As our attention has been for some time turned to carbonic acid gas as a motive power, and seeing an article in your paper of the 25th April, in reply to a correspondent on that subject, we take the opportunity of addressing you the following particulars on the subject. At the meeting of the British Association at Newcastle, Enjin 1S3S, Robert Adam*,-Esq., solidified carbonic acid gas by means of an apparatus consisting of a strong wrought-iron vessel, in appearance like a swivel gun, two feet long and six inches in diameter, suspended by trunnions on an iron frame ; also a vessel similar in form and size, but mounted perpendicularly on a flat stand : there were two pumps worked by powerful levers, together with the needful valves and connecting tubes. Into the generator, or suspended vessel, proper quantities ot bi-carbonate of soda and warm water were placed ; a long tube was also inserted, containing sulphuric acid, having its mouth closed with a screw valve. On the generator being rapidly whirled round on its trunnions, the sulphuric acid flows out and mixes with the solution of bicarbonate of soda. The carbonic acid disengaged, having no room to expand, was condensed into a liquid. So far the apparatus resembles that first employed for the same purpose by M. Thellussier, in Paris, but stopping short here, Thellussier could only make iise of about one-third of the carbonic acid disengaged, while Mr. Adams, by pumping it into the second vessel, obtained nearly th%whole; on allowing this liquid carbonic acid to escape through a box, or hollow brass cylinder into the atmosphere, the instantaneous evaporation of one portion, caused it to absorb so much caloric as to solidify the remainder. The solid carbonic acid resembles in appearance and texture newly fallen snow or Jsmall hail ; it evaporates rapidly, but not instantly, from the atmosphere of gas around it, preventing close contact; its intense coldness is not immediately felt, but the brass box m which it is collected, or the solid acid itself when long held, blisters the skin like hot iron; various experiments were tried with, it such as freezing large quantities of mercury, &c. But the circumstance of most consequence in relation to its practical employment, is, that it can be reduced to a liquid by a pressure of 36 atmospheres, or a column of mercury 90 feet, and at a temperature of 150 the liquid acid exerts an expansive force of 70 atmospheres, or 1050 lbs. on the square inch ; and every increase of a single ('egree of temperature aug ments the pressure by upwards of an atmosphere or 15 lbs. on the square inch. None of the carbonates gives so much carbonic acid as the bi-carbonate of soda orthe bi-carbonate ot potash ; 50 parts of the carbonate ot lime (marble and chalk) combines with 49 parts ot sulphuric acid, and gives 22 parts of carbonic acid. 76 parts of bi-carbonate of soda combines with 49 parts ot sulphuric acid and gives 44 parts of carbonic acid. A cubic foot of atmospheric air weighs 527 04 grains, one cubic foot of carbonic acid weighs 804'79008 grains. Having the same capacity for heat as atmospheric air, it embraces the principle of being as great an economizer of fuel as air. We consider that it CMI be economically employed as a motive power. As the carbonic acid can be used frequently, besides the sulphate of soda, formed in preparing the gas, could be sold to advantage, or the soda might be recovered and sold as washing soda. Some may object to the using of this gas on account of its deadly nature, but we are convinced that it can be used with as much safety as steam. And having ?uch an expansive force, an engine propelled by it would not require to occupy more than the o:ie-hundredth part of a caloric engine of the tame power, and about one tenth of the steam engine. You will perceive at once its great superiority over any other motive power (including hot air and electricity) for long voyages, or onlonglines of railroad, where fuel has to form a great proportion of the freight. Two CONSTANT READERS. Paterson, N. J., May 2nd, 1853. [The foregoing communication is a very interesting one, as it displays an acquaintance with chemistry, and at the same time it presents some plain practical information which may be new to many of our readers. When carbonic asid gas was first reduced to a liquid, Sir I. Brunei took out a patent to employ it in an engine, but it failed to realize his expectations. Mr. Salomon, of Cincinnati, took out a patent in our own country for an improved carbonic acid gas engine, two years ago, since which time no engine of the kind has been introduced here. It is no doubT: much superior to air as an economizer of fuel, but it has never been found to economize both machinery, fuel, and other expenses, as compared with steam, neither is it as economical as water.