598 



POTASH, PRUSSIATE OP 



dent, as far as any evolution of gas is concerned ; but if, previous to the addition of the 

 acid, the nitre be strongly fused, it will be found, as soon as the admixture takes 

 place, that red fumes are evolved. This arises from the fact, that nitrate of potash, 

 when subjected to strong ignition, is decomposed with evolution of oxygen, the nitrate 

 becoming gradually converted into the nitrite of potash, thus : 



KO.NO 5 = ISO NO 3 + 20. (2XX70' - 2KNO + O J ). 



This reaction acquires great interest from the circumstance, that to its correct 

 explanation was owing the commencement of the fame of the illustrious Swedish 

 chemist Scheele. A pharmaceutist, at Upsala, having heated some saltpetre to redness 

 in a crucible, happened, when it became cold, to pour vinegar over it, when, to his 

 surprise, red fumes were evolved. Gahn was applied to for an explanation ; but, 

 unable to comprehend the matter, he applied to Bergmann ; but even he was as much in 

 the dark as Gahn. The explanation which these eminent chemists were unable to 

 give was supplied by the pharmaceutist's apprentice, the young Scheele. Bergmann, 

 when informed by Gahn of Scheele's explanation, felt a strong desire to make his 

 acquaintance, and ultimately they were introduced to each other. 



Nitrite of potash has acquired some importance of late years, owing to the valuable 

 properties, as a decomposing agent, which have been found by chemists to reside in 

 nitrous acid. 



Preparation. Nitrate of potash is to be fused at a red heat for a considerable time. 

 When cold, the contents of the crucible are to be dissolved out with boiling water, 

 and the nitrate of potash remaining is to be removed as far as possible by crystallisa- 

 tion. The nitrite of potash may be obtained from the mother-liquor by evaporation 

 and subsequent crystallisation. It is a neutral salt which deliquesces on exposure to 

 the air. If a piece of strongly-fused nitre be put, when cold, into a solution of sul- 

 phate of copper, a very beautiful apple-green colour is produced, of a tint which is 

 seldom observed, except in solutions containing the nitrite of that metal. 



POTASH, PRUSSIATE OP. Fcrrocyanide of potassium or Yellow prussiate of 

 potash. K'FeCy' + 3HO (X'FeCy 6 + 3H 2 O). 



This salt occurs in a state of great purity in commerce, and is thus manufactured 

 on the large scale : 



Among the animal substances used for the preparation of this lixivium, blood 

 deserves the preference, where it can be had cheap enough. It must be evaporated 

 to perfect dryness, reduced to powder, and sifted. Hoofs, parings of horns, hides, old 

 woollen rags, and other animal offals, are, however, generally had recourse to, as 

 condensing most azotised matter in the smallest bulk. Dried funguses have been 

 also prescribed. These animal-matters may either be first carbonised in cast-iron 

 cylinders, and the residual charcoal may then be taken for making the ferroprussiate ; 

 or the dry animal-matters may be directly employed. The latter process is apt to be 

 exceedingly offensive to the workmen and neighbourhood, from the nauseous vapours 

 that are exhaled in it. Eight pounds of horn (hoofs), or ten pounds of dry blood, 

 afford upon an average one pound of charcoal. This must be mixed well with good 

 pearlash, (freed previously from most of the sulphate of potash, with which it is always 

 contaminated,) either in the dry way, or by soaking the bruised charcoal with a 

 strong solution of the alkali : the proportion being one part of carbonate of potash 

 to from 1 to 2 parts of charcoal, or to about 8 parts of hard animal-matter. 



The pot for calcining the mixture of animal and alkaline matter is egg-shaped, as 

 represented at a, fig. 1636, and is considerably narrowed at the-neck c, to facilitate 



the closing of the mouth with a lid t. It 



1636 ^fiSflBl^ta!^ is made of cast iron, about two inches thick 



in the belly and bottom ; this strength being 

 requisite because the chemical action of 

 the materials wears the metals away fast. 

 It should be built into the furnace in a 

 direction sloping downwards, (more than 

 is shown in the figure,) and have a strong 

 knob b, projecting from its bottom to sup- 

 port it upon the back wall, while its 

 shoulder is embraced at the arms c, c, by 

 the brickwork in front. The interior of 

 the furnace is so formed as to leave but 

 a space of a few inches round the pot. in 

 order to make the flame play closely over 

 its whole surface. The fire-door/, and the 

 draught-hole, z, of the ash-pit, arc placed iu tho posterior part of the furnace, in order 



