BURNING-GLASS. 



and which must be considered to be as 

 the friction of an engine, of which nature 

 they really partake. The solar rays re- 

 ceived on a circular surface of 2 feet 8^ 

 inches, when concentrated within the di- 

 ameter of an inch, will be 1056.25 times 

 its intensity, or this number of times 

 greater than the heat of the sun, as felt on 

 the surface of the earth. We will suppose 

 that, as the heat oi the air, in ordinary 

 summer weather, is 65, and in sultry 

 weather is 75, the average of which is 

 70, 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 the 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 1 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 reduc- 

 tion 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 

 th'S lens for X) minutes, was reduced to 

 6 grains ; during which operation it open- 

 ed, and foliated like the leaves of aflower, 

 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, not- 

 withstanding an exposure at intervals of 

 many hours : but what is remarkable, the 

 rest, or cupel, which was composed of 

 bone-ash, was tinctured with a beautiful 

 pink colour. 



The experiments on platina evince that 

 th<- specimens were in different states of 

 approach to a complete metallic form ; 

 several 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 un- 

 fused : an evidence of the effect of flux 

 on this metal. 



Several of the semi-crystalline substan- 

 ces, exposed to the tocal heat, exhibited 

 symptoms of fusion: such as the agate, 

 oriental flint, cornelian, and jasper; '>ut 

 as the probability is that these substances 

 were not capable of complete vitrification, 

 it <s enough that they were rendered ex- 

 ternally of a glassy form. Garnei com- 

 ?)evely fused on black-lead, in 12 /' 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 se- 

 conds, and at last formed into one globu* 

 lar garnet. The clay used by Mr. Wedg- 

 wood to make his pyrometric test run in 

 a few seconds into a white enamel. Seven 

 other kinds of clay sent by Mr. Wedg- 

 wood were all vitrified. Several experi- 

 ments 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 pro- 

 duced from one of the specimens, on 

 being put into the mouth, flew into a 

 thousand pieces, occasioned, it is presum- 

 ed, by the moisture. 



Some experiments were made in the 

 year 1802, with Mr. Parker's lens, with 

 the view of ascertaining whetherthe moon 

 communicated any heat to the earth, in 

 common with the reflected light from 

 which we derive so much advantage. 

 This experiment was attended by Sir Jo- 

 seph Banks, with several members of the 

 Royal Society, together with Dr. Craw- 

 ford, 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 di- 

 minution thereof; but this suspicion did 

 not lead them to a fair investigation of 

 the fact. Since this period some experi- 

 ments have been made, that evince the 

 power of communicating cold by reflec- 

 tion ; but as this fact has not yet been ex- 

 plained consistently with the present re- 

 ceived theory, we shall content ourselves 

 withtakingnotice of the experiment made 

 by M. Pictet. Two concave mirrors being 

 placed at the distance of 10 feet from 

 each other, a very delicate air thermome- 

 ter was put into one of the foci, and a 

 glass matrass full of snow in the other. 

 The thermometer sunk several degrees. 



