228 
Royal Irish Academy, 
cooling of the glass, being deposited in globules throughout 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 diy 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 
