Chemical Science. 159 



in cases where a great quantity of gas is absorbed by the 

 amraoniacal solution of baryta or lime, or where the mix- 

 ture stands for several days, are the carbonates entirely pre- 

 cipitated. 



Even lime water, when the lime is not entirely separated 

 by carbonic acid, retains some carbonate of lime in solution, 

 which will be found by heating such lime water to ebullition in 

 a closed retort, when the carbonate will fall to the bottom ; 

 and it was ascertained by a comparative experiment, that the 

 precipitate was not due to the separation of lime from the hot 

 water, but was really a carbonate of lime. 



Though both lime-water and barytes water absorb carbonic 

 acid, and readily deposit carbonates ; yet, when previously 

 mixed with ammonia, they are not in the least rendered turbid 

 by a small quantity of carbonic acid gas, and ebullition is re- 

 quired to perfect the precipitation. 



Lime-water acts in the same manner when poured into a 

 solution of alkaline carbonate of ammonia ; the transparency is 

 slightly clouded, and immediately after restored, and ebulli- 

 tion is required to obtain a precipitate. If a large proportion 

 of lime-water be added, a permanent precipitate is obtained. 



If, therefore, a muriate of barytes, or lime mixed with am- 

 monia, be employed to detect carbonic acid, it must be boiled 

 some time, to throw down the whole of the precipitate ; but 

 Dr. Vogel recommends, as the surest means to pass the gas 

 through barytic water, and to determine the volume of the 

 carbonic acid gas, from the weight of the carbonate, when dried. 



It is often of importance to obtain the carbonic acid from the 

 water at the spring head. To do this, we have sometimes 

 adopted the plan of adding such a quantity of barytes water 

 to a given volume of the spring water, as to precipitate all the 

 carbonic acid, and then ascertain the quantity of carbonic acid 

 in the precipitate in the laboratory, and we know of no objec- 

 tions to the plan. — Ed. 



10. Plumbago in Coal-gas Retorts. — The following descrip- 

 tion of an artificial plumbago, is by the Rev. J. J. Conybeare ; 

 he is speaking of the retorts in the Bath gas-works. The 

 unservicable retorts, on being withdrawn from their beds, are 

 found lined with a coating of plumbago, averaging the thick- 

 ness of four inches. This coating is thickest towards the bot- 

 tom of the retort. The general aspect of the predominant va- 



