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1829.] pure materials. The glass when finished and cold was of a deep purple colour: this was immediately referred to the manganese in the flint glass; a supposition proved by repeating the experiment with other flint glass, and then with flint glass of our own manufacture in which no manganese was used: the latter glass gave no purple colour; the former, a colour as deep as that produced by the first flint glass.

23. Thus it appears that this very heavy glass, the silicated borate of lead (and I find it to be the case with other heavy glasses), has the power of developing the colour of mineral substances far beyond what flint glass possesses; just as flint glass surpasses in the same property plate and crown glass. In the ease in question, the manganese, which did not give a sensible tint to the flint glass, produced a strong colour when diluted eight or nine times by the heavy glass, for the proportion of flint glass used was only $10⁄85$ths of the whole. On making a few experiments with iron, I find that the same strong development of colour is produced with it in these heavy glasses; so that the utmost care is necessary to preserve all the materials during their preparation, and the glass in every part of the process, from metallic contamination.

24. The use of flint glass even without manganese was also objectionable, because of the alkali in it, which, as before stated, was found to produce bad effects, and rendered the glass containing it very liable to tarnish.

25. Such are the materials from which the heavy optical glass has been latterly manufactured. When the composition had been determined upon, the proper proportions and quantities of each have been weighed out in a clean balance and vessels. Thus, for the silicated borate of lead glass, consisting of single proportionals of each substance, 24 parts of the silicate were taken, for they contained a proportional of silica equal to 16 parts, and in addition 8 parts of protoxide of lead: the proportional of oxide of lead has been taken as 112 parts; but there being 8 in the silicate, the quantity of nitrate of lead equivalent to 104parts only was required, and this is 154.14 parts: the equivalent of dry boracic acid is 24, which being contained in 42 parts of the crystals, that quantity was the one required. These proportions when heated and submitted to mutual action leave only 152 parts of glass, or thereabout; for