722 



CARBONATES 



poisonous, and advantage has been taken of this character to use it as a vermin- 

 destroyer. A small quantity of the bisulphide, enclosed in an air-tight chamber with 

 grain infested with -weevils -will destroy the insects and their larvae. It is curious that 

 the vapour of bisulphide of carbon passed over wood undergoing destructive distillation, 

 alters the character of the carbonaceous residue, and produces a material having high 

 conducting powers for heat and electricity, and extremely sonorous when struck. 



CARBONADO. A massive variety of diamond, found in small black pebbles in 

 Brazil, and used for polishing hard stones. 



CARBONATES. By this term is understood, in popular language, the salts 

 formed by the union of carbonic acid with bases. In conformity with the language of 

 modern chemistry, a carbonate may be defined as carbonic acid in which the hydrogen 

 is replaced by a metal. Thus, carbonate of potash, according to the old definition, is 

 a compound of carbonic acid gas (carbonic anhydride) with potash ; whilst, according 

 to the new definition it is merely carbonic acid (the true hydrate, not the anhydride), 

 in which potassium takes the place of hydrogen. See SAI/TS. 



The carbonates are among the most valuable of the salts, whether we regard 

 their physical, geological, chemical, or technical interest. Were limestone and 

 marble the only carbonates familiarly known, they would be sufficient to stamp this 

 class of salts as among the most important. The carbonates of lime, potash, soda, 

 ammonia, and lead, are articles of immense importance to the technologist, and are 

 prepared on a vast scale for various purposes in the arts. The carbonates of iron and 

 copper are among the most valued ores of those metals. Numerous processes of separa- 

 tion in analysis are founded on the various degrees of solubility in water and certain 

 reagents of the different carbonates. By taking advantage of this fact, baryta, 

 strontia, and lime, may be separated from magnesia and the alkalis. There are few 

 analytical problems which have attracted more attention than the accurate deter- 

 mination of the carbonic acid in the carbonates. This has partly arisen from the 

 frequency with which the potashes, soda-ashes, limestone, and other carbonates of 

 commerce, are sent to chemists for analysis. The number of instruments contrived 

 for the purpose is something extraordinary, especially when the simplicity and ease 

 of the operation is considered. Among them all, there is none more convenient or 

 easy to use than that of Parnell. ' It consists of a glass flask (fig. 425) of about two 

 ounces capacity, fitted with a sound cork, through which two tubes pass, one serving 

 to connect a chloride-of-calcium tube, a, while the other, b, will be described presently. 



A small test-tube, c, is so placed in the flask, and is 

 of such a size, that it cannot fall down, but its con- 

 tents may be made to flow out by inclining the 

 apparatus to one side. To perform the experiment, 

 a weighed quantity of the carbonate is placed in 

 the flask, and water added up to the level seen in 

 the figure ; the test-tube is then filled nearly to the 

 top with concentrated sulphuric acid, and is care- 

 fully lowered into the flask; the cork with the 

 tubes attached is then affixed, the aperturo b being 

 closed with a small cork. The whole apparatus is 

 now carefully weighed ; the flask is then to be in- 

 clined so as to allow some of the acid to flow out, 

 and, when the effervescence has subsided, a little 

 more, and so on until no more carbonic acid is 

 evolved. The flask is now to be so inclined as to 

 cause the whole of the acid to mingle with the 

 aqueous fluid, and thus cause a considerable rise of 

 temperature ; this expels the carbonic acid from 

 the liquid ; but as an atmosphere of the latter gas 

 fills the flask, it must be removed and replaced by 

 air, as the difference in density of the two is very 

 considerable. For this purpose, the cork b is removed and air is sucked out of d, 

 until it no longer tastes of carbonic acid ; the flask is then allowed to become per- 

 fectly cold, and, the little cork being replaced, it is then re-weighed : the difference in 

 the two weighings is the amount of carbonic acid in the specimen. On drawing air 

 for some time through the apparatus, it begins slowly to acquire weight, arising from 

 the moisture in the atmosphere being absorbed by the chloride of calcium, and 

 although the error introduced by this means is too minute to affect ordinary expe- 

 riment, it must not be neglected where, from the quantity of material in the flask 

 being limited, or other causes, a small difference has an important bearing on the 

 result. In this latter case another chloride-of-calcium tube is to be attached to the 

 aperture b, and the air must be drawn through by means of a suction-tube applied 

 at d.' C, G. W.'s Chemical Manipulation. 



