1. A capsule for pharmaceutical preparations for powder inhalers consisting of a capsule body and a capsule cap, which both consist of the same material and which can be attached to one another so as to form a stable, enclosed hollow space of defined volume, characterized in that the capsule material is a water-insoluble, hydrophobic synthetic material. 2. A capsule according to claim 1, characterized in that the walls of the cap and the body are 0.1 mm to 0.5 mm thick. 3. A capsule according to claim 1 or 2, characterized in that the capsule can withstand a force acting upon its longitudinal axis and its transverse axis of up to 15 N. 4. A capsule according to claim 1, 2 or 3, characterized in that the Shore hardness D of the synthetic material is in the range from 65 to 73. 5. A capsule according to claim 1, 2, 3 or 4, characterized in that the wall of the capsule has a steam permeability of less than 1.3 x 10<-14>kg/ (m<2>xsxPa). 6. A capsule according to one of the preceding claims, characterized in that the wall of the capsule has a steam permeability of 1.5 x 10<-16> to 2 x 10<-16>kg/ (m<2>xsxPa). 7. A capsule according to claim 1, 2, 3, 4 or 5, characterized in that the synthetic material is polyethylene, polycarbonate, polyester, polypropylene or polyethyleneterephthalate. 8. A capsule according to claim 7, characterized in that the synthetic material is polyethylene with a density of 9000-10,000 kg/m<3>. 9. A capsule according to one of the preceding claims 1, 2, 3, 4, 5, 6, 7 or 8, characterized in that one or more elevations or recesses are located on the inner jacket of the cap and one or more recesses or elevations are located on the outer jacket of the body, these elevations or recesses being arranged so that the elevations engage with the recesses when the capsule is closed. 10. A capsule according to one of the preceding claims, characterized in that a bulge runs in an annular shape around the outside of the body perpendicular to the connecting axis of the cap and the body, the side of the bulge which is orientated towards the open end of the body standing practically at right angles to the outer wall of the body. 11. Capsule according to one of the preceding claims, characterized in that the capsule body and capsule cap are both made of high density polyethylene and are both in the shape of a cylinder of round cross section with a convex closed end, so that the elongation of the capsule (distance from the closed end of the body to the closed end of the cap in relation to the diameter when the capsule is closed) is greater than 1. 12. Capsule according to one of the preceding claims 1 to 11, characterized in that the joint between the body and cap, or the gap, is sealed by welding, adhesive bonding, wrapping or covering the cap with a protective film. 13. Capsule according to one of the preceding claims 1 to 12, characterized in that the joint between the body and cap, or the gap, is sealed by filling with a pharmaceutically acceptable filler. 14. Capsule according to claim 13, characterized in that the filler is Eudragit. 15. Use of the capsule according to one of the preceding claims for a pharmaceutical preparation which contains cromoglycic acid, reproterol, beclomethasone, terbutaline, salbutamol, salmeterol, ketotifen, orciprenaline, fluticasone, ipratropium, dexamethasone, bambuterol, tiotropium, budesonide, fenoterol, clenbuterol, prednisolone, prednisone, prednylidene, methylprednisolone, formoterol, nedocromil, insulin, ipratropium bromide, tiotropium bromide, the salts or mixtures thereof or another cortisone preparation or atropine derivative suitable for inhalation purposes. 16. Use of the capsule according to one of the preceding claims in powder inhalers.