actual expansion of the space, and consequently no derangement of the quantity of motion in that space. LAW OF SPECIFIC HEAT. From this view it is evident that the specific heat of elastic fluids can only be considered as approximately determined. If equal spaces possess equal momenta, and the ethereal or _tomic_ matter be inversely as the weight of the atomic matter in the same space, it follows that the product of the specific gravities and specific heats of the simple gases should be constant; or that the specific heats should be inversely as the specific gravities,--taking pound for pound in determining those specific heats. If we test the matter by the data now afforded, it is best to obey the injunction, "_In medio tutissimus ibis_." In the following table, the first column are the values obtained by Regnault; in the second, the former values; and in the third, the mean of the two. Gases. Reg. specific heats. Former specific heats. Mean. Atmospheric air, .237 .267 .252 Oxygen, .218 .236 .227 Nitrogen, .244 .275 .260 Hydrogen, 3.405 3.294 3.350 The specific gravities of these gases, according to the best tables in our possession, are: Specific gravities. Mean. Products. Atmospheric air, 1.0000 × .252 = .252 Oxygen, 1.1111 × .227 = .252 Nitrogen, 0.9722 × .260 = .252 Hydrogen, 0.0745 × 3.350 = .249 As might be expected, there is a greater discrepancy in the case of hydrogen.