668 PUFF PASTE 



Its relative richness in the real ferroprussiate of iron may be estimated by the 

 quantity of potash or soda -which a given quantity of it requires to destroy its blue 

 colour. 



Sulphuretted hydrogen passed through Prussian blue diffused in water whitens it; 

 while prussic acid is eliminated, sulphur is thrown down, and the sesquicyanide of 

 iron is converted into the single cyanide. Iron and tin operate in the same way. 

 When Prussian blue is made with two atoms of ferrocyanide of potassium instead of 

 one, it becomes soluble in -water. Soluble Prussian blue is generally prepared by 

 adding perchloride of iron to excess of ferrocyanide of potassium. 



For the mode of applying this pigment in dyeing, see CALICO-PRINTING. 



A process for Prussian blue, which deserves notice, as the first in which that 

 interesting compound was made to any extent independently of animal-matter, 

 was introduced by Mr. Lewis Thompson, who received a medal from the Society of 

 Arts, in 1837, for this invention. He observed that in the common way of manufac- 

 turing prussiate of potash, the quantity of nitrogen furnished by a given weight of 

 nnimal-inatter is not large, and seldom exceeds 8 per cent. ; and of this small quantity, 

 at least one half appears to be dissipated during the ignition. It occurred to him that 

 the atmosphere might be economically made to supply the requisite nitrogen, if 

 caused to act in favourable circumstances upon a mixture of carbon and potash. He 

 found the following to answer : Take of pearlash and coke, each 2 parts ; iron turn- 

 ings, 1 part ; grind them together into a coarse powder ; place this in an open 

 crucible, and expose the whole for half an hour to a full red heat in an open fire, with 

 occasional stirring of the mixture. During this process, little jets of purple flame 

 will be observed to rise from the surface of the materials. When these cease, the 

 crucible must be removed and allowed to cool. The mass is to be lixiviated ; the 

 lixivium, which is a solution of ferrocyanide of potassium, with excess of potash, is 

 to be treated in the usual way, and the black matter set aside for a fresh operation, 

 with a fresh dose of pearlash. Mr. Thompson stated that one pound of pearlash, 

 containing 45 per cent, of alkali, yielded 1,355 grains of pure Prussian blue, or 

 ferrocyanide of iron, or about 3 ounces avoirdupois. 



Of Lite years several improvements have been introduced into the manufacture of 

 Prussian blue, relating chiefly to oxidation of the white precipitate thrown down from 

 a solution of green sulphate of iron and alum by means of yellow prussiate of potash. 

 Instead of oxidising this precipitate by exposure to the atmosphere, recourse may be 

 had to the oxidising power of chlorine ; thus nitro-muriatic acid (aqua regia) may bo 

 added to the precipitate. Perchloride of iron or perchloride of manganese may be 

 used with similar effect. 



Prussian blue may also be prepared from the ammoniacal liquor of gas-works, or 

 from the spent lime of gas-purifiers. 



PRUSSIAN BROWN. A fine deep brown colour obtained by adding the yellow 

 prussiate of potash (ferrocyanide of potassium) to a solution of sulphate of copper. 



PRUSSIATE or POTASH. See POTASH, PHTSSIATE OF. 



PRUSSIC ACID. See HYDROCYANIC ACID. 



PSIIiOIVZEIiANz:. An ore of MANGANESE, which see. 



PUDDLING OF IRON. See IRON. 



fUTf FASTI!. A preparation of flour and butter, which is in great demand, 

 not only at the pastry-cooks', but in almost every private family. Take a certain 

 quantity of flour, say half a pound, put it upon a wooden board, make a hole or de- 

 pression in the centre, and mix it with somewhat less than half a pint of cold water, 

 BO as to make a softish paste ; dry it off from the board by shaking a little flour over 

 and under, as is well known, but do not 'work it' more than you can help. Take 

 now a quarter of a pound of fresh butter, which should be as hard as possible (and 

 therefore it should be kept iu as cold a place as practicable, the ice closet, if pro- 

 curable, being the best place), and squeeze out all the water, or butter-milk which it 

 contains, by kneading it with one hand on the board. This operation is called in 

 French ' manier le beurre.' Eoll now the paste prepared as above into a flat, thick, 

 square slab, extending about 6 or 7 inches; lay the pat of butter, treated as above, in 

 the middle of the slab of paste, and so wrap the butter up into it by folding the sides 

 of the paste all round over it ; roll the whole mass gently with the rolling-pin, so as to 

 form a thick sheet, put it upon a tin plate-, or tray, cover it with a linen cloth wetted 

 with water as cold as possible, and leave the whole at rest for about a quarter of an 

 hour in a cold place. At the end of that time, roll the mass with the rolling-pin into 

 a sheet about 15 or 16 inches long, and fold it into three, one over the other; roll it 

 out again into a sheet as before, and again fold it into three, one over the other, as 

 before, and repeat this operation once more, making three times in alL Put the 

 square mass, with a wet cloth upon it, in a cold place for another quarter of an hour, 

 as before, and at the end of that time roll it out with the rolling-pin, and fold it into 



