Lond.

"Take of ferrocyanide of potassium, two ounces; sulphuric acid, an ounce and a half; distilled water, a pint and a half. Mix the acid with four fluid ounces of the water, and having put them into a glass retort, add the ferrocyanide of potassium dissolved in half a pint of the water. Pour f viij. of the water into a glass receiver, then having fitted to it the retort, distil f vj. of the acid, with the gentle heat of a sand bath, into this water. Lastly add f vj. of distilled water to this acid, or as much as suffices, that 12.7 grains of nitrate of silver dissolved in distilled water may be accurately saturated by 100 grains of this acid."

Diluted hydrocyanic acid may also be prepared when it is more immediately wanted, from forty-eight grains and a half of cyanide of silver, added to a fluid ounce of distilled water, mixed with thirty-nine grains and a half of hydrochloric acid. Shake all these in a well stopped phial, and after a short interval pour off the clear fluid into another vessel. Keep this for use, the access of light being prevented.