Yarns and like materials are treated with gaseous fluid under pressure by a process comprising supplying the fluid into a passage having a diverging part at such a rate that expansion of the fluid as it passes along the <PICT:0623308/IV (a)/1> <PICT:0623308/IV (a)/2> <PICT:0623308/IV (a)/3> diverging part gives rise to an increase in the velocity of the fluid, introducing the yarn or like material into the fluid in the region of the point of its maximum velocity, and causing the material to travel with the fluid along the part of the passage beyond this point, in which part the velocity of the fluid falls and its pressure rises so that the material is subjected to the action of the fluid under the increased pressure thereof. The process may be employed for stretching yarns or ribbons of cellulose acetate or other organic derivative of cellulose, or yarns or ribbons of polyamides or polyvinyl compounds in presence of a gaseous softening agent such as saturated or wet steam. The diverging part of the passage for the fluid may be entered by way of a convergent throat in which the fluid attains the velocity of sound. The velocity is increased beyond this value in the diverging part. In order to make the pressure of the fluid rise and its velocity fall beyond the point where the yarn enters, the passage converges for some distance beyond that point. The convergence may lead into a narrow tube through which the yarn passes while being treated with the fluid under pressure. If desired, the passage after converging may diverge again to secure a further fall of velocity and a rise of pressure which is maintained by providing a constricted outlet orifice from which the gaseous fluid and the material can emerge together at atmospheric pressure. After the convergence, the passage may be of constant diameter for some distance over which pressure and velocity remain constant. The process is such that flow of gaseous medium through the yarn inlet in a direction opposite to the direction of the yarn is prevented or minimized. The rapid flow of fluid at the point of entry of the yarn serves to draw the yarn forward into the inlet. As shown in Fig. 1, the apparatus comprises yarn input section 11, tube 12, parallel tube 13, and yarn output section 14. Steam enters under pressure through passage 24 and annular groove 23 to the space surrounding hollow needle 15. It then flows into the passage between the needle and the inner wall of the converging part 19, this passage diminishing in cross-section for a short distance till it reaches the throat of the passage at the shoulder 16 of needle 15. The passage then increases gradually until the tip of the needle is reached, at which point the area increases suddenly. The passage converges to the throat of tube 12 and then increases in size until the parallel tube 13 is reached. The treatment chamber is completed by the converging tube 28 and orifice 29. The treatment chamber is disposed between feed rollers and draw-off rollers which may be driven so as to stretch the yarn. As shown in Fig. 4, the treatment is carried out in a long narrow tube 65 having a converging entry 66 into which a yarn input nozzle 67 enters. This nozzle has a parallel portion and a tapered tip, the shoulder 68 formed between them entering the convergent entry 66. High pressure steam enters through passage 73 and annular space 74 to the passage between the cylindrical part of nozzle 67 and the flared entry 66 of tube 65. The steam flowing between shoulder 68 and the tip of the nozzle has an increasing velocity exceeding the acoustic velocity attained near the shoulder 68. Just beyond the tip of the nozzle 67, the pressure is at a minimum and the velocity at a maximum. The velocity then decreases and the pressure increases owing to the convergence of the passage until the narrow part of tube 65 is reached. Several stretching or other treatment chambers may be arranged in series and they may be built up as a single composite device, as shown in Fig. 5. If it is desired to subject the yarn to a period of relaxation, the stretching device may be associated with a similar device in which the fluid is directed in the reverse direction. If desired, the yarn input nozzle may be surrounded by two or more annular fluid-inlets. The supply pressure may be adjusted so that at least the major part of the stretching action is exerted by the flow of the steam itself, acting frictionally on the yarn. Specifications 438,584, 438,585, 438,655, 438,656, and 558,958, are referred to.ALSO:<PICT:0623308/IV (c)/1> <PICT:0623308/IV (c)/2> <PICT:0623308/IV (c)/3> Yarns and like materials are treated with gaseous fluid under pressure in apparatus in which the fluid is supplied to a passage having a diverging part at such a rate that expansion of the fluid gives rise to an increase in the velocity of the fluid, means being provided for introducing yarn or like material into the fluid in the region where the fluid attains its maximum velocity, and for causing the yarn to travel with the fluid along the part of the passage beyond this point, in which part the velocity of the fluid falls and its pressure rises so that the yarn may be subjected to the action of the fluid under the increased pressure thereof. The arrangement may be used for stretching yarn or ribbon of cellulose acetate, polyamides, or polyvinyl compounds, in presence of a gaseous softening agent such as saturated or wet steam. The diverging part of the passage for the fluid may be entered by way of a convergent throat in which the fluid attains the velocity of sound. The passage may converge for some distance beyond the point where the yarn enters, so as to make the pressure of the fluid rise and its velocity fall. The convergence may lead to a narrow tube through which the yarn passes while being treated with the fluid under pressure. After converging, the passage may diverge again to secure a further fall of velocity and a rise of pressure, which is maintained by providing a restricted opening for the gas and yarn to emerge from the enclosure. After the convergence, the passage may be of constant cross-section for some distance over which pressure and velocity of the fluid remain constant. The rapid flow of gas at the point of entry of the yarn serves to draw the yarn into the inlet, and there is substantially no flow of gas through the yarn inlet in the opposite direction to the yarn. As shown in Fig. 1, the device comprises yarn input section 11, tube 12, parallel tube 13, and yarn output section 14. Steam enters through passage 24 and groove 23 to the space around hollow needle 15. It then flows into the passage between the needle and the inner wall of the converging part 19, this passage diminishing for a short distance till it reaches the throat formed by the shoulder 16 of needle 15. The passage then increases gradually to the tip of the needle, at which point the cross-sectional area increases suddenly. The passage converges to the throat of tube 12 and then increases until parallel tube 13 is reached. The passage is completed by the converging tube 28 and orifice 29. The device is disposed between feed and draw-off rollers which may stretch the yarn. As shown in Fig. 4, the treatment is carried out in a long narrow tube 65 having a converging entry 66 into which a yarn input nozzle 67 enters. This nozzle has a parallel portion and a tapered tip, the shoulder 68 entering the convergent entry 66. High-pressure steam enters through passage 73 and annular space 74 to the passage between the cylindrical part of nozzle 67 and the flared entry 66 of tube 65. The steam flowing between shoulder 68 and the tip of the nozzle has an increasing velocity exceeding the acoustic velocity. Just beyond the tip of nozzle 67 the pressure is at a minimum and the velocity at a maximum. The velocity then decreases and the pressure increases owing to convergence of the passage until the narrow part of the tube 65 is reached. Several stretching or other treatment chambers may be arranged in series and they may be built up as a single composite device as shown in Fig. 5. The supply pressure may be adjusted so that at least the major part of the stretching action is exerted by the flow of the steam or other gas acting frictionally on the yarn. If it is desired to subject the travelling yarn to a period of relaxation, the stretching device may be associated with a similar device in which the fluid is directed in the opposite direction. If desired, the yarn input nozzle may be surrounded by two or more annular fluid-inlets. Specifications 438,584, 438,585, 438,655, 438,656 and 558,958 are referred to.