BURNING GLASS. 
(juence of the impediments that the glass 
of which it was made must occasion, as 
well as the four reflections and two more 
by way of diminution ; but we will endea- 
vour to appreciate it after making a full 
allowance for these deductions, which must 
necessarily result from every means of con- 
centrating the solar rays, and which must 
be considered to be as the friction of an 
engine, of which nature they really par- 
take. The solar rays received on a circular 
surface of 2 feet 8t inches, when concen- 
trated within the diameter of an inch will 
be 1056.25 times its intensity, or this num- 
ber of times greater than the heat of the 
sun as felt on the surface of the earth. We 
will suppose that as the heat of the air, in 
ordinary summer weather, is 65°, and in 
sultry weather is 75°, the average of which 
is 7 0°, and that we take this degree as the 
average effect, the accumulated power of 
the lens, on the supposition of an equal 
effect over the whole surface of the focus, 
will be equal to 73938°. 
It must be recollected by those who have 
had an opportunity of examining the effects 
of this lens, that tlie external part of the 
focal light was less intense than that part 
which was near the centre of it ; or rather, 
that the effect was very much accumulated 
in the centre ; but as it is possible that the 
refraction of the light and of the caloric 
fluid may not take place in the same angles, 
we think it safest to consider it as of an 
uniform effect, and after deducting one- 
fourth part thereof as a compensation, there 
remains 55454°, as the expression of its 
power, As the application of the second 
lens reduced the diameter of the focus to 
half an inch, the effect, without allowing 
for the reduction of its power, would be 
equal to 221816°, but deducting one-fourth 
for the second transmission, there remains 
166362°, as the expression of its power. 
Mr. Parker farther informs us that a--' 
diamond, weighing 10 grains, exposed to 
this lens for 30 minutes, was reduced to 6 
grains; during which operation it opened, 
and foliated like the leaves of a flower, and 
emitted whitish fumes, and when closed 
again it bore a polish and retained its form. 
Gold remained in its metallic state without 
apparent diminution, notwithstanding an 
exposure at intervals of many hours ; but 
what is remarkable, the rest, or cupel, 
which was composed of bone-ash, was tinc- 
tured with a beautiful pink colour, 
The experiments on platina evince that 
the specimens were in different states of ap- 
proach to a complete metallic form ; seve- 
ral of them threw off their parts in sparks, 
which in most instances were metallic. 
Copper, after three minutes exposure, was 
not found to have lost in weight. 
What is remarkable with regard to ex- 
periments on iron, is, that the lower part, 
i. e. that part in contact with the charcoal, 
was first melted, when that part which was 
exposed to the focus remained unfused : 
an evidence of the effect of flux on this 
metal. 
Several of the semi-crystalline substances, 
exposed to the focal heat, exhibited symp- 
toms of fusion : such as the agate, oriental 
flint, cornelian, and jasper; but as tlie pro- 
bability is that these substances were not 
capable of complete vitrification, it is 
enough that they were rendered externally 
of a glassy form. Garnet completely fused 
on black-lead in 120", lost -’th of a grain, 
became darker in colour, and was attracted 
by the magnet. Ten cut garnets taken 
from a bracelet began to run the one into 
the other in a few seconds, and at last 
formed into one globular garnet. The clay 
used by Mr. Wedgw^ood to make his pyro- 
metric test run in a few seconds into a 
white enamel. Seven other kinds of clay 
sent by Mr. Wedgwood were all vitrified. 
Several experiments were made on lime- 
stone, some of which were vitrified, but all 
of which were agglutinated ; it is, however, 
suspected that some extraneous substance 
must have been intermixed. A globule 
produced from one of the specimens,- on 
being put into the mouth, flew into a thou- 
sand pieces, occasioned, it is presumed, by 
the moisture. 
Some experiments were made in the year 
1802, with Mr. Parker’s lens, with the 
view of ascertaining whether the moon com- 
municated any heat to the earth, in com. 
mon with the reflected light from which 
we derive so much advantage. This expe- 
riment w'as attended by Sir Joseph Banks, 
with several members of the Royal Society, 
together with Dr. Crawford, who provided 
the most sensible thermometers ; but after 
applying them to the luminous focus, so 
far from a perceptible increase of heat, it 
was thought there was perceived rather a 
diminution thereof ; but this suspicion did 
not lead them to a fair investigation of the 
fact. Since this period some experiments 
have been made, that evince the power of 
communicating cold by reflection; but as 
this fact has not yet been explained con- 
sistently with the present received Jheory, 
