Thus it is that the naming and classification of coals is a perennial source of difficulty and controversy. The earliest and most widely used classification is based on the ratio between fixed (or non-volatile) carbon and volatile constituents, called the "fuel ratio." For this purpose "proximate" analyses of coal are made, in terms of fixed carbon, volatile matter, moisture, ash, and sulphur. Anthracite has a higher fuel ratio than bituminous coal; that is, it has more fixed carbon in relation to volatile matter. Similarly bituminous coal has a higher fuel ratio than lignite. The fuel ratio measures roughly the heat or calorific power of the coal, in other words, its fuel value. However, some bituminous coals have a higher calorific power than some anthracites, because a large part of their volatile matter is combustible and yields more heat than the corresponding weight of fixed carbon in the anthracite. The fuel ratio pretty well discriminates coals of the higher ranks, and gives a classification corresponding roughly with their commercial uses. For the lower ranks of coal it is not so satisfactory, because the volatile constituents of such coals contain large and varying percentages of non-combustible hydrogen, oxygen, and nitrogen. Also such coals contain larger and more variable amounts of moisture, which is inert to combustion and requires heat for its evaporation. Two coals of the lower ranks with the same fuel ratio may have very different fuel qualities and different commercial uses, because of their different amounts of inert volatile matter and of water. For these coals it is sometimes desirable to supplement the chemical classification by physical criteria. For instance, subbituminous coal may be distinguished from lignite, not by its fuel ratio alone, but by its shiny, black appearance as contrasted with the dull, woody appearance of lignite. Bituminous may be distinguished from subbituminous by the manner of weathering. Other classifications have attempted to recognize these difficulties and still maintain a purely chemical basis by considering separately the combustible and non-combustible volatile constituents. For this purpose, it is necessary to have not merely approximate analyses, but the ultimate analyses in terms of elements. Definitions of the principal kinds of coal by Campbell,[18] of the United States Geological Survey, are as follows: _Anthracite._ Anthracite is generally well known and may be defined as a hard coal having a fuel ratio (fixed carbon divided by the volatile matter) of not more than 50 or 60 and not less than 10.