POTASH 



591 



where great strength is required ; but, owing to the numerous cavities it contains 

 (produced by the empty casts of shells), it will not receive a close, even face, and is 

 therefore not so well adapted for many other purposes of a more ornamental descrip- 

 tion. . The rubbly bed is not much worked ; but the white or best bed, when accessible 

 is always quarried, and affords a white oolitic freestone, which takes a smooth, even 

 face, and works freely in all directions. 



The following analysis by Professor Daniell, gives the chemical composition of this 

 stone : 



Silica ......... 1-20 



Carbonate of lime ....... 95'16 



Carbonate of magnesia ...... 1'20 



Iron and alumina . ...... - 50 



Water and loss ...... " . T94 



Bitumen ........ trace 



100-00 



The other principal localities where the Portland stone is quarried are the Isle of 

 Purbeck in Dorsetshire, where it is called Purbeck-Portland ; and the Vale of Wardour, 

 where (as at Chilmark and other places) it affords a freestone of a superior description. 

 From 56,000 to 60,000 tons of stone are raised annually in the Isle of Portland. 

 H. W. B. 



POST. A North of England term for any bed of firm rock. 



POTASH, or POTASSA. (Potasse, Fr. ; Kali, Ger.) This substance was 

 so named from being prepared for commercial purposes by evaporating in iron pots 

 the lixivium of the ashes of wood-fuel. In the crude state it consists, therefore, of 

 such constituents of burned vegetables as are very soluble in water, and fixed in the 

 fire. The potash-salts of plants which originally contained vegetable acids will be 

 converted into carbonates ; the sulphates will become sulphites, sulphides, or even 

 carbonates, according to the manner of incineration ; the nitrates will be changed into 

 pure carbonates, while the muriates or chlorides will remain unaltered. Should quick- 

 lime be added to the solution of the ashes, a corresponding portion of caustic potassa 

 will be introduced into the product, with more or less lime, according to the care taken 

 in decanting off the clear lye for evaporation. 



In America, where timber is in many places an incumbrance upon the soil, it is 

 felled, piled up in pyramids, and burned, solely with a view to the manufacture of 

 potash. The ashes are put into wooden cisterns, having a plug at the bottom of one 

 of the sides under a false bottom ; a moderate quantity of water is then poured on the 

 mass, and some quicklime is stirred in. After standing for a few hours, so as to take 

 up the soluble matter, the clear liquor is drawn off, evaporated to dryness in iron pots, 

 and finally fused at a red heat into compact masses, which are grey on the outside and 

 pink-coloured within. 



Pearlash is prepared by calcining potashes upon a reverberatory hearth, till the 

 whole carbonaceous matter, and the greater part of the sulphur, be dissipated : then 

 lixiviating the mass in a cistern having a false bottom covered with straw, evaporating 

 the clear lye to dryness in flat iron pans, and stirring it towards the end into white 

 lumpy granulations. 



The best pink Canadian potashes, as they are imported in casks containing about 

 5 cwts., contain pretty uniformly 60 per cent, of absolute potassa; and the best 

 pearlashes contain 50 per cent. ; the alkali in the former being nearly in a caustic 

 state ; in the latter carbonated. 



All kinds of vegetables do not yield the same proportion of potash. The more 

 succulent the plant, the more does it afford ; for it is only in the juices that the vege- 

 table salts reside, which are converted by incineration into alkaline matter. Herb- 

 aceous weeds are more productive of potash than the graminiferous species, or shrubs, 

 and these than trees ; and for a like reason twigs and leaves are more productive 

 than timber. But plants in all cases are richest in alkaline salts when they have 

 arrived at maturity. The soil in which they grow also influences the quantity of 

 saline matter. 



The following Table exhibits the average product in potash of several plants, 

 according to the researches of Vauquelin, Pertuis, Kirwan, and De Saussure : 



In 1000 parts 

 Pine or fir 

 Poplar 

 Trefoil . 

 Beeclnvood 

 Oak 



Potassa 

 . 0'45 

 . 075 

 . 075 

 . 1'45 



In 1000 parts 

 Boxwood. . 

 Willow . . 

 Elm and maple 

 Wheat-straw . 



. 153 [Bark of oak-twigs 



Potassa 

 . 2-26 

 . 2-85 

 . 3-90 

 . 3-90 

 . 4-20 



In 1000 parts 

 Thistles . 

 Flax-stems 

 Small rushes 

 Vine-shoots 

 Barley-straw 



Totassa 

 5'00 

 5'00 

 5 '08 

 5 - 50 

 5'80 



