BICARBONATE OF SODA. 4(>9 



easily obtained of good quality. The operator must also make 

 him self familiar with the difference between the slight reddening 

 of his test paper^ by carbonic acid, which is disengaged, and the 

 unequivocal reddening produced by a strong acid, which last is 

 the indication to guide him. 



Bicarbonate of soda; HO, CO 2 + NaO, CO 2 ; 1056.2 or 

 84.64. This salt is prepared by transmitting a stream of carbo- 

 nic acid through a saturated solution of the neutral carbonate ; 

 it is then deposited as a farinaceous powder, but may be ob- 

 tained in crystals from a weaker solution. It requires 1 3 times 

 its weight of cold water to dissolve it. Although containing 

 two equivalents of acid, this salt is alkaline to test paper, but its 

 taste is much less unpleasant than the neutral carbonate, and 

 indeed is scarcely perceived when combined with a little common 

 salt. The proportion of alkali in bicarbonate of soda is 37-0 per 

 cent, but the salt of commerce generally contains upwards of 40 

 per cent, owing to the presence of neutral carbonate in the state 

 of protohydrate, which last salt may be separated by a small 

 quantity of water. 



The bicarbonate of soda is also obtained otherwise by an in- 

 teresting reaction. Equal weights of common salt and carbo- 

 nate of ammonia of the shops are taken ; the former is dissolved 

 in three times its weight of water, and the latter added in a 

 state of fine powder to this solution, the whole stirred well toge- 

 ther, and allowed to stand for some hours. The bicarbonate of 

 oxide of ammonium present reacts upon chloride of sodium, 

 producing the sparingly soluble bicarbonate of soda, which preci- 

 pitates in crystalline grains and causes the liquid to become 

 thick, and chloride of ammonium (sal ammoniac), which re- 

 mains in solution : 



HO, CO 2 + NH 4 O, CO 2 and Na Cl= 

 HO, C0 2 + NaO, C0 2 and NH 4 Cl. 



The solid bicarbonate of soda is separated from the liquid by 

 pressure in a screw press ; but retains a portion of chloride of 

 sodium. Messrs. Hemming and Dyer, who first observed this 

 reaction, have proposed to found upon it a process for obtaining 

 carbonate of soda from common salt. 



Sesquicarbonate of soda; 2NaO-}-3CO 2 + 4HO ; 2061 or 

 163.15. This salt presents itself occasionally in small prismatic 

 crystals, but cannot be prepared at pleasure. It is unalterable 

 in the air, but is decomposed in the dry state, I find, by a less 

 degree of heat than the bicarbonate, notwithstanding its con- 



