Divers apparatus for liquefying carbon anhydride have been devised, those of Thilorier and Natterer being the beat. Thilorier employs two strong wrought-iron cylinders, into one of which is introduced about 51h. of bicarbonate of soda and 7 pints of water. A copper tube containing 2i lb. of sulphuric acid is then lowered into the mixture and set on end, the top of the being firmly closed with a cap, into which the delivery tube, fitted with a carefully constructed stopcock, is introduced. By inclining the retort or " generator," for which purpose it may be con veniently swung in an iron frame, the acid is allowed to run gradually out of the tube and mix with the bicarbonate. In the meantime the second cylinder or " receiver," kept cool by means of a freezing mixture or ice, has been connected with the delivery tube of the generator, and when the sulphuric acid has set free gaseous carbon anhydride from the bicarbonate of soda, the cocks are opened, and the gas allowed to rush over into the receiver, where it condenses by its own pressure.
At Vienna another plan, that of Professor Beims, has been tried, but too great a quantity of fuel is required to allow of its being commercially successful. By this method the gaseous anhydride is freed from bicarbonate of soda by heating the latter up to 371° (700° F.) in closed strong iron vessels, and condensed, as in the case of Thilorier's apparatus, by its own pressure.
A very excellent process is the following :—a mixture of chalk and water is introduced into the generator, which is fitted with an agitator., The acid is. run in gradually from a vessel placed above, and thorough admixture secured by agitation. When liberated, the gas is conducted through a vessel called the " washer," containing water, into the receiver, where it is kept until a pressure of about 100 lb. to the square inch is attained. It is then taken through a set of refrigerating pipes into the compressing cylinders, and from thence into the " holders," which are carefully surrounded by a freezing mixture. The holders should be made of thin sheets of steel placed in successive layers with overlapping joints, and soldered together with pure tin, the outer case being carefully rivetted. Considerably less work is manifestly put upon the compressing engine by the gas being stored in the receiver until it can be introduced into the compressing cylinder at an already high pressure.
Faraday first liquefied, carbonic anhydride by decomposing carbonate of ammonia by sul phuric acid iu a sealed hent glass tube. Gore has proposed to condense, after a somewhat similar manner, in strung glass tubes closed with guttapercha stoppers. By Natterer's process, to which
reference has been made, the gas generated by the action of sulphuric acid upon bicarbonate of soda is pumped by means of a force-pump into a strong wrought-iron vessel, in a similar manner to the pumping of air into the receiver of an air gun. As soon as the volume of gas pumped in amounts to about thirty-six times the volume of the receiver, every stroke produces condensation. This apparatus has also been used for liquefying nitrous oxide gas.
When liquid enrbon anhydride is suddenly freed from pressure, it is instantaneously converted into the gaseous form, and by the operation such intense cold is produced, that a portion of the liquid is frozen. It then forms the solid anhydride, a white, snow-like mass. Though this sub stance has so low a temperature as — 78°, it can be handfed without inconvenience, owing to a constant volatilization, which keeps it from actual contact with the hand. Wetted with ether, solid carbonic anhydride forma the moat effectual freezing mixture known, a temperature of —110° being obtainable, and mercury instantly solidified into a lead-like masa. If when liquefying the gas at a temperature of —87° the preasure is increased to four atmospheres, a solid mass is obtained, which has the transparency of ice, and may be divided into crystals, which have the action of intense heat when pressed between the fingers.
It remains only to notice the aqueous solution of carbon dioxide, the only real acid form of the compound. The gaseous anhydride dissolves in its own volume of water, giving a solution of 1.0018 ap. gr. with a sharp acid taste, and possessing decided acid properties. By boiling this nqueous solution, however, the gas is evolved, and litmus no longer reddened. It is owing to this that hard water can be softened by boiling, the carbonate of lime yielding up its carbon dioxide, and being deposited in the kettle or boiler in the shape of " fur." If the gas be simply pissed into water, only about two-thirds of the bulk of the solvents is taken up; but if the pressure is increased or the temperature diminished, the solubility is very much greater. Hence water impregnated with the gas at a high pressure immediately parts with it when the pressure is removed. For further details of this part of the subject sea " Aerated Waters." The following table gives the volumes of carbonic anhydride absorbed by 1 volume of water under a pressure of 760 tome., and at the temperatures indicated :— The column marked a In the annexed table shows the volume of as absorbed at the ordinary temperature of the air, and under the pressures indicated by the column marked P.