the reduction of viscosity with shear velocity which enables rapid flow of such oils through small orifices polysiloxane oils may have viscosities ranging from 0 6 to 10 7 centistokes depending on molecular weight this property enables the oil to be chosen according to the requirements of the embodiment in particular embodiments that require flow of the compressible substance through passages of small cross sections as is the case for the embodiment shown in fig1 the other embodiments may be provided with a compressible substance in the form of an elastic solid such as vulcanised silicon rubber for example of the type silgel u00ae 6 2 manufactured by wacker chemie having volumetric compressibility only about 25 lower than low viscosity polysiloxanes as an example monomer hexamethylsiloxane ch 3 6 sio may be elastically compressed under a pressure of approximately 2000 bars with a volume reduction of about 15 if the volume of the liquid to be injected is 0 1 ml and the minimum pressure at the end of injection is chosen to be 1000 bars the non compressed volume of polysiloxane is 1 3 ml the device according to the invention is not only extremely compact but enables the injection of liquid at pressures well above those available in conventional systems which makes possible the production of.