BURNING INSTRUMENTS. 



in 



menu with 

 Crudaiue's 



tigfiy, an honorary member of t In- K oyal Academy of 

 '.ces. This gentleman, whose liberality and zeal 

 deserve to be recorded, engaged to be- at the ex pence 

 of a large burning glass, formed under tin- direction 

 of several commissioners named by the Academy. 

 This lens consisted of two spherical segments 8 

 radius and 8 lines thick. The lenticular cavity was 4 

 feet in diameter, and 6 inches and ft lines thick at the 

 centre, and was filled with pirit of wine, of which it 

 held no less than 1 M) pints. The focal length of a 

 zone at the cirumfcreuce, about () or 7 lines broad, 

 was 10 fcetO inches and 6 lines, the focal length of a 

 portion at the centre, about ( inches in diameter, was 

 10 feet 7 inches and 5 lines, and the diameter of the 

 focus was 14| lines. When the whole surface was cover- 

 ed, except a zone at the circumference of C or 7 lines, 

 the following were the foci of the different rays : 



Fetft. Inch. Lines. 



Violet 9 6 44 from the centre of the lens. 

 Blue 9 

 Yellow 10 

 Orange 10 

 Red 10 



The 'following experiments were made in Octo- 

 ber 1774, in the Jardin del' Infante, by M. M. Tru- 

 daine, Macquer, Cadet, Lavoisier, and Brisson, the 

 commissioners appointed by the Academy. 



1. The burning power of the anterior half of the 

 lens was much greater than that of the exterior half. 



2. On the 5th of October after mid-day, the sky- 

 not being very clear, two farthings, placed upon 

 charcoal, were completely melted in half a minute. 



3. In order to melt forged iron, it was found ne- 

 cessary to concentrate the rays by a second lens 8- 

 inches diameter, 22 inches 8 lines in focal length, and 

 placed at 8 feet 7 inches from the centre of the great 

 lens. At this place, the cone of rays was 8 inches in 

 diameter, and the burning focus, now reduced to 8 

 lines in diameter, was 1 foot from the small lens. 



4. In the focus of the small lens, upon a piece of 

 hollow charcoal, small pieces of forged iron were 

 placed, which were instantly melted. After fusion, 

 the metal bubbled up, and fumed like nitre in fusion, 

 and then sent off a great number of sparks. This 

 effect (which was observed during the experiments 

 with Tschirnhausen's lens,) always took place after 

 the fusion ef iron, forged iron, or steel. 



5. In order to try the effect upon greater masses, 

 pieces of forged iron, and the end of a nail, were ex- 

 posed to the focus, and were melted in 15 seconds. 

 A piece of nail, 5 lines long and ^ of a line square, 

 which was added to the rest, was instantly fused* 

 and the same was the case with a screw that had a 

 round head, and was 8 lines in length. 



6. Some days afterwards, a bar of steel, 4 inches 



Burning 

 Inttru- 



menu 



long, and 4 lines square, was exposed, o as to receive 

 the focal image upon the middle ol ih, This 



part was completely melted in 5 mimitet, after hav- 

 ing begun to run at the end of the second minute. 



i'latina, in grains, appeared to draw together, to 

 diminish in bulk, and to prepare- for fusion. A little af- 

 ter, it bubbled up and smoked. All the grains were 

 united into one mass, without however forming a sphe- 

 rical button like other melted metal*. After the 

 platina had undergone this B/.-mifu iion, it was not at- 

 tracted by the magnet as it wa b*forc the operation. 



8. A portion of platina, deprived of the iron which 

 it contained, and therefore not affected by the mag- 

 net, lost a part of its bulk, smoked, and formed one 

 mass, which was extended under the hammer.* 



[). Several experiments were made, in order to find 

 the lens that was most proper for collecting the ray 

 after refraction by the large lens. A spirit of wine 

 lens 2 feet in diameter and 4 feet focus, a solid lent 

 18 inches diameter and 3 feet focus, and another 13 

 inches in diameter, were successively tried, but none' 

 of them produced such a powerful effect a the lens 

 8 inches in diameter, and 22 inches and 8 lines 

 focus, though it was full of vesicles and striae. 



Messrs Cadet and Brisson made a number of expe- 

 riments on the refractive power of different fluids, by 

 inclosing them in the lens of M. Trudaine, and ob- 

 serving the variations in its focal length. The ob- 

 ject of their experiments was- to find a fluid that pos- 

 sessed a greater refractive power than spirits of wine, 

 and was at the same time sufficiently cheap and trans- 

 parent to be used between the glass segments. Li- 

 quid turpentine was the most refractive fluid that 

 they employed ; but as they found that its dispersive 

 power was to that of crown-glass as 84 to 28, this 

 fluid was obviously, on this account, unfit for the pur- 

 pose, f The fluid which they preferred was a satu- 

 rated 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, burning 

 After a great number of experiments, and an expence len - 

 of above L. 700, this able artist succeeded in com- 

 pleting a burning lens of -flint-glass three feet in dia- 

 meter. This powerful instrument is represented in 

 Plate CVI. Fig. 1. The large lens, which is placed 

 in the ring at A, is doubly convex, and when fixed 

 in its frame, it exposes a surface of 2 feet 8.J- inches. 

 It is 3} inches thick at the centre ; its focal distance 

 is 6 feet 8 inches ; the diameter of the burning foetid 

 one inch; and the weight of the lens 212 pounds. 

 The rays that were refracted by this lens, were re- 

 ceived (according to the method of Tschirnhausen) 

 upon a second lens B, whose diameter is 16 inches 

 out of the frame, and 13 inches in the frame ; its cen- 

 tral thickness is !* of an inch. The length of its 

 focus is 29 inches, the diameter of the focal image 



PiA-rr 

 CVI. 

 F,. I. 



" Messrs Macquer and Beaume are said to have melted small grains of platina by a concave glass, 22 inches in diameter, 

 and 28 inches focus. 



f Cadet and Brisson, in the course of their ex|>crimcnts, were led to the discovery of achromatic fluid object -glasses, a dis- 

 covery which has hitherto been referred to a much later date. This discovery is most distinctly contained in the following pas- 

 sage. " Comme la tcrebinthc, cause un dispersion de rayons assez dirterente decelle quo cause k- verre, comine nous, nous en 

 sommes assures par 1'experiencc, ne pourroit on pas faire des objectifs dans lesqucls, pour les rcndre uchromatiques, on feroi*. 

 usage dc cette resine, a la place du flint-glass, matiere si difficile a se procurer d'nnv densite uni forme, et sans defauts, surtrwit 

 en grands morceaux, mais le dsvdoppcment de cette Idee nous cacueroit trop loin, ct ne fait pas partie de notre sujet actuel '' 

 Alt*. Acad. Par. 1777, p. 551. 



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