preferred embodiments of the present invention will now be described . in particular , the embodiments of the present invention include a chiller as shown in fig3 - 4 , a reaction vessel as shown in fig5 - 11 and the overall apparatus for conducting reactions at a low temperature as shown in fig1 . the skilled person will appreciate that whilst the chiller and reaction vessel of these embodiments are described as being used together , each can also be used with alternative additional components . fig3 shows a cross - section through a chiller 10 according to an embodiment of an aspect of the present invention . the chiller 10 comprises a cavity 12 which can be of any size and shape but typically , as shown , is essentially cylindrical with a capacity of approximately 2 litres . the cavity 12 contains , in use , a plurality of lumps of a coolant substrate 14 . these lumps of coolant substrate are preferably irregularly shaped so that , when placed in the cavity 12 , they do not fit perfectly and have gaps between them to allow for gas flow through the coolant substrate as a whole and around the individual lumps 14 as indicated by the arrows g . the cavity 12 has an entry port 16 located near the top of the cavity and an exit port 18 located near the bottom of the cavity to allow for flow of gas through the cavity and past the coolant substrate . preferably the exit port 18 is located close to , but not directly at , the lowest point of the cavity when the chiller is placed on a level surface , so that the exit port 18 is not blocked by any dirt or other solid matter that may end up in the cavity 12 . whilst the chiller 10 shown in fig3 has the entry port 16 at the top of the cavity 12 and the exit port 18 at the bottom , and this is the preferred arrangement for thermal circulation reasons , it is not necessary for this arrangement to be adopted . indeed , the entry and exit ports could be reversed . it is preferable that the entry and exit ports are located at opposite ends of the cavity 12 so that the gas passing through the cavity 12 has to pass over the maximum amount of coolant substrate 14 before exiting the cavity 12 , thereby achieving the greatest cooling effect possible on the gas . the preferred coolant substrate is dry ice ( solid carbon dioxide / co 2 ). however the coolant substrate may be made from different substances , depending on the desired output temperature of the chilled gas and the resources available . for example , for reactions in conditions only slightly below ambient , and above 0 ° c ., ice could be used as the coolant substrate . alternatively , the coolant substrate could be metal or ceramic which has been cooled to a predetermined temperature prior to being placed in the chiller . the advantages of using dry ice as the coolant substrate are that it has a well - defined and unchanging surface temperature (− 78 ° c .) at which the solid co 2 sublimes to gaseous co 2 . in contrast , a non - expendable coolant substrate , such as a metal or a ceramic , will have a varying surface temperature over the time it is in use as heat is transferred to it from the passing gas . further , since co 2 sublimes directly to a gaseous form , the evaporated co 2 can be immediately entrained in the gas flow over the remaining coolant substrate and out of the chiller , and so does not block the exit port 18 . in contrast , if ice was used as the cooling substrate , it would melt and the resultant liquid water would have to be periodically removed from the cavity 12 to ensure that the gas flow was not obstructed . it has been further found that using dry ice as the coolant substrate allows a highly uniform temperature of the cooled gas exiting the chiller and being conducted to the reaction vessel , and that an exit gas temperature of approximately − 76 ° c . can be achieved even after the gas has travelled up to 1 metre along an insulated pipe from the exit port 18 . this can only be achieved if the exit port 18 has a minimum diameter of 3 . 5 mm . as a result of having an exit port 18 with this minimum diameter , it has been found that the flow of gas over the dry ice not only results in a cooling , by conduction , of the gas itself , but in the entraining of small particles of dry ice . these entrained particles subsequently sublime to gas and , in doing so , maintain a low temperature of the cooled gas . the cavity 12 has an opening 20 at the top of the chiller 10 to allow the coolant substrate 14 to be placed into the cavity 12 , and the cavity 12 to be cleaned if required . this opening 20 can be sealed , for example by a screw - in lid 22 as shown in fig3 , both to seal the cavity 12 at a pressure above atmospheric and to insulate the cavity 12 . the lid 22 is insulated . fig4 shows a perspective view of the upper portion 30 of the chiller 10 with the flask portion removed . in the embodiment shown , two exit ports 18 are provided to promote uniform gas flow through the chiller 10 and reduce the amount of stagnation in the lower portions of the chiller 10 . further exit ports can be provided , although additional ports will require additional pipes 32 which conduct the chilled gas from the exit ports 18 to the reactor vessel , and the space at the top of the chiller 10 may restrict the number of such pipes 32 . the exit ports 18 are formed as angled cuts through the end of pipes 32 which conduct the chilled gas from the inside of the chiller 10 to the reactor vessel . this configuration of the exit ports 18 minimises the risk of all of the exit ports 18 being blocked by the coolant substrate 14 . the cavity 12 is insulated by an outer shell 24 . in the preferred embodiment shown in fig3 , this outer shell 24 encloses a vacuum chamber 26 which surrounds the cavity 12 and insulates it from the surrounding air . alternatively or additionally , the outer shell may be made from insulating material ( material with a low heat conductivity ) such as a solid foam . the inside surface 28 of the cavity 12 may have a reflective coating to further insulate the contents of the cavity 12 . typical chillers according to embodiments of the present invention have cavities with capacities of between 1 and 5 litres , particularly between 2 and 3 litres . a chiller with a capacity of 2 litres or similar according to an embodiment of the present invention can therefore be provided which can be set on the laboratory bench adjacent the reactor vessel in which the low temperature reaction is being conducted . this is not only convenient in that it avoids pipes carrying coolant from running across other parts of the laboratory , but also minimises the importance of insulation of such pipes and / or the temperature gained by the coolant between its point of lowest temperature and the reaction vessel . furthermore , if it is desired to conduct multiple low temperature reactions in series , multiple chillers can be provided without significant constraints on space . the output from the reaction chamber of one chilled reaction vessel can be fed into the reaction chamber of a second chilled reaction vessel , and each reaction vessel cooled by a supply of chilled gas from a separate chiller according to an embodiment of the invention . as shown in fig1 , the entry port 16 is connected by a pipe 34 to a regulator 32 which is connected to the pressurised gas source 36 . the regulator 32 acts to reduce the pressure of the gas from the pressurised gas source 36 to a lower pressure . in the present embodiment , the regulator 32 acts to reduce the pressure from approximately 10 bar in the pressurised gas source ( the pressure supplied from a typical pre - regulated compressed gas cylinder — nitrogen in cylinders is typically stored at 230 bar , but regulated at the cylinder exit to 10 bar ) to 0 . 5 bar gauge pressure ( i . e . the overpressure above atmospheric ) for supply to the chiller 10 and onwards through the apparatus . there are two principal reasons for the use of the regulator 32 . the first is that , in many jurisdictions ( including the uk ), there are safety regulations which place requirements on any sealed vessel which is intended to contain a gas above a certain pressure ( the applicable uk regulations are the simple pressure vessels ( safety ) regulations 1991 , which set a limit of a gauge pressure of 0 . 5 bar , above which the vessel must satisfy additional requirements as to its construction and certification ). since the apparatus of this embodiment uses the pressure of the supplied gas to cause flow through the apparatus , but does not require pressures above 0 . 5 bar gauge pressure to achieve this flow , the chiller can be made simpler , cheaper and safer by regulating the pressure of the gas supply at the start of its flow through the apparatus . the second reason for regulating the pressure of the gas supply is to provide for efficient use of the gas supply . if the gas supply was connected to the apparatus and not regulated , then the volume flow rate of gas through the apparatus would be significantly greater . this would not only deplete the pressurised gas source 36 faster ( necessitating more regular changes of the gas source where it is an individual cylinder , or more regular refills of a larger store ), but would also reduce the cooling effectiveness of the gas in the reaction vessel , since the gas would spend a shorter time in contact with both the coolant substrate 14 and the surfaces of the reactor chamber 56 in the reaction vessel 50 . therefore , whilst regulation of the input gas to a gauge pressure of less than 0 . 5 bar is applicable in the uk , alternative pressures may be selected for use of the apparatus in different countries , but it is advantageous in any event that the regulator reduces the pressure of the pressurised gas source to a gauge pressure of less than 1 . 0 bar , more preferably to a gauge pressure of less than 0 . 7 bar . operating at a gauge pressure of 0 . 5 bar , and maintaining a demand temperature of − 40 ° c . for the reaction chamber , an apparatus such as that shown in fig1 , and described below , can operate using only approximately 0 . 2 kg of dry ice per hour . any appropriate source of pressurised gas can be used with the chillers and reaction vessels of the present invention . ideally the pressurised gas is dry , so that there are no problems with icing when the gas is cooled below 0 ° c . the preferred source of pressurised gas is compressed nitrogen . compressed nitrogen is readily available in cylinder form and already in use in most laboratories and is dry and does not contain any component which will solidify or condense in the temperature range of reactions generally being considered . compressed nitrogen is also stored in larger quantities by some companies . alternatively , compressed dry air may be used . as shown in fig1 , the exit port 18 from the cavity 12 is connected by an insulated pipe 98 to the reaction vessel 50 . fig5 shows a perspective view of a reaction vessel 50 according to an embodiment of the invention with the outer casing and supporting structure removed . fig6 is the same perspective view , but with the reactor chamber removed to show further detail of the inner components . fig7 is the same perspective view as fig5 , showing the temperature sensor . fig8 is a plan view of the reaction vessel 50 with the outer casing removed . fig9 shows a cross - section through the reaction vessel 50 along the line marked a - a in fig8 and showing the reaction vessel 50 in complete form . reaction vessel 50 is generally cylindrical and has a base 51 on which a set of concentric components are mounted . viewed as a whole in fig9 , the reaction vessel 50 comprises , from the inside outwards , a central chamber 52 , a centrifugal fan 54 , a reaction chamber 56 which is an elongate tube wound around a series of posts 58 , an exhaust cavity 60 , an inner shell 62 , an insulating vacuum cavity 64 , an outer shell 66 , and a support casing 68 . the volume contained inside the inner shell 62 will be referred to as a circulation chamber 70 . a cooling medium , which may be liquid nitrogen , crushed dry ice or cold gas from a chiller ( preferably a chiller such as that described in the embodiment above ) is supplied to supply port 94 in the reaction vessel 50 which feeds into the central chamber 52 where it cools the gas within that central chamber , for example by evaporating or subliming , or by mixing with gas within the central chamber . the size and position of the supply port 94 may differ in particular depending on the form of the cooling medium being used . the centrifugal fan 54 is driven by an electric motor 72 and causes gas from the central chamber to be forced outwards past the reaction chamber 56 to the exhaust cavity 60 . a return passage 74 allows gas arriving in the exhaust cavity 60 to be returned to the central chamber 52 thereby creating a circulation pathway indicated on fig9 by arrows f of cold gas around the circulation chamber 70 and a constant flow of cold gas past the reaction chamber 58 . exhaust ports 90 are connected to the exhaust cavity 60 and allow a flow of gas out of the circulation chamber 70 through an exit cavity 92 to exit ports 95 to the outside , thereby keeping the pressure inside the circulation chamber 70 constant . the centrifugal fan 54 is made from a conductor such as aluminium ( or any other metal , although metals with lower specific heat capacities are preferable as they allow for more rapid changes of operating temperature ), which ensures that it is at uniform temperature and once it has been initially cooled to the temperature of the gas circulating in the circulation chamber 70 it does not affect the temperature of the gas passing through it . the fan 54 rotates at high speed ( preferably between 1 , 000 and 5 , 000 rpm , preferably between 2 , 000 and 4 , 000 rpm and typically at 2 , 500 rpm ) and as well as forcing circulation of the gas inside the circulation chamber 70 agitates the gas to ensure that the flow in the circulation chamber is relatively uniform over all parts of the reaction chamber 56 , and is non - laminar , thereby ensuring uniform temperature distribution of the outer surface of the reaction chamber 56 ( and consequently along the length of the inside of the reaction chamber 56 ). the reaction chamber 56 is an elongate tube through which the reagents flow whilst the reaction is being conducted . by using an elongate tube as the reaction chamber 56 , the time for which the reagents are mixed in the reaction chamber 56 can be controlled by controlling the flow rate of reagents into the chamber or by changing the reaction chamber for one having a different internal volume . the elongate tube provides for a large surface area of the reaction chamber 56 , which provides for the maximum area for heat transfer between the cold gas in the circulation chamber 70 and the reaction chamber 56 . this ensures that the reaction chamber 56 as a whole as well as the contents of the reaction chamber 56 are maintained at the desired temperature by the cold gas , and that there is no temperature gradient within the reaction chamber 56 . furthermore , the reaction chamber 56 can be wound in a helical manner to form a cylindrical structure as shown in fig5 , 7 and 9 , thereby allowing the reaction chamber 56 to be arranged in a compact manner and the concentric arrangement of the components of the reaction vessel achieved . as shown in fig5 , 7 and 9 , the reaction chamber 56 is wound in a weave pattern around the posts 58 . this weave arrangement maximises the flow of the cold gas in the circulation chamber 70 over the surface of the reaction chamber 56 , allowing the cold gas to pass between the coils of the reaction chamber 56 , without the need to provide additional structural support to ensure separation of the coils of the reaction chamber 56 . the weave arrangement also interrupts the direct flow of the gas from the centrifugal fan 54 and further ensures that the gas flow over the reaction chamber 56 is agitated and non - laminar . in the reaction vessel 50 shown in fig5 - 11 , the central chamber 52 contains two mixing vessels : a pre - mixing vessel 76 and a quench vessel 78 . as these mixing vessels are within the circulation chamber 70 , they are also cooled to the desired reaction temperature by the passage of the gas around the circulation chamber 70 . the pre - mixing vessel 76 allows combination and mixing of the reagents prior to their passing through the reaction chamber 56 . an output 80 from the pre - mixing vessel 76 is therefore connected ( connection not shown ) to the reaction chamber 56 and the pre - mixing vessel 76 has a plurality of inputs 82 for the supply of reagents . the quench vessel 78 allows the reaction being carried out in the reaction chamber 56 to be quenched before the reaction mixture leaves the low temperature environment of the reaction vessel 56 , thereby preventing incomplete reactions suddenly being accelerated by the passage of the reaction mixture to an area of higher temperature . the quench vessel 78 has an input ( not shown ) which is connected to the output end of the reaction chamber 56 , an input 84 which can be connected to a supply of quench solution and an output 86 , which is the ultimate output of the reaction vessel 50 . the quench solution is also passed through a pre - cooling tube ( as described below for reagents ) to ensure the quench solution is at the temperature of the reactor . just as it is undesirable for the reaction mixture to leave the low temperature environment of the reaction vessel 50 before the reaction has either completed or been quenched , it is also undesirable for the reagents or quench solution to be mixed at a higher temperature than the reaction is intended to be carried out at ( e . g . room temperature ) before they enter the pre - mixing vessel 76 or the reaction chamber 56 as any reaction ( which may be different to the desired reaction , or may occur at an undesirable rate ) is likely to commence immediately on mixing . accordingly , the reaction vessel 50 may also have a plurality of pre - cooling coils ( not shown ) which are coils of tubing contained within the exit cavity 92 . the cool gas exiting the exhaust cavity 60 via the exhaust ports 90 enters this exit cavity 92 and cools the reagents flowing in the pre - cooling coils before they enter the pre - mixing vessel 76 or the reaction chamber 56 ( in an arrangement in which there is no pre - mixing vessel ). in alternative arrangements , the pre - cooling coils may pass through other parts of the circulation chamber , for example in the central chamber 52 , particularly if the neither of the mixing vessels 76 , 78 is provided in that chamber , or the exhaust cavity 60 . reagents and the quench solution enter the reaction vessel 50 through input tubes 97 a and 97 b , and the output from the quench vessel 78 exits the reaction vessel 50 through output tube 97 c . in the embodiment shown , there are two input tubes 97 a for the input of reagents , a single input tube 97 b for the quench solution and a single output tube 97 c , but alternative arrangements are envisaged in which there are further input or output tubes depending on the type of reaction to be conducted in the reaction vessel 50 . power to the motor 72 driving the fan 54 is supplied through power lead 91 . the outer shell 66 and the vacuum cavity 64 insulate the circulation chamber 70 from the surrounding environment . base 51 has similar insulating properties , but uses solid insulation . the skilled person will appreciate that alternative arrangements may be provided to insulate the circulation chamber . however , insulation of at least part of the reaction vessel 50 using a vacuum cavity 64 is particularly advantageous as , when provided in conjunction with either a transparent outer shell 66 and support casing 68 ( e . g . made from glass or transparent plastic ) or viewing windows in the outer shell 66 and support casing 68 , it allows visual inspection of the circulation chamber 70 and in particular the reaction chamber 56 whilst the reaction is being carried out . to connect the support casing 68 and the base 51 of the reaction vessel , the base 51 has a number of locking lugs 67 which interact with the support casing 68 to hold the base and support casing together . a temperature sensor 88 is provided which senses the temperature of the reaction chamber 56 . the preferred arrangement uses a platinum thin film pt100 sensor , which has been found to offer a lower cost solution than a thermocouple , and also provide a greater degree of noise immunity . the temperature sensor 88 is inserted into an entry port 90 in the outer shell 66 and positioned so as to abut the outer wall of the reaction chamber 56 . in this arrangement the temperature sensor 88 does not contact the reactants flowing in the reaction chamber 56 . however , an extremely accurate temperature measurement of the temperature of the fluid passing through the reaction chamber 56 can be achieved due to the direct contact with the wall of the reaction chamber . the entry port 90 for the temperature sensor and the temperature sensor 88 interact so as to seal the entry port when the temperature sensor is in place and thus prevent chilled gas circulating in the circulation chamber 70 from escaping through the entry port 90 . a cover or similar mechanism may be provided for sealing the entry port 90 if the temperature sensor is not connected . the temperature sensor 88 provides an electrical output through cable 99 which forms an input into a controller 40 , as shown in fig1 , which is arranged to control the temperature of the reaction chamber 56 at a desired temperature for the reaction being conducted . the desired temperature can be set by a data entry into the controller 40 . this may be means of a manual input device such as a dial or buttons , or through entry of an appropriate value on a computer which is connected to or forms part of the controller 40 . in the embodiment shown , the controller 40 controls the temperature of the reaction chamber 56 by controlling the input of gas to the chiller 10 . in alternative embodiments , where the cooling medium is not supplied to the reaction vessel 50 from a chiller 10 , the controller 40 may control the temperature of the reaction chamber 56 by controlling the flow of cooling medium to the supply port 94 . this may be achieved in a number of ways . the preferred way of controlling the temperature where a chiller 10 is used is to control the amount of dry gas being input to the chiller 10 and thereby control the amount of cool gas exiting the chiller and entering the reaction vessel 56 . in the embodiment illustrated , this is achieved through the controller 40 controlling a valve 45 which permits , restricts or prevents flow of the dry gas into the chiller 10 to the reaction vessel 50 . arrangement of the valve 45 in the gas flow prior to the chiller 10 is preferred as it avoids the problems with icing of the valve ( s ) in the cold part of the gas flow . in particular , the flow of gas through the valve 45 may be pulsed , and the overall amount of gas flowing may be controlled by changing the time ratio or proportion of the “ on ” and “ off ” pulses to each other . similar pulsed arrangements may be used for the supply of liquid or gaseous cooling medium to the reaction vessel 50 from other sources , particularly where the sources are pressurised . alternatively , for a liquid cooling medium , a pump may be used which is controlled by the controller 40 to deliver known quantities of cooling medium in a continuous adjustable rate or through a pulsed arrangement . a further alternative , for a solid cooling medium , is a augur filled with said solid cooling medium , the augur being controlled by the controller 40 to deliver known quantities of the cooling medium in a continuous , adjustable rate or in a quantised manner , for example by using a stepper motor to drive the augur . fig1 shows a graph of temperature measurements taken during testing of the apparatus shown in fig1 . in particular , fig1 shows three traces which represent the measured temperatures at the chiller 10 and two readings from temperature sensors located in the reaction vessel 50 . except where the readings substantially overlap ( readings up to 55 minutes on the x - axis of fig1 ), the upper reading is that from the chiller 12 . the temperature profiles of fig1 show the performance of the apparatus , starting from an ambient temperature of 25 ° c ., in response to a series of demand temperatures , as set by the controller 40 , of respectively − 70 ° c . ( from time zero on the x - axis of fig1 ), − 60 ° c . ( from time 45 minutes ), − 40 ° c . ( from time 55 minutes ), − 20 ° c . ( from time 75 minutes ) and 0 ° c . ( from time 110 minutes ). from fig1 it can be seen that the apparatus as a whole reaches an operating temperature of − 70 ° c . in approximately 20 minutes from the demand for that temperature being set . this is a considerable improvement over known chiller apparatuses , where such temperatures in a reaction vessel can only be achieved 30 - 120 minutes from the low temperature being demanded . the effect of pulse control of the gas input to the chiller can be seen in the “ saw - tooth ” temperature readings at each of the steady temperatures . furthermore , each demand temperature is reached without appreciable overshoot in either the positive or negative direction , and so the apparatus of this embodiment demonstrates the high degree of temperature control that can be achieved by using an apparatus according to the above embodiment . in order to increase the response rate of the apparatus to demands for increases in the temperature in the reaction chamber , it is possible to provide an intermediate heater between the chiller and the reaction vessel , rather than relying on heating from the surrounding environment to raise the temperature . however , as the response times achievable are generally comparatively rapid , such an arrangement is generally not necessary . the rapid responses and accuracy of the temperature control result from the use of chilled gas as a coolant , which is much more controllable than a liquid coolant , and in particular has a lower unit heat capacity , so the gas in the circulation chamber of the reaction vessel is able to change temperature rapidly without using an additional heater . also , by arranging the temperature sensor as a contact sensor on the outer wall of the reaction chamber , the controller is able to accurately control the input of chilled gas to regulate the temperature of the gas circulating in the reaction vessel .