1829.] Oft the Manufacture of Optical Glass. 243 



luble impurities : but to prevent excessive solution of the nitrate, 

 the same portions of water may be used for several basins of 

 crystals washed at the same time, by making it pass from one 

 to another in succession. Being thus cleansed, they are to be 

 drained, put over the sand-bath, stirred and dried, and finally pre- 

 served in glass bottles. By this process much iron and sulphate 

 of lead are excluded ; and the purified nitrate is found to yield 

 a glass very far superior in colour to that prepared with the 

 ordinary oxides of lead, and to exert not the slightest action 

 on the platina: its use put an end to all the accidents and 

 failures which resulted from the presence of metallic lead in 

 the oxide. 166 parts by weight are to be considered as equi- 

 valent to one proportional or 112 parts of protoxide of lead. 



18. Boracic acid. The boracic acid for these experiments 

 was obtained pure from the manufacturer, but before being 

 used was carefully examined. It was rejected unless it was in 

 white or bluish-white crystals, clean and entirely soluble in 

 water. Its solution was tested for iron by the ferro-prussiate 

 of potash and a drop of sulphuric acid, and also for other me- 

 tallic impurities by a little solution of sulphuretted hydrogen. 

 An ounce or two were heated and dissolved in a little water ; 

 and when cold, the soluble part separated and examined for 

 sulphuric acid, by a few drops of nitrate of baryta and a little 

 nitric acid. It was also examined for soda by dissolving three 

 or four ounces in hot water, adding ten or fifteen drops of sul- 

 phuric acid, and allowing the whole to cool and crystallize ; 

 expressing the mother-water from the crystals ; concentrating 

 it ; again crystallizing, and then acting upon the mother-liquor, 

 obtained at the second time, by strong alcohol ; continuing 

 to wash with the latter fluid until all was dissolved or an inso- 

 luble part left. If the latter circumstance occurred, the inso- 

 luble substance was examined for sulphate of soda, which, if 

 in any sensible quantity, occasioned the condemnation of the 

 boracic acid. This care respecting alkali in boracic acid was 

 taken in consequence of observing certain bad effects produced 

 in glasses, which appeared referable to its presence. 



19. When the boracic acid was acknowledged as pure, 36 

 parts by weight of the crystals were considered as equivalent 

 to 24- parts or one proportional of the dry substance. 



20. Silica. This material is in its most convenient state 



R2 



