The above-mentioned Mr. John Houghton, who every week, in the commercial paper edited by him, published an article on some technical or scientific subject, in the spring of 1696 devoted a series of these ‘leaders’ to the subject of glass. After some general reflections on the substance, when we are told, among other things, that ‘Vitrification is the last mutation of bodies of which Nature is capable and from which there is no going back,’ in his issue of May 2 he takes up the main subject. ‘According to my information,’ he tells us, ‘we are of late greatly improved in the art of _Glass-making_. For I remember the time when the Duke of _Buckingham_ first encouraged _glass-plates_, and _Mr. Ravenscroft_ first made _Flint-glass_.[250] Since then we have mended our Window-glass and outdo all abroad. And what e’er may be said against _Stock-Jobbery_, yet it has been the Means to raise great Summs of Money to improve this Art.’ Again, on May 16 we are given a carefully classified list of ninety glass-houses existing in England. Of these, twenty-four were in London, nine at Bristol, seventeen at Stourbridge, and eleven at Newcastle. These glass-houses he divides into those for looking-glass plates, for bottles and for ‘_Flint, Green, and Ordinary_.’ Now the rational inference from all this seems to me to be that Houghton, _who was in a position to know_, knew nothing about lead-glass. The flint-glass houses are classed together with the ‘green’ and ‘ordinary,’ and flint-glass for him was glass made from flints. So, as we have seen, Haudicquer de Blancourt, writing in France a few years earlier, knew nothing of lead-glass other than that used for objects of _verroterie_. It is at least evident that if our own glass-makers had mastered the art before the end of the century, the secret was well kept.[251] But before proceeding further, it may be well to form some definite idea of the composition of lead-glass and of the physical properties that led to its replacing in great measure the soda-lime glass of Venetian type. In the first place, as I have said, it is essential that the alkali in this glass (in the manufacture of hollow ware, at least) should be potash, and it was, perhaps, the fact that the lead was at first used along with soda that so long delayed the production of a ‘metal’ suitable for the manufacture of blown-glass. Again, the potash in the case of lead-glass must be something quite different from the impure material employed for the old green glass; this crude alkali contained, among other bases, a large percentage of lime. Saltpetre appears to have been used in the first place, and then a more carefully lixiviated form of vegetable ashes known as pearl-ash. The amount of lead oxide may vary from 28 to 40 per cent., and the specific gravity of the resultant glass from 2·8 to 3·6.