Explosives vary in their composition and the manner of combustion. To go off, they must be ignited according to their peculiar requirements. For example, gunpowder needs to be confined to explode, but the majority of explosives, such as nitroglycerine and dynamite, need not be confined. Since the outside of a bomb, as a rule, gives no indication of its contents or the method of ignition, utmost care is needed in handling. Often, rough usage alone can cause ignition. Therefore, if officers on duty find a parcel which they suspect to be an explosive, they should not touch it without consideration, overturn it, or try to open it. Such a parcel should be deposited in an open space and protected by light material (e.g., sawdust) placed around it but not upon it. A list of the police forces which used the book is included.