IDENTIFICATION SYSTEM FOR SECURITY PURPOSES Technical Field 5 The present invention relates to an identification system for security purposes. Background Art 10 Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field. Over the last century or more, a huge variety of systems and devices has been 15 proposed to assist in identifying perpetrators of crimes, especially thieves and robbers. However, many of these systems are potentially harmful to the person thus identified or to innocent bystanders. Further, many of the known systems are complex and therefore expensive to manufacture and install. 20 An additional drawback of many of the known systems is that they must be directly operated by a person on the spot; this has the practical disadvantage of requiring action to operate the system from a person who is being (for example) held up by an armed robber, and thus would be risking his life to operate the system control. It is of course safer to provide comprehensive real-time remote monitoring, so that an 25 operator can use the system from a safe place; unfortunately, this type of monitoring is extremely expensive and completely beyond the reach of small business owners. Disclosure of Invention 30 An object of the present invention is the provision of an identification system for security purposes which does not harm the person being identified and which is straightforward and inexpensive to manufacture and install. 35 A further object of the present invention is the provision of an identification system 1 capable of operating semi-automatically, without the requirement for remote monitoring. The present invention provides an identification system for security purposes, said 5 system including providing apparatus which includes: a reservoir for containing a supply of identification liquid; a source of pressurised gas; means for mixing the pressurised gas and the identification liquid to pressurize the identification liquid; a nozzle controlled by valve means, for spraying the pressurized identification liquid; remotely operable control means for controlling the mixing means; at least one motion 10 sensor arranged to control said valve means; wherein operation of said system includes the following steps: - placing the apparatus in an armed state by operating said control means such that said mixing means mixes the pressurised gas and the identification liquid; - opening said valve means to release a spray of pressurised identification liquid 15 through said nozzle only when said motion sensor senses motion in a first pre determined direction. Preferably, the apparatus includes two motion sensors arranged to control said valve means and wherein one of said motion sensors senses motion in said first 20 predetermined direction and is arranged to open said valve means when motion in said first predetermined direction is sensed; and the other of said motion sensors senses motion in the opposite direction and is arranged to keep said valve closed when motion in said opposite direction is sensed. 25 Preferably also, operation of said system also includes the step of cancelling said armed state by operation of said remote control means. Preferably, the source of pressurised gas is a sealed gas capsule and the mixing means includes a servo motor controlled by said remote control means and arranged 30 to drive a pin to pierce the sealed gas capsule, together with a one way valve arranged to allow gas released from the capsule to enter the reservoir of identification liquid and pressurise the liquid. 2 Brief Description of Drawings By way of example only, a preferred embodiment of the present invention is described in detail below with reference to the accompanying drawings, in which: Figure 1 is a block diagram showing the sequence of events included in the system of 5 the present invention; Figure 2 is a side view of the apparatus of the present invention; Figure 3 is a cross-section through part of the apparatus of Figure 2, on a larger scale; and Figure 4 is a detail of part of Figure 3. 10 Best Mode for Carrying Out the Invention Referring to the drawings, Figure 2 shows a typical mounting arrangement for the apparatus of the present invention:- a housing 10 is mounted on top of a door frame 15 (typically the main entry door of a shop or similar area) with the spray nozzle 11 extending downwards so as to be able to spray a person passing through the door. A servo-motor 38 and a solenoid valve 13, both mounted on the housing 10, are connected to a suitable power supply by wires (not shown); the power supply may be a battery power supply and/or a mains power supply. 20 The housing 10 is shown in greater detail in Figure 3. The housing 10 is a solid block formed with two parallel cavities 14,15. The cavity 14 forms a fluid tank, and cavity 15 receives a capsule 16 which contains a pressurised gas, typically pressurised carbon dioxide. However, any non-toxic non-explosive gas may be used. 25 The cavities 14,15 are interconnected by a passage 17 which contains a non-return valve 17. The non-return valve is of known type and consists of a chamber providing a small inlet 19 which normally is blocked by a ball 20 under pressure of a biasing spring 21. However, when the pressure in the cavity 15 exceeds that the pressure exerted by 30 the biasing spring 21, the ball 20 is moved away from the inlet 19, allowing gas to flow through the inlet 19 and out through the outlet 22 into the cavity 14. The inner end of the cavity 15 within the housing 10 is closed. The outer end of the cavity 15, at the edge of the housing 10, is closed by a removable screw-threaded plug 35 23 which carries the actuator 12, which is shown in detail in Figure 4 only. The 3 actuator 12 includes a pair of spaced support plates 30, formed with aligned apertures 31 in which a rotatable cylinder 32 is journalled. The cylinder 32 provides a cam surface 33 and a plunger 34 is biased by a spring 35 into contact with the face of the cam surface. 5 The plunger 34 can be moved in the direction of arrow A to push a pin 24 into the adjacent end of capsule 16. This pierces the end of capsule 16, to release the compressed gas from the capsule 16 into the cavity 15. 10 Movement of the plunger 34 on the direction of arrow B withdraws the pin 24 from contact with the capsule 16. One end of the cylinder 32 is secured to an arm 36, the other end of which is connected to the reciprocating arm 37 of a servo motor 38 of known type. When the 15 servo motor 38 is driven to extend of the arm 37 in the direction of arrow C, the arm 36 is pivoted in the direction of arrow D to rotate the cylinder 32 and thus push the plunger 34 in the direction of arrow A. When the servo motor moves the arm 37 in the direction of arrow E, the arm 36 pivots 20 in the direction of arrow F and the return spring 35 withdraws the pin 24 from the end of the capsule. The inner end of the cavity 14 within the housing 10 is closed. The outer end of the cavity 14, at the edge of the housing 10, is closed by a removable screw thread plug 25 25 which can be removed when it is necessary to refill the cavity 14 with liquid. A pipe 26 extends down the length of the cavity 14 from a point a short distance away from the plug 25 to the end of the housing 10, where the adjacent end of the pipe 26 is closed by a plug 27. The end of the pipe 26 adjacent the plug 25 is open. 30 Two ports, 28,29 open into the portion of the pipe 26 which extends beyond the cavity 14. The ports 28,29 are connected to the solenoid valve 13 which incorporates a pressure sensor and which is connected to the spray nozzle 11. The spray nozzle 11 is pivotally mounted on the housing 10 so that the angle of the nozzle can be adjusted over a wide range, to accommodate various locations. 35 4 The solenoid valve 13 is connected to, and controlled by motion sensors 30,31 which are located so as to detect, respectively, persons entering or persons leaving the area being monitored. 5 The above described system operates as follows: before positioning the apparatus, the plug 25 is removed, the cavity 14 is filled with a liquid containing a dye and the plug 25 is replaced. The dye may be any of a wide range of non-toxic dyes and preferably is a visible dye capable of dying both skin and clothes and persisting for at least several hours, preferably several days, even if washed repeatedly. Alternatively, 10 the liquid may instead be a solution which possesses an odour which can be sensed by dogs, so that police dogs can identify the person who has been sprayed; a fluid of this type may be used alone or combined with a dye. Further, the canister 16 is positioned in the cavity 15 as described above. 15 The apparatus is then positioned above the doorway into the space to be monitored, as described above; the sensors 30,31 are arranged so as to detect persons respectively entering or leaving the space to be monitored. 20 The apparatus is now ready for use, and is controlled by a remote control, as described below. In the event that the user is suspicious about a person who has entered the space being monitored, and believes that he may need to use the system, the user presses 25 the panic button on the remote control. When the panic button is pushed, the pin 24 of the actuator 12 is driven to the position of Figure 3, to penetrate the end of the cylinder 16 and release the compressed gas into the chamber 15. Once the gas has been released from the cylinder, the pressure 30 in the chamber 15 becomes great enough to open the non-return valve 17 and this allows the gas in the chamber 15 to pass into the chamber 14, where the gas mixes with the liquid contained in the chamber 14, and pressurises that liquid. This increases the pressure in the chamber 14 and forces pressurised liquid down in the tube 26, through the port 28 and 29 and to the solenoid valve 30. The increasing 35 pressure in this portion of the system triggers the pressure sensor on the solenoid 5 valve 13, which lights up. The apparatus is now armed and ready to operate. It will be appreciated that pressing the panic button makes no noise, and apart from the fact that the pressure sensor 5 lights up, there is no indication that the system has been armed. If preferred, the pressure sensor on the solenoid valve 13 could be dispensed with completely, or could be positioned so that it is not readily visible from the space being monitored. Once the system has been armed in this way, as soon as the sensor 31 senses that 10 the person is leaving the area being monitored, the sensor 31 opens the solenoid valve 13 and a spray of pressurised liquid is released through the nozzle 11, spraying the person who is leaving. A spray is released from the nozzle 11 each time the sensor 31 is triggered, until the supply of liquid is exhausted. However, if a person enters the area being monitored, the sensor 30 is triggered to hold the solenoid valve 15 13 closed, so that no spray is emitted through the nozzle 11. In the event that the system has been armed, but the user then realises that there was no need for concern and the system will not be needed, the user simply holds the panic button down for five seconds to deactivate the system so that triggering the 20 sensor 31 will not release a spray of liquid through the nozzle 11. The apparatus may then be re-armed by pressing the panic button again. After the apparatus has been used, the liquid in cavity 14 is replenished and a fresh cylinder 16 is inserted in cavity 15. 25 It will be appreciated that the above described apparatus is relatively inexpensive to manufacture, install and maintain and is extremely simple to operate. Further, the fact that the system operates automatically once it has been armed means that it is particularly safe for the user: the user is able to arm the system if the circumstances 30 give rise to any concern, and does not have to press a button to operate the system when a robbery is actually in progress. If the system is not in fact needed, then it may be stood down unobtrusively, without causing any embarrassment to the user. It will be appreciated that the apparatus of the present invention could be used in 35 combination with a remote monitoring system if preferred. 6