Marble is the purest carbonate of lime, containing about 44 per cent, of its weight of carbonic acid, and is when ground a valuable material for the production of carbonic acid. White marble is the best, and, as analysis has proved, yields the purest carbonic acid. Colored marble, gray, black, etc., yields impure carbonic acid. It contains bitumen and metals, and the gas would be impure and sensibly affect the flavor of any beverage. The waste of white marble in marble works is ground and profitably sold to the mineral-water trade. Marble dust is less violently effervescive with acids than the less compact forms of carbonate of lime, and therefore approved by most bottling establishments. There is much difference in marble dust regarding the quantity of carbonic acid it contains, and inferior and even adulterated marble dust is in the market, which causes oftentimes very much annoyance to the manufacturer, as such marble dust soon becomes hard and clings to the sides of the generator.

We found in one shipment of marble dust different grades of it, some barrels differing widely from the others, and yielded by far less carbonic acid gas than some of the lot. The results prove beyond doubt that inferior grades of marble dust are sold with superior grades at the price of the latter, a practice which is a disadvantage to the carbonator, who sometimes wonders why he can bottle with one charge a greater amount of water than with another. He also wonders sometimes when his beverages have a particular smell, and is inclined to lay the fault on the extracts, etc., or other materials used, while in fact his marble dust may be the cause of it. To buy from reliable houses is the only safeguard against such practice, and by all means to wash the gas carefully to insure against any fumes that may evolve from the carbonate.

Marble dust is the cheapest carbonate at present, and yields as much and as pure a carbonic acid gas as any other, except bicarbonate of soda. A compound of half marble dust and half whiting was tried in some cases, but this is generally abandoned now and the American carbonate, the native product of marble dust, is predominating over its foreign competitor, the whiting.

Marble dust appears in commerce ordinarily in three grades: coarse, medium, and fine; the first resembles coarse sand, the second sifted sand, and the third pulverized sugar. It is a curious fact, that the finer it is ground the greater its bulk becomes; consequently, a barrel of the coarse marble dust weighs more than a barrel of the medium, and considerably more than a barrel of the fine marble. It is another curious fact that the finer the stone is ground the whiter the dust is, even black marble becoming almost white if ground exceedingly fine. By reason of this the finest grade of marble dust is also the whitest and handsomest, and is therefore often preferred by those who do not understand the matter fully. It weighs the least, however, and of course yields the least gas, and is open to the further objection that its minute particles are apt to be acted upon too quickly by the acid, thus causing a troublesome boiling and foaming in the generator unless great care is exercised. Indeed, when muriatic acid is used, the marble should be in small fragments, else a violent foaming would take place.

The coarse marble dust is decidedly to be preferred where much car-bonating is done, as it yields the most gas to the barrel and produces it in an even, steady volume. The generation is slow, however, and where there are only one or two fountains to be charged at once, and the operator cannot afford to spend much time over the work, the medium grade is the best. The numerous globules of air lodged in fine marble dust are also objectionable, and the fineness of the grinding renders it doubly difficult to detect adulteration.

There is no commercial standard of purity and strength for ground marble, and no gauge for determining its quality (except by chemical analysis). The flint or silica that sometimes occurs in chalk is removed by the process of manufacturing the whiting, but any silica that may be found in the marble is ground up with it and forms not infrequently as much as 15 per cent. of the mass. It is not actively harmful, but only inert and useless. Those who object to it may generally detect its presence by examining the dust under a strong microscope. The silica has a sharp, flinty fracture, very distinct from the obtuse angles of the particles of genuine carbonate. It is quite unaffected by the acid, and may be even more readily detected in the refuse from the generator.

Iron is also sometimes present in the marble, and yields hydrogen in the generator, not at all to the advantage of the carbonator. This mineral may be suspected if the dust is of a brown color, and, other things being equal, the whiter the ground marble the more likely it is to be a pure carbonate.

Marble is also liable to intentional adulteration with the odds and ends of mineral around the mill where it is ground, marble dust being the easiest medium through which to work them off. These additions, like the silica, are generally merely inert. The easiest way to avoid this petty swindle is to buy of some one who makes a specialty of supplies for carbonating, and grinds no mineral but marble.

Marble dust is being decomposed almost exclusively by the action of sulphuric acid (vitriol), producing carbonic acid gas and a residue: sulphate of lime.