In 1802, under Napoleon, a mirror showing four square metres of surface cost 3,644 francs, or very nearly three times the present cost of a mirror, not tinned like the mirrors of 1802, but silvered, of twice and a half that size. While new markets are constantly opening to this great industry all over the world, the progress of chemical science and of mechanics is as constantly suggesting new economies and new improvements in the manufacture of glass, and St.-Gobain, though one of the most thoroughly French of all French 'institutions,' shows no Chauvinism in its incessant study and prompt appropriation of these economies and these improvements. During the invasion of 1814 the workmen of St.-Gobain marched off to Chauny to resist the advance of the Prussians, and the manufactory had to pay a heavy fine for its patriotism. But it avails itself as readily of German as of French science to-day, and I found M. Henrivaux entirely and minutely familiar with the very latest phenomena of the great change which is coming over the glassworks, as well as all the other industries, of Pittsburg, through the use there of natural gas instead of coal gas and coal. All the most recently invented furnaces--English, German, American--have been tried and tested here as soon as they were made; and the latest American 'crushers' and 'regulators' get to St.-Gobain as soon as they do to Pittsburg. The materials which go to the making of a plate-glass mirror pass through seven processes before the original heap of pebbles, dust, and ashes is transformed into a sheet of splendour and light. A hundred years ago more than ten days were required to complete these seven processes, from the crushing and mixing and putting into the furnace of the soda and the silicious sand and the charcoal and the lime and the broken glass, called here _calcin_, through the fusion, and the moulding, and the squaring, and the smoothing, and the washing, and the polishing. Now this is all done in half the time--127 hours instead of 246.