referring to fig1 it will be seen therefrom that in the operative condition of the apparatus , disposed in the upper or front portion 3 of a housing 2 , representing a second container , of the injection apparatus , which can be heated as by an electrical heating means 33 is a sleeve member or tube 1 which represents a first container . the tube 1 is provided with a closable opening 4 and 5 at each end . the tube 1 is introduced into the housing 2 of the injection apparatus through a charging opening 6 . in the illustrated position the tube 1 is carried in the upper or front portion 3 of the housing 2 , or more precisely , the space 7 within same , with such an accurate fit therein that there cannot be any flow around the outside wall surface of the tube 1 . the tube 1 is fixed in the space 7 by means of a clamping ring 9 which can be screwed in through the charging opening 6 while the latter is open . a space 8 inside the housing 2 , below or rearwardly of the ring 9 , is also accessible through the opening 6 which can be closed by means of a high - pressure closure arrangement 10 . the discharge opening 4 of the tube 1 is aligned with an injection opening 11 for the discharge of sample and injection agent . from the discharge opening 4 , a feed conduit leads by way of a highpressure screw connection 12 to the interior of the reaction vessel ( not shown in fig1 but shown at 25 in fig3 ) which in operation is under a high pressure and at a high temperature . provided in a lower or rearward portion 13 of the housing 2 , in the side wall thereof , is a flow opening or port 14 for the intake of the injection agent , the port 14 also being provided with a high pressure screw connection as indicated at 15 for connecting a feed conduit thereto . the port 14 communicates with the space 8 inside the housing 2 , upstream of the tube 1 or the intake opening 5 thereof , as considered in the direction of flow of the injection agent as indicated at 16 . referring also at this point to fig3 disposed between the high - pressure screw connection 12 and the reaction vessel 25 are a high - pressure valve 35 and , possibly , in conjunction therewith , a check valve 36 which is temporarily opened for carrying out the injection operation . the injection apparatus can be fixedly connected to a container for the injection agent , for example oil when injecting coal , by way of the screw connection 15 . the injection agent is essentially a conveyor agent or flow medium for the sample to be injected . the connection 15 also has a further valve ( not shown ) with which the container for the injection agent can be switched on and off as required . referring to fig2 shown therein on a larger scale is the sleeve member or tube 1 which is used for carrying out the radioactive irradiation operation and possibly freezing the sample , in a storage container 17 . the tube 1 comprises a cylindrical metal tube which is slightly tapered inwardly at its upper end which has the discharge opening 4 to provide a constriction in the flow therethrough . the second or intake opening through which the injection agent passes into the tube 1 at the other end thereof is indicated at 5 . shown within the tube 1 is a sample 18 to be injected . in a specific situation of use , the substance involved may be for example radioactively marked coal powder or a coal - oil suspension which is also marked . the openings 4 and 5 of the tube 1 are closed by closure plugs or stoppers 19 and 20 which can be easily melted and which comprise a suitable material for example wax . if the sample 18 is formed by a coaloil suspension , the plugs or stoppers 19 , 20 may be omitted if the sample 18 has a sufficiently high solidification temperature . the storage container 17 in which the tube 1 is put into intermediate storage after the sample 18 has been radioactively irradiated also comprises a metal tube and can be closed at its one end by a screw - in plug or stopper 21 which has a shoulder portion 22 and a seal 23 , which project into the opening 5 of the tube 1 and in so doing seal it off . disposed at the other end of the container 17 , within the container , is a further plug or stopper 24 which has a screw - threaded bore 34 for connection to an ejector member , for example a rod or bar . the screw - threaded bore 34 is a blind bore so that a sealing action is produced between the plug or stopper 24 and the tube 1 , in the region of the edge 37 . the plug or stopper 24 is mounted movably in the storage container 17 so that , after the plug or stopper 21 has been removed , the tube 1 can be easily pushed out of the container 17 for unloading same , through the opening 31 of the container 17 . it will be seen that the container 17 has an inwardly extending flange at its end adjacent the stopper 24 to hold the latter in the container 17 . the plug or stopper 24 can be braced with respect to the sleeve or tube 1 , for sealing purposes . that can be effected in a simple manner by cooling , as is required in many situations in any case , as when using a coal - oil suspension as the sample . the severe cooling action results in shrinkage of the container 17 both in the peripheral direction and also in the longitudinal direction , the lengthwise shrinkage meaning that the plug or stopper 24 is pressed firmly against the end face of the sleeve member or tube 1 , that has the discharge opening 4 therein . fig3 shows the injection apparatus in conjunction with a reaction vessel for converting a substance in a high - pressure process . the substance to be converted flows through the reaction vessel 25 in the direction indicated by the arrow 26 . beside the reaction vessel 25 , there are measuring devices 27 , 28 and 29 and possibly further measuring devices which are arranged in a distributed array along the longitudinal axis of the reaction vessel 25 and by means of which , after a radioactive substance has been injected , the path of movement thereof , the residence time and so forth in the product flow in the reaction vessel can be followed . the kind and nature of substance to be injected generally depend on the nature of the phase in which the product flow to be investigated occurs . depending on whether the product flow is in gaseous , liquid , possibly pasty or pulpy or solid form , the above - described apparatus can also be used for injecting gas , liquid or solid . an embodiment of the process provides the following mode of operation , using the injection apparatus shown in fig1 and the storage container 17 : the steps of : casting the closure plug or stopper 19 of a fusible material in the upper slightly conical portion of the tube 1 , with subsequent setting of the plug or stopper ; introducing coal powder into the tube 1 ; closing the tube 1 at the other lower end by means of a wax plug or stopper 20 , with subsequent setting thereof , in which connection the plug 20 can be prevented from falling out by means of a groove formed in the tube member , thus providing a form - locking or positive securing action ; introducing the tube 1 into the container 17 ; and closing same by means of the screw plug 21 . it will be appreciated that the tube 1 can also be charged when it is already in the container 17 . the steps of : dropping the suspension when of high viscosity into the tube 1 which has been introduced into the storage container 17 ; closing the tube 1 with the closure plug or stopper 21 ; when using a low - viscosity suspension , cooling the outer casing of the container to cause the suspension to solidify , while at the same time , due to the prestressing effect , producing a metallic sealing action at the opening 31 , by the pressure applied by the plug or stopper 21 to the tube 1 , between the two components . 2 . irradiation of the container 17 with the tube 1 therein , in a neutron field , to activate the sample 18 which is serving as a marking substance . when the wax closure members 19 and 20 are employed , there is no need for cooling ( for example with dry ice ); opening of the plug or stopper 21 ; pushing the tube 17 with its content out of the container 1 by means of the plug or stopper 24 using a suitable , tool ( not shown ) through the end opening 31 of the container 17 ; inserting the tube 1 with its discharge opening 4 leading into the housing 2 of the injection apparatus by means of a pair of pliers or the like with the high - pressure closure arrangement 10 in an open condition , through the opening 6 and into the upper or forward portion 7 of the space 8 within the apparatus ; and fixing the tube 1 in the housing 2 by a clamping ring 9 . the steps of : introducing the container 17 with tube 1 into a dismantling apparatus ( not shown ); cooling the content of the tube 1 by surface contact with a cooling jacket whereby the suspension representing the sample solidifies ; dismantling the container 17 ; and introducing the tube . the steps of : opening the feed line for the injection agent which is to be introduced through the port 14 and applying a preliminary pressure of the order of magnitude of 20 bars by means of the injection agent in the space 8 within the housing ; heating the injection apparatus 2 by heating means 33 ; after the closure plugs or stoppers 19 and 20 have melted , increasing the pressure of the injection agent by way of the connecting line and the port 14 in the space 8 within the housing to a pressure which is higher than the pressure in the reactor 25 ; opening the connecting valve 35 ; and injecting the substance 18 by means of the injection agent which flows through the tube 1 . when using a coal - oil suspension which has set within the tube 1 , the heating means 33 causes that suspension to melt , instead of the closure plugs or stoppers 19 and 20 . in other respects the manner of performing the injection operation corresponds to the manner of performance when using coal powder . when injecting a gas , the mode of operation may be the same as when injecting a powder . in that respect , the tube or sleeve member 1 may also be closed by means of diaphragms instead of the plugs or stoppers 19 and 20 . after the tube has been introduced into the housing 2 of the injection apparatus and after closure thereof , the diaphragm can be ruptured by a shock pressure from the port 14 by means of an auxiliary agent , for example an inert oil , whereupon the gas is injected into the reaction vessel or some other piece of apparatus . it will be seen from the foregoing description that the process and the apparatus described are simple and straightforward , permitting easy handling of the materials involved . the apparatus is of such a nature as to provide for safe and easy handling of the radioactive substance in the environment of a hot reaction vessel which is under a high internal pressure , while also complying with the relevant regulations to be observed in relation to the use of radioactive materials . various other modifications and alterations may be made in the above - described process and apparatus without thereby departing from the scope of the invention as defined by the appended claims .