a mixing vessel 8 is positioned above a buffer vessel 16 . the mixing vessel 8 can be filled via a water feed 12 and via an electrically operated valve 11 , using a metering pipe 13 . when the water level in the mixing vessel 8 has risen above a defined level , the vessel is emptied by the siphon effect using a siphon line 14 . the siphon line 14 is arranged in the cover of the buffer vessel 16 . a metering device is arranged above the mixing vessel 8 , which metering device comprises a housing 5 , an which a casing 2 is positioned via a connection opening with a seal 1 . a metering slide 7 can be moved in the horizontal direction in the housing 5 using a drive 9 . a metering opening 6 is formed in the metering slide 7 , which opening can communicate in sequence with an opening in the housing 5 into the mixing vessel 8 and an opening in the housing 5 into the connection opening 1 holding the casing 2 . a stack of tablets 3 which fit into the metering opening 6 in the metering slide 7 is positioned in the casing 2 . by moving the metering slide in the horizontal direction , in each case one tablet is moved from the casing 2 into the mixing vessel 8 . a seal 4 is arranged around the metering slide 7 , in the housing 5 . as a result , it is impossible for any moisture to penetrate along the metering slide 7 into the casing 2 when the metering opening 6 has been moved to above the mixing vessel . on its open side , the casing 2 is provided on the outside with a smooth rim which interacts with the seal of the connection opening 1 , so that it is also impossible for any moisture to penetrate into the casing 2 by this route . as a result , the tablets 3 in the casing 2 remain as dry as possible . the device functions as follows . one or more tablets 3 are moved into the mixing vessel 8 via a control system 18 using the drive 9 . this is achieved by moving the metering slide 7 in the horizontal direction , with the result that one tablet each time falls into the mixing vessel 8 . via an operating cable 10 , a signal is then transmitted to the electrically operated valve 11 , which consequently opens , causing water to flow out of the water feed 12 into the mixing - vessel 8 . this water is preferably hot water at a temperature of approximately 80 to 90 ° c ., with the result that the tablets 3 dissolve rapidly . after the mixing vessel 8 has been almost filled , meaning that approximately one litre of hot water has flowed in , and after the tablets 3 have been dissolved , the liquid will flow out of the mixing vessel 8 , via the siphon line 14 , into the buffer vessel 16 . the buffer vessel 16 is then filled to the desired level by keeping the valve 11 open for a sufficient period of time or by filling the buffer vessel 16 with hot water in a different way . the buffer vessel 16 has a volume of , for example , from 70 to 120 litres and is connected in a known manner to a milking system , which is not shown in more detail and may be designed in a known manner . the liquid can be removed from the buffer vessel 16 in a known manner and circulates through the milking system 17 , cf . also in this regard the abovementioned application ep 0 399 604 , for example . the milking system 17 may be a simple milking installation , in which case the user starts the cleaning cycle himself , but may also be an automatically operating milking system which is periodically cleaned using an automatic system , as is employed , for example , with a milking robot . to clean a milking system , acidic cleaning and alkaline cleaning are employed in sequence . for acidic cleaning , which takes place , for example , every 2 to 3 days , sulfamic acid is used , for example 3 tablets containing a total of 60 grams of acid are used . for the alkaline cleaning , use is made of a combination of caustic soda , for example 40 grams , and , for example , 40 grams of disinfectant , such as sodium dichloroisocyanate . the alkaline cleaning is carried out daily and is alternated with an acidic cleaning which takes place , for example , every 4th cleaning operation . the tablets employed have a diameter of 34 mm and each weigh approximately 20 grams . the internal diameter of the casing 2 in which the tablets 3 are stored is , for example , approximately 40 mm . wetting agents are added to the caustic soda tablets , so that a fat - emulsifying action is generated . this is important in particular for milking installations in which milking continues throughout the day , such as for example in the case of automatic milking devices in which the milking cups are automatically attached to the cows &# 39 ; udders using a robot . since the buffer vessel 16 has to be provided with various cleaning agents , it is possible for a plurality of metering systems to be positioned above the buffer vessel . in this case , use may be made of , for example , three identical metering systems with three mixing vessels 8 , with different tablets positioned in each metering system . it is also conceivable to position , for example , three casings with three metering slides 7 above a mixing vessel , or to design the metering slide 7 in such a manner that different tablets can be taken from different casings at the same time or in sequence . according to another embodiment of the device , the cleaning agents which are used for one cleaning cycle are combined to form a single combination tablet . this means that in this case , by way of example , the caustic soda and the disinfectant are combined in a single tablet , with separating means , for example , such as a separating layer of neutral substance being arranged between the caustic soda and the disinfectant . it is also possible to package the acidic and alkaline cleaning agents in one casing , in which case the different substances are packaged , for example , in water - soluble film in order to avoid undesirable reactions between the substances . in this case , the tablets 3 are placed in the casing 2 in the desired order , with one dose ( for example of three tablets ) of acidic cleaning agent being positioned after every three doses ( for example of three combination tablets ) of alkaline cleaning agent . as a result , the desired cleaning takes place regularly in the desired order . in order to avoid errors when metering cleaning agent , the device may also be provided with a system which indicates how much cleaning agent is still present . this may be a simple mechanical indicator displaying the number of tablets in the casing ; if appropriate , it may also be a mechanical counter which indicates the number of times that the metering slide has metered tablets . another way of displaying the quantity can be realized using the control system 18 which records how many tablets have been metered since the last time that a new casing containing tablets was put in place . it is also possible for means which indicate how much powder is still present to be present in systems which use powder instead of tablets . in another embodiment of the metering device , the mixing vessel 8 is omitted and the tablets are metered directly into the buffer vessel 16 . in this embodiment , it is assumed that the tablets 3 dissolve immediately in the hot water , and if appropriate additional measures are employed , such as a separate mixing chamber in the buffer vessel 16 or , if appropriate , an agitator .