Cite as: 526 U. S. 865 (1999)
Opinion of the Court
The process in which peat transforms into coal is referred to as coalification. Ibid.
The coalification process generates methane and other gases. R. Rogers, Coalbed Methane: Principles and Practice 148 (1994). Because coal is porous, some of that gas is retained in the coal. CBM gas exists in the coal in three basic states: as free gas; as gas dissolved in the water in coal; and as gas "adsorped" on the solid surface of the coal, that is, held to the surface by weak forces called van der Waals forces. Id., at 16-17, 117. These are the same three states or conditions in which gas is stored in other rock formations. Because of the large surface area of coal pores, however, a much higher proportion of the gas is adsorped on the surface of coal than is adsorped in other rock. Id., at 16-17. When pressure on the coalbed is decreased, the gas in the coal formation escapes. As a result, CBM gas is released from coal as the coal is mined and brought to the surface.
III
While the modern science of coal provides a useful backdrop for our discussion and is consistent with our ultimate disposition, it does not answer the question presented to us. The question is not whether, given what scientists know today, it makes sense to regard CBM gas as a constituent of coal but whether Congress so regarded it in 1909 and 1910. In interpreting statutory mineral reservations like the one at issue here, we have emphasized that Congress "was dealing with a practical subject in a practical way" and that it intended the terms of the reservation to be understood in "their ordinary and popular sense." Burke v. Southern Pacific R. Co., 234 U. S. 669, 679 (1914) (rejecting "scientific test" for determining whether a reservation of "mineral lands" included "petroleum lands"); see also Perrin v. United States, 444 U. S. 37, 42 (1979) ("[U]nless otherwise defined, words will be interpreted as taking their ordinary, contemporary, common meaning" at the time
873
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