153 
SUN HEAT. 
plied in the focus, immediately burned, and consumed in flame, smoke, 
and ashes. 
A little vessel of water placed in the focus, immediately boiled. 
Thin pieces of metal applied in the focus did not immediately melt, 
Lut, growing gradually hot, at length ran. If they were too thick for 
the power of the focus to peuetrate, they scarcely melted thoroughly. 
Tiles burnt or dried in the sun, also talc and other bodies grew red 
hot in a moment, and soon after turned to glass. 
Sulphur, pitch, and rosin, being covered over with water, melted under it. 
A piece of slender wood being put under water in the focus, in the 
summer time, and kept there awhile, appeared to remain entire when 
viewed on the outside, but upon breaking, the wood was found, on the 
inside, turzed to coal and burnt. 
If the matter intended to be changed were exposed in a black receiver, 
the power of the focus was surprisingly increased. 
If metals or other bodies examined by this fire, were laid on a coal made 
of green wood, and not thoroughly dried, they instantly melted, emitted 
sparks, and flew off. Lead and tin quickly melted, and not only fumed, 
but calcined, vitrified, and were lost. 
The ashes of all vegetables presently vitrified. 
All metals exposed in an unglazed china vessel, vitrified, provided 
the china were gradually heated, to prevent its breaking by too sudden a 
fire, and were likewise thick enough to prevent its fusing. 
If nitre were exposed in a large glass vessel, and the focus carefully 
directed within the cavity of the vessel so as to fall on the matter to be 
changed, but not on the glass, it became wholly volatile in a moment's 
time, and turned to volatile spirit of nitre. 
The utmost light of the full moon collected by this glass, vielded a most 
lucid focus, but no heat at all. 
It moved and agitated almost all bodies, even in vacuo itself, though 
often not without great danger. 
From all which, and many other experiments, it appears 
that this focus, made by M. Tschirnhausen’s lens, is 
weaker than that of M. Villette’s speculum, and yet fitter 
for examining the nature of fire by its effects. 
Mr. Parker’s Lens. 
The most powerful burning-glass that has yet been con 
structed, was made by Mr. Penn, of Islington, for Mr. Parker, 
a glassman, of Fleet Street. After a great number of expe 
riments, the able artist succeeded in completing a burning 
lens of flint-glass, three feet in diameter. 
This powerful instrument is represented in fig. 91. The large lens, 
which is placed in the ring at a, is double convex, and when fixed in 
its frame, it exposes a surface of 2 feet 8 inches 4. It is 3 inches 4 thick 
at the centre, its focal distance is 6 feet 8 inches, the diameter of the 
burning focus one inch, and the weight of the lens, 212 pounds. 
The rays tbat were refracted by this lens were received, according to 
the method of Tschirnhausen, upon a second lens, b, whose diam eter 
is 16 inches out of the frame, and 13 inches in the frame, its central 
thickness is 1 inch 5/8. The length of its focus is 29 inches, the