242 METHODS BY WHICH SODA IS PREPARED*. 



and drawn oft* to other. When all was cool, which in the winter was gene* 



Stfciftcm! rall} ' the cafe m threG ***** a pIu & in the bottom of the cif- 

 tern was drawn in order to let off the fluid, and the cryftal- 

 lized fait was taken from the flips of lead. The bottom exhi- 

 bited a rock of fait, which was detached by chiflel and mallet. 

 On this account it is that the lead which lines the ciflern 

 muft be thick, in order to guard againft accidents. For if the 

 metal be perforated, the faline folution creeps between it and 

 the ivood, and in a very fliort time detaches the lining, and it is 

 befides extremely difficult to find out the place where the de- 

 at a temperature feet really is. The temperature where the foda is left to cry- 

 Thefoda is then ft^h'ze ought not to exceed 55° Fah. In this ftage of the pro- 

 walhed with cold cefs the whole of the fait is wafhed in the fame cittern with 

 water > cold water, to clear it of impurities ; after which it is trans- 



then diflolved ferred again into the boiler, diflblved in cleas water, and eva- 



tfl toJhSt? P orated D y heat - As r ° on as a ftron S pellicle is formed, it is 

 furlercd to cool fo far that the hand may be dipped in the 

 fluid without injury, and the heat is kept at that temperature 

 Pellicles of fal- a s long as effectual pellicles continue to be formed over the 

 are* formed ^nd wno * ie furface of the boiler, and then fall to the bottom, 

 fall downj When no more pellicles are formed, or at leatt only by blow- 



ing with the mouth upon the furface, the fire is withdrawn, 

 and the fluid is ladled out into the cittern to cryftallize. The 

 fulphate of potath, &c. which had been deposited, is then 

 which ceafing, taken out of the boiler and put afide. If the fluid be fuffered 

 llrawnofftocrv- *° C00 ^ P ret ty l° w before it is fuffered to run into the ciilern, 

 ftallize. very little fulphate of potafli is found in the foda ; but in ge- 



neral the rocky mafles of foda met with in the market contain 

 100 parts of ful- a confiderable quantity. By this procefs from 136 to 139 lb. 



afford ttltf 0f foda ma ^ be obtained from 100lb - of fulphate of foda, if 

 carbonate. the foda be eryftallized in large cryftals ; if fmall eryftallized it 



yields lefs ; it fells now at 52s. or 54s. per cwt. and is retailed 

 at 8d. per lb. 

 The two cryftal- We might be inclined to fuppofe that the firft operation was 

 hzationsare unneceflary, and that the foda might be feparated at once 

 ceffary. from the fulphate of potath at the inftant of its formation ; but 



practice will convince the operator otherwife. A confiderable 

 lofs is mantfefted if the procefs be not conducted in this man- 

 ner ; though the difcovery of the caufe may perhaps be not fo 

 S&ii\y accompliflied as the proof of the fact. 



Other 



