After the successful application of steam to propel machinery, ether, alcohol, and various vapors were proposed as substitutes, because it was supposed that liquids which boiled at a lower ,heat than water—-gave off their vapor then":\vouJd , economize fuel This opinion was ente rtained by both scientific and unscientific men, and although Mr. Ainger pointed out this error iI] an article read before the Royal Society in 1830, tl.\e very last number of the Franklin Journal copies an artiA from the London Chemical Gazette, by J. Apjqh,n, Professor of Chemistry, Trinity College, Dublin, in which he proves to hi$ own satisfaction that all fluids which boil at a Io:W,er temperature than watej must necessarily economize fuel if applied as substitutes for-steam in propelling machinery. We will point out his error, and in doing so establish the principle that although water floes no t boil at such a low temperature as many other fluids, its vapor possesses a greater elastic force just in proportion to the heat applied to it. The principle which Mr. Apjohn lays down to prove that alcohol and ether which boil at a lower temperature than water, are more economical in fuel, to exert a force in propelling machinery is this :—” The specific and latent heat of water combined, is 11'29'0o that of al cohol 875-5°, that of ether 534'7°.” “The mere inspection of these figures,” he says, '-is sufficient to show that with alcohol about three-fourths, and with ether somewhat less than one-half the caloric required for water will suffice to produce the same mechani cal effect.” ' What reason does he adduce > Here it is, “ the vapors of different liquids have at their respective boiling points the same elastic force, equal volumes of them will produce equal mechanical effects,” This is a grave error to be propagated by a professor of chemistry; it is not the basis upon which to found any proposition for proving the economy of one liquid over another to produce mechanical effect and we will show why. The mechanical effects of vapors are inversely in proportion to their densities; thus although alcohol floats on water, and ether on alcohol, yet the vapor of water (steam) floats above the vapor of alcohol, and the vapor of alcohol above that of ether. The density of water is 10, alcohol 8, ether “1; the density of their vapors is water 6, alcohol 10. ether 25. M. Cagniard de la Tour put some water into one glass tube, ether into another, and alcohol into another, and hermefical] y sealed them. By applying heat ether became gaseous in a space scarcely double its volume., at a temperature of 320°, and exerted a pressure of no more than 38 atmospheres; alcohol became gaseous at a temperature of 4044 in a space of thrice its volume with a pressure of 139 atmospheres; water acted on the glass chemically, but by adding some carbonate of soda to it, it becomes gaseous :1t a temperature of 648° in a space four times its volume, consequently, as an increase of a double vol ume in al cohol vapor increased the pressure nearly four times, from 38 to 139 atmospheres, the pressure of the vapor of water would be in the same proportion 53G atmospheres ; less elastic, according to the pressure to be sure, but under the same pressure there can be no doubt, that according to i s latent and specific heat, it would exert a force in proportion over alcohol and ether. Water vapor has 2'5 times more iatent heat than alcohol vapor, but the specific gravity of the lat- 'ter is 2'5 times greater, this shows that the same bulk of vapor will be produced from them both—alcohol and water—with the same expenditure of heat; hence there can be no advatage—no economy in substituting alcohol for water as a sourc e of vapor in the steam engine. The error of Mr. Apjohn lies in taking his deductions from the product—vapor— of eat and a Jl uid, not from the heat and fluid fir§t. It is the case with too many people, they do not go to th e root of the matter, hence their deductions, from laying down a false proposition, may look very plausible, but at the -saine time be very erroneous. Alcohol, etherj carbonic acitJ. gas, &c., are more expensive and troublesome to obtain than the vapor of #atel';. Soffl.EI'of them would act chemically on the. -''ti'lachinery also . . They doii6t possess the tjlSWity of being so easily and suddenly condensed as steam, and thus they have iot the same qualities to recommend them as substitutes for it. This is the reason why volatile fluids which boil at a lower temperature than water,"'\Vh'en"applied in engines (and thfljre have been many of such engines,) have al. ays failed t(lecqrnpete with steam. We intended to produce some reasons why hot air engines haNlalsofatled to compete with steam but this must leave till next week.