Mechanics' Society.—A letter from W. B. Robinson, of Chippewa, Canada West, informs us that a mechanics' mutual society lor improvement has been formed in that place, and that at the opening of it, several articles were read with approbation from the Scientific American. The objects of the society are the cultivation of knowledge in science, practical mechanics, and chemistry. O. T. Mack-lin, proprietor of the foundry and machine shop in that place, has given the society the privilege of a room, and the use of such works as Tredgold on the Steam Engine, and a number of other uselul works. A rule of the society is that the members I shall read an original article, and occupy no more than one hour every week, or select a good article from some mechanical work, the time of the meeting after that to be devoted to asking and answering questions, and giving opinions upon the subject. The objects of this society are good, and we like the rules for acquiring knowledge, they are the best we could advise. We hope that perseverance will characterize the conduct of its members. We have noticed that many mechanics' societies have run well for a while until the charm of novelty was worn away; let no such conduct be exemplified by the Chippewa Mechanics' Society. RED COTTON—Some new materials have lately been received in Manchester, England, Irom the south-west coast of Africa, among which is a fibrous substance sent by a missionary at Abeokuta, as a kind of red cotton.' It came from a country lurther to the north, and is found in great quantities. It turns out, however, to be a species of silk, and is of a deep red color, but it has been dyed; probably it is a species of silk grass, like the kind which is so abundant in the East Indies, which is very beautiful, and will endure for a great number of years, but must not be brought in contact with steam or warm water. PARKERS'; WHEEL CASES.—J. Sloan, Esq., of Sloan's Mills, Ky., informs us that the airtight cases for Parkers' wheels must not be of less area than ten times the area of the wheel issues under low falls, nor over fifteen times the area of the issues under high falls. There must at least be fifteen times the area at the discharging end of the air tight cases as the area of the issues of the wheel; the vent under the penstock must be similar. Less than this under the lorebay is a serious disadvantage. This is for one wheel. For two or more wheels in one pit, the same proportions must be preserved. The inlet sluice must have the same area as the issues of the wheel. The discharge of the air-tight cases must be one inch below the surface of the tail water at the lowest stage of water when the wheel is standing. WROUGHT IRON CAR WHEELS.—We have been informed that some of our eastern rail-jxd-..companies have commenced employ-' Ingwrought-iron in place of cast-iron car wheels. The problem to be solved is, whether, in the long run, the wrought or cast-iron wheels are the best; whether the wear of the one kind exceeds or not the expense ol renewing the cracked and broken of the other. CARBONIC ACID GAS, ITS USES.—A correspondent enquires ot us, " how carbonic acid is obtained, how it is affected with heat, and whether or not carbonic acid gas engines have ever been employed." Carbonic acid gas is produced by pouring vitriol on marble dust or chalk. It is thus obtained lor making soda water. It is also the product of the perfect combustion of pure charcoal. Its capacity for heat is the same as that ol air,near-ly one half less than steam. Having the same capacity for heat as air, it embraces the principle of being as great an economizer of fuel as air, and as it can be reduced to a fluid it presents mechanical advantages of a superior character. Sir Humphrey Davy threw out the idea that when carbonic acid gas was first reduced to a liquid, that its prodigious elastic force, under a low temperature eminently fitted it for moving machinery. Sir I. Brunnel took out a patent some years ago for a carbonic acid gas engine, in England, and Professor Salomon, of Cincinnati, secured a patent for an improved carbonic acid gas engine, about two years ago, in our own country. How he has succeeded we cannot tell., Carbonic acid gas can be condensed into liquid by a pressure of 40 atmospheres. This gas is so volatile that when reduced to a liquid, it exerts a pressure of 750 lbs. on the square inch at 45 Fah.— We believe that it cannot be employed sc economically as steam.