12 
MR. FARADAY ON THE 
17. The good crystals are to be washed in three or four waters, to remove the 
last portion of deposit and adhering soluble impurities : but to prevent excessive 
solution of the nitrate, the same portions of water may be used for several ba- 
sins 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 preserved in glass bottles. By this pro- 
cess 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 con- 
sidered as equivalent to one proportional or 112 parts of protoxide of lead. 
1 8. 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 metallic 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 sulphuric 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, and continuing to wash with the latter fluid 
until all was dissolved, or an insoluble part left. If the latter circumstance 
occurred, the insoluble substance was examined for sulphate of soda, which if 
in any sensible quantity occasioned the condemnation of the boracic acid. The 
care respecting alkali in boracic acid was taken in consequence of observing 
certain bad effects produced in glasses which appeared referrible to its 
presence. 
10. 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. 
