35 



by a method which forms the subject of the second section of this 

 paper. 



When a plate of glass is brought to a full red heat, and is then 

 cooled by placing its edge on a bar of cold iron, the same fringes of 

 colour are developed during cooling as by placing cold glass upon 

 hot iron ; and in this case the glass retains the property given to it 

 even after it is completely cold. 



The author delineates various configurations of colours produced 

 by plates of different forms thus cooled. Among many others, a 

 parallelogram of glass exhibits an inscribed parallelogram, with lines 

 from each angle to the angles of the plate ; and when the plate has 

 been divided longitudinally by a diamond, each of the portions again 

 exhibits an inscribed parallelogram, just as if the parts had been se- 

 parately heated ; and in this respect they present a property analo- 

 gous to that of a divided magnet, each part of which has opposite 

 poles as the entire bar. 



A circular plate of glass, cooled with its centre resting on a piece 

 of cold iron, or a cylindrical rod of glass cooled in the open air, when 

 examined by polarized light in the direction of its axis, each present 

 the same appearance of a black cross through their centre, and con- 

 centric fringes parallel to their circumference. 



Since it is obvious that in these cases of rapid cooling, as well as 

 those of rapidly heating, there must be progressive variations of den- 

 sity of the glass proceeding in a direction from the source of heat or 

 of cold, and since the phenomena exhibited by many crystallized bo- 

 dies, when examined in the direction of their axis, are precisely similar, 

 Dr. Brewster infers that there exists in these crystallized bodies also 

 a corresponding variation of density, proceeding toward their axes, 

 which will afford an easy explanation of the fringes they exhibit. 



Farther Experiments on the Combustion of explosive Mixtures confined 

 by Wire-gauze, with some Observations on Flame. By Sir Humphry 

 Davy, LL.D. F.R.S. V.P.R.I. Read January 25, 1816. [Phil. 

 Trans. 1816,^. 115.] 



In these experiments, the author examines what magnitude of wire 

 and of apertures in the metallic gauze of his lamp is consistent with 

 security against explosion of mixtures externally. 



When the gauze is made of wire one fiftieth of an inch in diameter, 

 and at intervals of one tenth, so as to make 100 apertures in the square 

 inch, explosion may take place, either from intense ignition of the 

 top of the lamp, or from lateral currents of air forcing the flame 

 through the interstices. 



When the intervals of the same wire were only one fourteenth, 

 though the danger from lateral motion was obviated, still ignition of 

 the wire caused explosion. With intervals of one sixteenth, still 

 there was danger from the same source ; but when the distances 

 were reduced to one twenty-fourth on 576 apertures in the square 



n 2 



