Messrs. Cadet and Brisson made a number of experi ments on the refractive power of different fluids, by in closing them in the lens of m. Trudaine, and observing the variations in its focal length. The object of their experiments was to find a fluid that possessed a greater refractive power than spirits of wine, and was at the same time sufficiently cheap and transparent to be used between the glass segments. Liquid turpentine was the most refractive fluid that they employed ; but as they found that its dispersive power was to that of crown-glass as 34 to 28, ti,is fluid was obviously, on this account, unfit for the purpose.* The fluid which they preferred was a saturated solution of sal ammoniac or distilled water.
The most powerful burning-glass that has yet been constructed, was made by Mr Parker of Fleet•street. After a great number of experiments, and an expense of above 7001., this able artist succeeded in completing a burning lens of flint glass three feet in diameter. This powerful instrument is represented in Plate CVI. Fig. I. The large lens, which is placed in the ring at A, is dom bly convex, and when fixed in its frame, it exposes a sur face of 2 feet 8 inches. It is 31 inches 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 that were refracted by this fens, were received (according to the method of Tschirn hausen) upon a second lens B, whose diameter is 16 inches out of the frame, and 13 inches in the frame ; its central thickness is I; of an inch. The length of its focus is 29 inches, the diameter of the focal image 4 of an inch, and the weight of the lens 21 pounds. The combined focal length of these lenses is 5 feet 3 inches, and the diameter of the focus half an inch. These lenses are placed at the extremities of a truncated conical frame, consisting of twelve ribs of wood. Near the smaller end B is fixed a rack D, which passes through the pillar L, and is moveable by means of a pinion within the pillar, driven by the handle E. A bar of wood F, fixed at G between the two lower ribs of the cone, car ries an apparatus H, which turns on an universal joint at K, and also moves to or from F in a chased mortice. This apparatus, which carries the iron plate I for hold ing the substances to be examined, may thus be placed exactly in the focus of the lens B. The conical framing
is supported by pivots upon a strong iron bow AC, which rests upon a mahogany frame LL, with three feet MAIM furnished with castors. Friction wheels are placed under the table N, to facilitate the horizontal motion.
The following experiments with this lens were made in thcpresence of major Gardner, and of several mem bers of the Royal Society.
Substances fused, with their Weight in Time in weight and time of fusion. grams, secnnds.
Common slate, 10 2 Scoria of wrought iron, . . 12 Gold, pure, 20 3 Platina, do. 10 Nickel, • 16 3 Cast iron, a cube, 10 3 Silver, pure, 20 4 Crystal pebble, 7 6 Terra ponderosa, or barytes, 10 7 Lava, 10 Asbestos, 10 10 Steel, a cube, 10 12 Bar iron, do 10 12 • Garnet, 10 17 Copper, pure, 13 20 Onyx, 10 20 Zeolites, 10 23 Substances fused, with their Weight in Time in weight and time of fusion. grains. seconds.
Pumice stone, 10 24 An oriental emerald, . . . 2 25 Jasper, 10 25 White agate, 10 30 Flint, oriental, 10 30 A topaz or chrysolite, . . . 3 45 Common lime-stone, . . . 10 55 Volcanic clay, 10 60 Cornish moor stone, . . . . 10 60 White rhomboidal spar, . . 10 60 Rough cornelian, 10 75 Rotten stone, 10 80 A diamond of 10 grains, when exposed to the lens for 30 minutes, was reduced to 6 grains. It opened, foliated, and emitted whitish fumes, and when again closed, it bore a polish, and kept its form.
Gold retained its metallic state, though exposed for many hours.
The specimens of plating were in different states of approach to a metallic form.
Copper did not lose any of its weight after an exposure of three minutes.
Iron steel shear melted first at the part in contact with the charcoal, while the other part exposed to the focus was unfused.
Iron scoria melted in much less time than the turnings of iron.
Cale of iron from vitriolic acid, precipitated by mild fixed alkali, weighed 5 grains before exposure, and 51 after it.
The remains of regulus of zinc, after it had melted and was nearly evaporated, were magnetic.
Regulus of cobalt was completely evaporated in 57".
Regulus of Bismuth, exposed in charcoal, was nearly evaporated :—in black lead it began to melt in 2", and was soon after completely fused : Iron, on exposure of 180", lost only half a grain ; when placed on bone ash, it fused in S".