reference will now be made in detail to the present preferred embodiments of the invention which are illustrated in the accompanying drawings . in accordance with the invention , the present invention for a silica manufacturing apparatus , fig1 schematically depicts a system for delivering liquid siloxane feedstock and , optionally , dopant - providing compounds to a conversion site where the liquid siloxane feedstock is converted into silica . the conversion from liquid feedstock to silica occurs preferably in an area proximate a burner 10 , which is preferably a combustion burner . a liquid siloxane feedstock such as a polymethylcyclosiloxane is stored in feedstock tank 11 . feedstock tank 11 is connected to liquid feedstock injector 15 at the reactant introduction site via a liquid feedstock transporting conduit system that can , if desired , include metering pump 12 , optional filter 13 , and preheater 14 . said liquid feedstock transporting conduit has a first terminal end 17 connected to the liquid feedstock tank 11 , and a second terminal end 18 proximate the liquid feedstock injector 15 . the siloxane feedstock liquid from tank 11 is transferred through the liquid feedstock transporting conduit by pump 12 through filter 13 to optional preheater 14 . the liquid delivered through filter 13 is under sufficient pressure to substantially prevent and inhibit its volatilization in preheater 14 , which is optionally employed to warm the liquid reactant prior to its introduction into burner 10 and avoids the high temperatures of a vaporizer which normally promote gel formation . the burner preferably is conventionally provided with inner shield gas , outer shield gas , and a mixture of methane and oxygen for the flame , as described , for example , in u . s . pat . no . 4 , 165 , 223 to d . r . powers , which is hereby relied upon and incorporated by reference . the liquid reactant is conveyed from optional filter 13 or optional preheater 14 through second terminal end 18 to liquid injector 15 , which delivers the liquid to burner 10 . injector 15 comprises a device for delivering the liquid reactant , either as a liquid stream or as atomized liquid particles , directly into the flame of burner 10 . we generally refer in the discussion to the reactant as being in &# 34 ; liquid &# 34 ; form . what we mean by that expression is that the reactant is in a substantially liquid state . some small portion of the reactant may be in vapor form , particularly where preheater 14 is employed , or where a nitrogen blanket over the liquid is employed . a small portion of the reactant can be in vapor form as delivered to the combustion site without adversely affecting the operation of the invention . liquid injector 15 can comprise , for example , a syringe provided with a fine needle , by which a liquid stream can be injected at high velocity into the burner flame . although a syringe can be used on a small scale , commercial operations will require a reasonable large scale equivalent , e . g ., an atomizer . several types of atomizing means capable of forming very small particles of liquid are known in the atomization art as disclosed in atomization and sprays , by arthur h . lefebure , hemisphere publishing co ., 1989 , which is incorporated herein by reference . atomizers can be operated by various energy sources such as liquid , gas , mechanical , electrical and vibrational , and may be categorized as , for example , jet , swirl , jet - swirl , pneumatic , rotary , acoustic , ultrasonic , and electrostatic . preferably , a jet atomizer is used ; even more preferably , the jet atomizer is a swirl - jet atomizer , which swirls the liquid and then , as atomizers generally do , squirts the liquid at high velocity out of a small orifice . various types of atomizers are discussed in liquid atomization , by l . bayvel and z . orzechowski , taylor & amp ; francis , ( 1993 ), which is hereby incorporated by reference . another preferred type is a pneumatic atomizer operated by inert gas , oxygen gas , a mixture of oxygen and an inert gas , air , or combustible gas pressure . by inert gas , we mean a gaseous element such as nitrogen , argon , or helium , which is nonreactive under ordinary conditions . by combustible gas , we mean a gas that is typically used in the combustion of silica precursors , such as methane , or a mixture of methane and oxygen . in particularly preferred embodiments , the atomizer can be incorporated into the structure of the combustion burner . the atomized particles of the siloxane reactant compound are combusted in a burner fueled by a combustible gas , preferably , a combination of methane and oxygen . the atomized reactant particles can be conveyed from the atomizer to the burner flame by a carrier gas such as an inert gas , oxygen , a mixture of an inert gas and oxygen , or a combustible gas . fig2 is a cross - sectional view of an embodiment of the burner and atomizer apparatus used with the present invention . here , burner 40 incorporates within its structure atomizer 41 , which injects very finely atomized liquid reactant particles 42 into flame 23 . amorphous soot 25 produced by combustion of the liquid reactant may be collected on rotatable mandrel 26 to form a soot blank or preform which may be used to fabricate an optical waveguide . in an alternative embodiment , silica soot may be collected for subsequent consolidation , or the silica soot may be collected in a collection chamber in which the soot is immediately consolidated into its desired shape ( not shown ). as shown in fig2 burner 40 comprises a series of concentric channels surrounding atomizer 41 . liquid siloxane is delivered through atomizer 41 . a stream of an inert gas such as nitrogen gas , a mixture of oxygen gas and nitrogen gas , or oxygen gas alone delivered through channel 43 atomizes the liquid feedstock by the kinetic energy of the flowing gas to create liquid projections 42 which are converted into soot reactant particles in burner flame 23 . the area proximate to the burner face 53 and flame 23 thus acts as a conversion site for converting liquid projections 42 into soot reactant particles . oxygen gas may be delivered to flame 23 through channels 45 and 46 . an inert gas , such as nitrogen , argon or helium is delivered through channel 44 to inhibit reaction of the liquid feedstock and soot build - up on burner face 53 . applicants have found that when oxygen or a mixture of oxygen and inert gas is used as the atomizing gas , better results are obtained by delivering an inert gas through channel 44 . a premix of oxygen and a fuel such as methane is conducted to the flame through outermost channel 47 . in embodiments in which the silica soot blank is used to form optical waveguide fibers , a burner fitted with an atomizing injector , such as the embodiment depicted in fig2 produces a wide soot stream , which achieves improved concentricity of core and cladding regions of the subsequently formed optical waveguide fiber . a preferred atomizer unit 40 of the invention as shown in fig2 comprises a pneumatic atomizer . with such a pneumatic atomizer , the liquid siloxane feedstock is atomized by the kinetic energy of a flowing gas stream through the channel 43 . high velocity gas is utilized in atomizing the feedstock . this produces atomized liquid projections 42 with a gas velocity in the range of 0 . 5 to 50 . 0 m / sec . an inert gas , such as nitrogen or argon gas , oxygen gas , a combustible gas , or a mixture of oxygen and an inert gas may be used as the atomizing gas . with the use of the pneumatic atomizer in the invention , the high velocity blast gas is deployed in an effective manner to achieve a beneficial level of atomization of the siloxane proximate the burner face and in the flame . in practicing the invention , even though it is preferred to have atomizer unit 40 as an integral part of the burner unit , it is possible to use a pneumatic atomizer which is spatially separated from the burner , as with atomizers shown in fig2 and 3 of copending application ser . no . 08 / 767 , 653 . referring now to fig3 a preferred embodiment of the apparatus of the present invention includes a burner 40 , with the details of the burner similar to the burner shown in fig2 which produces a deposition flame 23 to decompose the liquid projections 42 produced by the atomizer ( not shown ) into soot particles . a heater mount 60 supports a heater 62 , which helps vaporize the liquid projections 42 , consequently allowing for the velocity of the atomizing gas velocity to be reduced . the additional heat provided by heater 62 will aid in vaporizing the larger liquid projections in the spray size distribution produced by the atomizer . the heater 62 may be a ceramic ring or tube which surrounds the flame 23 , and may further have an auxiliary heating source such as an electrical heater or a gas - fired heater . tube heater temperatures from about 800 ° c . to about 1500 ° c . are preferred , with a more preferred range from about 950 ° c . to about 1250 ° c . using a ceramic ring or tube heater with the heater temperature at about 1000 ° c . to about 1200 ° c . allows for the atomizing gas velocity to be reduced by about 50 % compared with a burner in which no heater was used . fig4 shows an alternative embodiment of the present invention . a burner 40 , produces deposition flame 23 , which will decompose the atomized liquid projections 42 exiting from the atomizer ( not shown ). in this embodiment , heater mount 60 supports an enclosure 64 , which retains heat from the burner flame 23 , and thus enclosure 64 functions as a heater . enclosure 64 can be made out of any suitable heat retaining material such as a ceramic material or refractory brick . in still another embodiment ( not shown ), the burner flame could be surrounded by a ring or tube heater having an additional heater source , and the entire structure could be enclosed by an enclosure made from a heat retaining material . in addition to effectively reducing liquid projection size in the spray , additional heat provided by the heater 62 can increase soot capture rate by increased thermophoresis . thermophoresis is the process by which soot is attracted to the preform . in fact , it produces the driving force which moves the particles towards the cooler preform . the hot gases from the burner pass around the preform during laydown ; the soot particles do not have sufficient momentum by combustion alone to strike the preform . thermophoresis moves particles in a temperature gradient from hot regions to cooler regions . the burnt gases from a burner are hotter than the preform . as these gases pass around the preform , a temperature gradient is produced . hot gas molecules have higher velocity than cold gas molecules . when hot gas molecules strike a particle , they transmit more momentum to the particle than a cold gas molecule does . thus , particles are driven towards the colder gas molecules and , in turn , toward the preform . it is believed that the soot particles , after being provided with additional heat from the heater , will have a larger driving force to attract them to a relatively cooler target . the heater also reduces the lizard skin defect on the surface of a soot blank or preform used for optical waveguides . by using a heater with a liquid burner in accordance with the present invention allows for reduced atomizing gas velocity which reduces flame turbulence providing for better quality soot and soot preforms . the apparatus can also be provided with dopant supply tank 16 , shown in fig1 which contains a compound capable of being converted by oxidation or flame hydrolysis to p 2 o 5 or to a metal oxide whose metallic component is selected from groups ia , ib , iia , iib , iiia , iiib , iva , ivb , va , and the rare earth series of the periodic table . these oxide dopants combine with the silica generated in the burner to provide doped silica glass , which can be subsequently formed into optical waveguide fibers . the compound that provides the silica glass dopant can be provided to feed tank 11 from dopant supply 16 of fig1 . alternatively , the dopant can be delivered from supply 16 to liquid injector 15 via a separate metering pump and optionally a filter ( not shown ) analogous to the delivery system used for the silicon - containing compound . in accordance with the invention , the preferably halide - free , silicon - containing reactant compound preferably comprises a polyalkylsiloxane , for example , hexamethyldisiloxane . more preferably , the polyalkylsiloxane comprises a polymethylcyclosiloxane . most preferably , the polymethylcyclosiloxane is selected from the group consisting of hexamethylcyclotrisiloxane , octamethylcyclotetrasiloxane , decamethylcyclopentasiloxane , dodecamethylcyclohexasiloxane , and mixtures thereof . the invention has been described in detail for the purpose of illustration , but it is understood that such detail is solely for that purpose and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention , which is defined by the following claims .