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It seems to follow, from the whole, that bodies of theglassy kind are changeable into porcelain by baking, onlyin so far as their vitrescent state has been procured bymeans of saline substances; that those which hold the leastsalt are the most easily changed, and that the porcelainthey afford is the most perfect ; and that thole, in whichthe proportion of salt is large, are very difficultly, if at all,made to undergo this change. Agreeably to these remarks,M. de Reaumur observes, that the very worst glass makesthe best porcelain ; and suggests, that in order to the per-fection of this kind of ware, it may be necessary for theglass-maker to acquire a habit of blowing vessels frommore refractory forts of glass than those which are com-monly worked. Perhaps the same end might be obtainedmore advantageously on another principle, which the fore-going observations point out, viz. by forming the glassof certain earthy compositions more disposed to vitrefythan those commonly employed ; so that a very little saltshall be sufficient for their vitrification in the furnace ofthe glass-house, and that the glass they afford, instead ofbeing more refractory, shall be even more fusible thanthe common green glass.

The different effects of cementation on different kindsof glass may perhaps afford some light into the cause ofthe change which coarse glass undergoes, and some usefulcharacters and discriminations of different vitreous and se-mivitreous bodies.

SECT. V.

Observations on the cause of the changes which green Glassundergoes by baking.

T H E most obvious way of accounting for this extra-ordinary change is, to suppose the earthy or otherunvitrescible particles of the matters, with which the

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