was long fince 
obferved; by the Academicians del Cimento, that, fpirit 
of wine could not be fired by any burning-glafs which they 
ufed ; and, notwithftanding the great improvements thefe 
indruments have fince received, M. Nollet has not been 
able, by the mod powerful bunting mirrors, to fet fire to 
any inflammable liquors whatever. However, a large 
burning lens, for filling and vitrifying fuch fubdances as 
refid the fires of furnaces, and especially for the applica¬ 
tion of heat in vacuo, and in certain other circumftances 
in which heat cannot be applied by other means, has long 
been a defideratum with perfons concerned in philofpphi- 
pal experiments; and this it appears is now accomplilhed 
by Mr. Parker, an ingenious glafs manufadlurer in Fleet- 
dreet, London. His lens'is made of flint glafs,. and is 3 
icet in/li4meter, but when fixed in its frame expofes a 
furface of 32 inches in the clear ; the length of the focus is 
6 feet 8 inches, and its diameter one inch. The rays from 
this large lens are received and tranfmitted through a 
fmaller, of 13 inches diameter in the clear within the 
frame, its focal length 29 inches, and diameter of its focus 
3-Sths of an inch : fo that this fecond lens increafes the 
power of the former more than 7 times, or as the f qua re- 
of 8 to the fquare of 3. From a great number of experi¬ 
ments made with this lens, the following are feledled to 
ferve as fpecimens of its pow ers : 
•Subftances fuled; with their 
of fufion. 
weight, and time 
Time in 
feconds- 
Weight 
in grs. 
Scoria of wrought iron - 
_ 
2 
I 2 
Common Date - 
- 
_ 
2 
IO 
Silver, pure - 
- 
3 
20 
Piarina-, pure 
- 
- 
3 
IO 
Nickeil - 
3 
l6 
Caff iron, a cube 
_ 
_ 
3 
IO 
Keartli - 
- 
3 
IO 
Gold, pure 
- 
4 
20 
Cryftal pebble 
6 
7 
Cauk, or terra ponderofa 
- 
. 
7 
IO 
Lava - 
- 
7 
IO 
Albeftos . - - 
- 
- 
10 
IO 
Bar iron, a cube 
_ 
I 2 
IO 
Steel, a cube 
- 
- 
I 2 
10 
Garnet - 
- 
17 
IO 
Copper, pure 
- 
• 
20 
33 
On)x - 
- 
20 
IO 
Zeolites - 
- 
- 
23 
IO 
Pumice (lone - 
- 
24 
IQ 
Oriental emerald 
- 
- 
2 5 
2 
Jafper - 
- 
2 S 
10 
White agate 
_ 
- 
30 
10 
Flint, oriental 
_ 
3° 
IO 
Topaz, or cluyfolite - 
- 
- 
45 
3 
Common limeftone 
- 
55 
IO 
White rhomboidal fpar 
- 
- 
60 
10 i 
Volcanic clay - 
- 
60 
IO ; 
Corniffi moorffone 
- 
. 
60 
to 
Rough cornelian 
- 
7 5 
10 ; 
Rotten ffone 
- 
- 
BO 1 
10 
In the annexed Plate, fig. 1, is. an elevation of Mr 
Parker’s lens. A is the large lens of the diameter men¬ 
tioned : thicknefs in the centre, 3 inches and i-4th; weight 
212 pounds; length of the focus, 6 feet 8 inches ; diame¬ 
ter of ditto, 1 inch. B, the fmaller lens, whofe diameter 
in the frame is 16 inches, and fftevvsin the clear 13 inches; 
thicknefs in the centre 1 inch 5-S'ths; weight 21 pounds ; 
length of focus, 29 inches; diameter of ditto, 3-8ths of 
an inch. When thefe two lenfes are compounded toge-. 
ther, the length of the focus-is 3 feet 3 inches; diameter 
ot ditto half an inch. C, a truncated cone, compofed of 
twenty-one ribs of wood ; at the larger end is fixed the 
great lens A, at the fmaller extremity the leffer lens B ; 
near the fmaller end is alfo fixed a rack, D, palling thro’ 
the pillar L, moveable by a pinion-fuming in the pillar,, 
by means of the handle E, tints giving a vertical motion 
to the machine. F, a bar of wood, fixed between the two. 
lower ribs of the coneat G; having within a chafed mor¬ 
tice in which it moves an apparatus, H, witii the iron, 
plate, I, fixed thereto ; and this part turning on a ball and 
focket, K, a method is thereby obtained of placing the 
matter under experiment, fo as to be adted upon by the 
focal rays' in the mod diredb and powerful manner. L, a 
mahogany frame, moving on caftors. Immediately under 
the table M, are three fritIion.-wh-ee.ls, by which the ma¬ 
chine moves horizontally. N, a ff rong.iron bow, or frame, 
in which the lens and.the cone hang. 
Fig. 2, is .a fedtion of the fame machine ; where a is 
the great lens marked A in the elevation ; b, is the frame 
which contains the Ie-ns; c, the fmall lens marked B. 
d, The frame which contains the fmall lens, ee, The 
truncated 
