Phosphorus pentoxide and phosphoric acids are obtained by mixing phosphatic materials, e.g. calcium phosphate and siliceous materials so balanced as to form a fusible slag with excess of carbonaceous material, and so charging the mixture into a reverberatory furnace where the charge is heated to a temperature sufficient to drive off free phosphorus, for example 1200 DEG C., that an atmosphere free from oxygen or carbon dioxide is maintained within and immediately adjacent to the charge whilst the furnace is fired in such a manner as to produce through the furnace a current of oxidizing gases which sweep generally parallel to the surface of the charge without impinging thereon, wherewith the phosphorus evolved mixes and is oxidized. Alternatively, phosphorus may be obtained either alone or in admixture with phosphorus pentoxide by maintaining an oxygen and carbon dioxide free atmosphere throughout the furnace. The carbonaceous material in the charge is prefer <PICT:0387329/III/1> <PICT:0387329/III/2> <PICT:0387329/III/3> ably one containing volatile matter, e.g. bituminous coal. The process may be carried out in a reverberatory furnace 10 of Figs. 1 and 4, provided with inclined side walls 38 and a series of side charging hoppers 11 through which the charge is fed. The furnace may be fired by oil, gas, lump coal or powdered coal say from a hopper 12 via a worm conveyer 46 to a burner or burners 43, air being supplied from a pipe 13 via a header 44 and valves 45. Additional air may be supplied through a byepass 14. A greater part of the charge is fed into the furnace along the firing half thereof and a layer of phosphorus free slag 41 upon which the excess of carbonaceous material floats and a layer of ferrophosphorus 42 form and are withdrawn through the offset apertures 15, 16 respectively. The phosphorus pentoxide containing gases pass by a flue 17 to a heat exchanger 18 which preferably comprises a brickwork chamber with a series of loops 19 of iron or other metal pipe through which the air for the burners 43 passes. The metal pipes either by pre-conditioning or through the working of the furnace become covered by a protective coating comprising flue dust and phosphorus pentoxide with possibly calcium phosphates, meta-phosphoric acid, &c. This coating has a low heat conductivity compared to the metal it protects. The heat-exchanger is provided with a byepass 22. A flue dust having a high phosphoric acid content and similar to mono-calcium phosphate collects in the heat-exchanger and is suitable for fertilizers. From the heat-exchanger the gases pass through an air cooler 24 and a series of washing towers 25, 35, and 36 (Fig. 2) irrigated with phosphoric acid solution in which the gases are progressively cooled. The conditions prevailing in the first tower 25 are such that the remaining flue dust and a portion of the phosphoric acid in a form containing less water than orthophosphoric acid, e.g. pyrophosphoric acid, are condensed. The condensed material may be employed as a fertilizer. In the second tower 35 phosphoric acid containing less water than orthophosphoric acid, e.g. pyrophosphoric acid, is obtained and in the last tower 36 orthophosphoric acid is obtained. Water coolers 36<1> are provided for the gases between the towers and water coolers 32 for the liquids circulating in the towers. The acid condensed in the third tower may be allowed to overflow by a pipe 37<1> into the bottom of the second tower where it is converted in pyrophosphoric acid by the evaporation of water or condensation of acid. The hydrogen present in the firing fuel or the carbonaceous material in the charge provides water for the formation of phosphoric acids. Electrical precipitation may be employed after the washing towers to give a final cleaning action to the gases or may be employed instead of the towers on the Cottrell system, a series of chambers being employed having adjusted temperatures and impressed voltages to form a series of differential products. After the furnace has started heated air only may be supplied, the combustion of the reducing material from the charge providing the necessary heat. Specification 235,924, [Class 1 (ii), Inorganic compounds, other than &c.], is referred to.ALSO:A flue dust having a high phosphoric acid content and similar to mono-calcium phosphate collects in a heat exchanger following a reverberatory furnace in which phosphorus pentoxide is obtained by heating phosphates, silica and carbonaceous materials. The gases containing phosphorus pentoxide pass from the heat exchanger to a series of washing towers irrigated with phosphoric acid. The remaining flue dust together with some of the phosphorus pentoxide is condensed in the first tower and this material may be also employed as a fertilizer.