[ <M4 ]

tried the weaker one) were pure cubic nitre, which defla-grated on a burning coal without crackling, ahdhad notthe least mixture of common salt. Some have reported,that though a pretty strong spirit of nitre was used in morethan double the weight of the fait, the residuum after thedistillation consisted chiefly of marine salt unchanged, withonly a small proportion of cubic nitre intermixed. Onwhat cause the failure depended, the few experiments Ihave made on this head do not enable me to judge: per-haps it may be necessary that the nitrous spirit should bevery strong, for a concentrated acid may produce decom-positions, as well as dissolutions, which the same acid, di-luted, is incapable of effecting.

II. Cubic nitre may be obtained also in the process ofmaking silver into luna cornea, which is the most effectualway of 'purifying silver. Solution of common salt inwater being dropt by degrees into a solution of silvermade in aquafortis, so long as any cloudiness ensues, themarine acid precipitates with the silver, as the vitriolic didwith chalk in No.. 2 of the foregoing article, and the re-maining liquor is a solution of cubic nitre, blended withthe copper which the silver contained. How far thiscopper may be injurious in the intention for which cubicnitre is here wanted, has not been fully examined.

III. The strong affinity of the vitriolic acid to calca-reous earth affords a method of obtaining cubic nitre,more eligible than either of the foregoing.. Spirit of saltis commonly prepared by distillation with the vitriolicacid; and in this case, what remains in the retort is acombination of that acid with the al call of the marine salt.This compound is common in the shops, under the nameof Glaubers salt or sal mirabile. If a saturated'solution ofsal mirabile be made in water, and a solution of chalk inaquafortis added by degrees so long as it 1 occasions ahy

cloudiness*