228 Royal IrisJi Academy. 



cooling of the glass, being deposited in globules througbqut its 

 interstices, (at least the appearance presented by the glass seems to 

 favour such an opinion.) 



" The colours produced by the fusion of metals with glass, being 

 different in many cases from those obtained when their oxides were 

 employed, and presenting the dull untransparent appearance which 

 is so remarkable in ancient glass, led me to suppose that the ancients 

 did not employ any colouring matter unknown at the present day, 

 but that, being unacquainted with the mineral acids, they employed 

 the metals either in the metallic state, in filings, or else in an im- 

 perfect state of oxidation. To determine the probability of this 

 conjecture, I selected three specimens of mosaic glass, analysed by 

 Klaproth ; and substituting for the oxides, in the same relative pro- 

 portion, the metals in a minute state of division, I obtained coloured 

 glasses of nearly the same colour as the mosaics, while the colours 

 produced when the oxides were employed were not only perfectly 

 different, but the glasses were clear and transparent. 



" One of a lively copper red, opake and very bright, contained, 

 in 200 grains, silica 142, oxide lead 28, copper 15, iron 2, alumina 

 5, lime 3. 



" Another, of a light verdigris green, contained, in 200 grains, 

 silica 130, oxide copper 20, lead 15, iron 7, lime 13, alumina 11. 



"A specimen of blue glass contained, in 200 grains, silica 163, 

 oxide iron 19, oxide copper 1, alumina 3, lime ^." 



December 9, 1839. — Mr, Clarke read a supplement to his paper 

 " on Atmospheric Electricity." 



The author gave in this supplement a more detailed description 

 than he had before done of the mode of insulating the apparatus 

 for experiments on atmospheric electricity, which he had used in 

 the course of his recent researches. 



He then described an experiment by which he had shown the 

 absence of decomposing agency in the electricity of serene weather, 

 and stated his opinion of the cause. 



Mr. Clarke next directed attention to the fact, that the curve 

 representing the diurnal variation of the barometric column was 

 the reverse of the electric, thermometric, and hygrometric curves. 

 He considered that such a result was to be expected ; for the baro- 

 metric column should naturally be lower from midday to 3 p. m. 

 than at midnight, in consequence of the greater quantity of aqueous 

 vapour which exists in the atmosphere at the former than at the 

 latter time, — air charged with aqueous vapour being known to be 

 of less specific gravity than dry air. Thus the barometric and 

 hygrometric curves would be the inverse of each other, the maxima 

 of the one corresponding to the minima of the other ; and as the 

 author had previously shown that the hygrometric, thermometric, 

 and electrometric curves were in accordance, the barometric curve 

 would be the inverse of the thermometric and electrometric curves 

 also. The author remarked, that if this character of the horary 



