Fluids of different densities are passed in opposite directions through a rotating spiral conduit of progressively increasing radius, the lighter fluid being introduced at the outer end and caused to travel in the direction of the inner end, and the heavier fluid being caused to travel in the direction of increasing radius by the action of centrifugal force. The spiral conduit may be flat or conical, and a number of flat spiral conduits may be arranged side by side on a common spindle. The process may be employed for the fractional distillation of liquids such as crude petroleum, or the absorption of constituents of gases or liquids by other liquids, or for effecting reaction between liquids of different densities, such as sulphuric acid and alcohol or oil. As shown in Fig. 2, vapours from a boiling liquid contained in a still are passed up a vertical pipe 32 and a space 35 to the outer end of a conical spiral tube 25 rotating at high speed about a vertical axis. The vapour passes up the tube 25 to a chamber 17 and from there into a stationary pipe 46. After passing through a stationary reflux condenser, the remaining constituents of the vapour are condensed and collected. Condensate from the stationary condenser is returned through a pipe 52 to pipe 46 and the spiral 25. The condensate entering and formed within the spiral is prevented from blocking the passage of the vapour by means of centrifugal force which urges it to the outer side of the tube and along the spiral in the direction of increasing radius. It finally leaves the lower outer end of the spiral and passes down the stationary tubes 38, 32 to the still. The conical spiral tube 25 is supported by a conical shell 11 with baseplate 12 and downwardly tapering portion 13. Mercury seals 36, 53 are provided between the stationary and rotating parts of the apparatus. When the apparatus is at rest, the mercury in the seals flows back into the annular troughs 40, 55. As shown in Fig. 4, vapours from a boiling liquid in a still 77 pass up a tube 76, along a radial tube 75 to the outer end of a flat-spiral tube 60. They pass through the spiral to the inner end, which is upturned vertically, through a chamber in a stuffing box and then up a stationary vertical pipe 99. The spiral tube with its insulating casing 62, and the still 77 are rotated at high speed by means connected with the pulley 67. Condensate forming in the apparatus is urged by centrifugal force to the outer end of the spiral 60 and to the chamber 178 of a diaphragm pump. The chamber 178 is provided with ball check-valves. Operation of the pump causes the condensate to flow along a tube 195 which passes radially to the tube 76 and then downwardly through it into the still. A pulley 193 causes rotation of a sleeve 190 and cam 189 at a lower speed than that of the casing 62. A piston 187 is thus caused to reciprocate with a frequency corresponding to the difference in speed of the two pulleys. Pulsations are communicated to the bellows 183 and the condensate is forced along the pipe 195. The apparatus shown in Fig. 9, comprises a number of rotating spiral tubes 102, <PICT:0401484/III/1> <PICT:0401484/III/2> <PICT:0401484/III/3> <PICT:0401484/III/4> <PICT:0401484/III/5> apparatus is used for contacting two liquids, a separating chamber is secured to the pipe 114. The heavier liquid is withdrawn from the bottom through an outlet provided with a float-operated valve. As shown in Figs. 14 and 15, two similar heating vessels 130 are fixed at opposite sides of a rotating table 128 which also supports a flat spiral tube 125. The vessels 130 are heated electrically. Vapours of disc form, mounted in a casing 105 on a hollow shaft 103. Vapour or light liquid is passed under pressure through the inlet 114 into the outer ends of the spiral tubes 102 through which it passes in countercurrent to condensate or heavy liquid. It is led away through the pipe 111. Heavy fluid may be supplied at the interior of the spiral tubes through the perforated pipe 112. When the enter the outer end of the spiral 125 through inlets 132, 133 and pass in countercurrent to condensate which is urged by centrifugal force towards the outer end of the spiral and the vessels 130. From the inner end of the spiral, the vapours pass up an axially disposed vertical tube 138 and then into a stationary still head 143. The tube 138 may be cooled by air which is supplied through the pipe 144 to the jacket 142. The tube 138 is tapered in order that centrifugal force may assist gravity in urging the condensate towards the bottom and into the spiral tube.ALSO:<PICT:0401484/IV/1> <PICT:0401484/IV/2> <PICT:0401484/IV/3> Fluids of different densities are passed in opposite directions through a rotating spiral conduit of progressively increasing radius, the lighter fluid being introduced at the outer end and caused to travel in the direction of the inner end, and the heavier fluid being caused to travel in the direction of increasing radius by the action of centrifugal force. The spiral conduit may be flat or conical, and a number of flat spiral conduits may be arranged side by side on a common spindle. The process may be employed for the fractional distillation of liquids such as crude petroleum, or the absorption of constituents of gases or liquids by other liquids, or for effecting reaction between liquids of differrent densities, such as sulphuric acid and alcohol or oils. As shown in Fig. 2, vapours from a boiling liquid contained in a still are passed up a vertical pipe 32 and a space 35 to the outer end <PICT:0401484/IV/4> <PICT:0401484/IV/5> of a conical spiral tube 25 rotating at high speed about a vertical axis. The vapour passes up the tube 25 to a chamber 17 and from there into a stationary pipe 46. After passing through a stationary reflux condenser, the remaining constituents of the vapour are condensed and collected. Condensate from the stationary condenser is returned through a pipe 52 to pipe 46 and the spiral 25. The condensate entering and formed within the spiral is prevented from blocking the passage of the vapour by means of centrifugal force which urges it to the outer side of the tube and along the spiral in the direction of increasing radius. It finally leaves the lower outer end of the spiral and passes down the stationary tubes 38 and 32 to the still. The conical spiral tube 25 is supported by a conical shell 11 with base plate 12 and downwardly tapering portion 13. Mercury seals 36, 53 are provided between the stationary and rotating parts of the apparatus. When the apparatus is at rest, the mercury in the seals flows back into the annular troughs 40, 55. As shown in Fig. 4, vapours from a boiling liquid in a still 77 pass up a tube 76, along a radial tube 75 to the outer end of a flat-spiral tube 60. They pass through the spiral to the inner end, which is upturned vertically, through a chamber in a stuffing box and then up a stationary vertical pipe 99. The spiral tube with its insulating casing 62, and the still 77 are rotated at high speed by means connected with the pulley 67. Condensate forming in the apparatus is urged by centrifugal force to the outer end of the spiral 60 and to the chamber 178 of a diaphragm pump. The chamber 178 is provided with ball check-valves. Operation of the pump causes the condensate to flow along a tube 195 which passes radially to the tube 76 and then downwardly through it into the still. A pulley 193 causes rotation of a sleeve 190 and cam 189 at a slower speed than that of the casing 62. A piston 187 is thus caused to reciprocate with a frequency corresponding to the difference in speed of the two pulleys. Pulsations are communicated to the bellows 183 and the condensate is forced along the pipe 195. The apparatus shown in Fig. 9, comprises a number of rotating spiral tubes 102, of disc form, mounted in a casing 105 on a hollow shaft 103. Vapour or light liquid is passed under pressure through the inlet 114 into the outer ends of the spiral tubes 102 through which it passes in counter-current to condensate or heavy liquid. It is led away through the pipe 111. Heavy fluid may be supplied at the interior of the spiral tubes through the perforated pipe 112. When the apparatus is used for contacting two liquids, a separating chamber is secured to the pipe 114. The heavier liquid is withdrawn from the bottom through an outlet provided with a float-operated valve. As shown in Figs. 14 and 15, two similar heating vessels 130 are fixed at opposite sides of a rotating table 128 which also supports a flat spiral tube 125. The vessels 130 are heated electrically. Vapours enter the outer end of the spiral 125 through inlets 132, 133 and pass in countercurrent to condensate which is urged by centrifugal force towards the outer end of the spiral and the vessels 130. From the inner end of the spiral, the vapours pass up an axially disposed vertical tube 138 and then into a stationary still head 143. The tube 138 may be cooled by air which is supplied through the pipe 144 to the jacket 142. The tube 138 is tapered in order that centrifugal force may assist gravity in urging the condensate towards the bottom and into the spiral tube.