220 PHILOSOPHICAL TRANSACTIONS. [anNO 1/92. 



ignition of the particles by attrition proceeds from the compression, and conse- 

 quent diminution of the capacity for heat. 



I am not certain that the increase of brightness in the gold plate, exper. g, 

 must be attributed to its transparency: it may arise from the gold being suddenly 

 heated to a white heat by the light of the explosion ; or the force of the explo- 

 sion may condense its parts, and diminish its capacity for heat or light. There 

 is however a strong analogical argument for the transparency of the gold; every 

 body whatever, when extremely thin, is pervious to light in such quantity as to 

 be perceptible to our eye-sight: thus gold, perhaps the most opaque of all bodies, 

 platina excepted, when beaten into leaf gold, is so pervious to the green rays, 

 that, if held close to the eye, all objects are seen through it with considerable 

 distinctness, appearing of a deepish green hue. Now the particles of matter in 

 the gold plate being much separated from each other, if not more regularly ar- 

 ranged by the heat; and the intensity of the light in the explosion of the grains 

 of gunpowder being so great: it is not improbable that some few rays may be 

 transmitted through the gold. 



After some reflection on the curious result of exper. 1, I am inclined to think 

 that the blackened cylinder does not begin to shine at so low a temperature as the 

 polished one; and consequently, that the commencement of ignition is not, in 

 all cases, a certain indication ot a particular temperature. For, when the 2 cy- 

 linders were removed from the ignited crucible (see fig. 6) the blackened one 

 looked of a brighter red than the polished, and yet, in the course of cooling, 

 disappeared in about -«- of the time that the polished one continued to shine, 

 without any apparent reason for its cooling at a faster rate. Should it not there- 

 fore seem that it requires a greater heat to make it shine.'' 



I am well aware, that these appearances may be differently explained; and, to 

 determine this point, I would propose the following experiment. Put larger cy- 

 linders into the tube; and, having made them red hot, drop them separately, 

 each at the instant of its disappearing, into cups of weighed water, of the tem- 

 perature of between 211 and 212° of Fahrenheit: then, as any addition of heat 

 will expand the water into steam, the loss of weight of each vessel will give an 

 exact measure of the heat of the cylinders at the time of immersion. 



XIV. A Narrative of the Earthquake felt in Lincolnshire, and the Neighbouring 

 Counties, Feb. 25, 1792. Bj Edm. Tumor, Esq., F.R.S. p. 283. 

 This narrative consists of short extracts from the letters of several persons 

 in different places. As might be expected, the accounts are various as to 

 duration, direction, &c.; but tiiey agree respecting the circumstances of noise, 

 shaking, heaving, undulation, &c. Mr. Turnor concludes his narrative with 

 the following remarks. 



