(Mêtric= có khoảng cách) này và có xu hướng làm tăng tốc độ giãn nở của vũ trụ.[1] Giả thiết về sự tồn tại của năng lượng tối là cách phổ biến nhất để giải thích về các quan sát gần đây rằng vũ trụ có vẻ như đang giãn nở với một tốc độ ngày càng tăng. | 90 energy a beginner s guide converted by new techniques or to development of new non-fossil sources of energy. coal the first fossil fuel Coals the plural is more accurate because of considerable differences in the composition and quality of these solid fuels are sedimentary rocks dominated by combustible organic material and containing varying amounts of incombustible mineral matter and water. All coals originated as plants deposited in swampy environments partially decomposed covered by other sediments and subjected to high pressures and temperatures for up to 350 million years. Many of these plant species are still around albeit in drastically smaller forms next time you see foot-tall horsetails and thumb-tall club mosses growing in wet sylvan spots imagine them as ten to thirty meter tall trees in the swampy forests inhabited by the first reptiles and large winged insects. More recently ten to twenty million years ago the buried trunks of ashes maples and willows began to be transformed into younger coals. COAL TYPES AND COMPOSITION Differences in the initial plant composition and the subsequent metamorphic processes explain the wide variation in quality apparent not only between coals from different fields but also between coals from a single field or even a single seam. The best coals the jet-black anthracites have their origin in the Carboniferous period 354-290 million years ago as do the goodquality bituminous hard black coals. Some lignites Europeans call them brown coals covered by only shallow sediments and hence not subject to high pressures and temperatures date from the Carboniferous era but most are much younger as they were produced by the transformation of plant material during the Tertiary period 65 million years ago . The poorest lignites lighter colored and crumbly have an energy density lower than wood because most of their mass is moisture and ash. In elemental composition the poorest lignites are less than fifty per cent carbon anthracites
