Explosives may be classified in various ways, according to the purpose of the classification, but the great majority of them fall naturally into two main divisions: propellants and high explosives. Propellants explode comparatively slowly, and are used to propel projectiles from fire-arms. High explosives are much more rapid in their action, and are used for bursting and shattering. Propellants are of two sorts, according as they are intended for use in shot-guns or rifled fire-arms. Those for shot-guns burn more rapidly than those for the latter, but both practically always contain a considerable proportion of nitrocellulose, gelatinised by means of such solvents as acetone or ether-alcohol, according as it is of high or low nitration. Some contain also nitroglycerine, and are then called nitroglycerine powders, whereas those that do not contain this substance are termed nitrocellulose powders. Many powders also contain other ingredients, as may be seen from the compositions given in this dictionary. Of high explosives an important class is used for charging shells and bombs. As a rule, but not necessarily, these are not the same as the explosives used for mining operations and other general blasting purposes. Another important class is that of the coal-mine explosives, which are designed to give only a short and comparatively cool flame so as to diminish the danger of igniting fire-damp and coal-dust. Nearly half the explosives in this dictionary are coal-mine explosives. The reason for this large number is that no finality has yet been reached as to the best and safest explosives to use in coal mines. When more experience has been gained it is probable that the number of these explosives on the market will be reduced. In England the Permitted List has recently been cut down considerably. In Great Britain these coal-mine explosives have to be submitted to the Inspectors of Explosives, and are subjected to tests for safety and strength. If they pass they are placed on the list of “Permitted Explosives,” and the compositions are published in the Explosives in Coal Mines Orders. In these Orders the upper and lower limits of the percentage of each constituent are given, but in this dictionary intermediate percentages are given so that the sum for any explosive amounts to 100. In the Explosives in Coal Mines Orders the percentage of such a substance as wood, meal or starch, is given in the dried condition, but here it is given in the air-dry state on the assumption that it then contains about 10 per cent. of moisture.