chamber in fig4 through fig7 the liquid having passed through the drill plate 15 is throttled by the slit plate 16 since however the slit is not restricted in the widthwise direction the liquid flows while expanding in the widthwise direction reference numeral 23 in fig6 designates the low concentration part of the liquid flow explaining now other embodiments of the drill plate 15 and the slit plate 16 than the embodiment shown in fig4 and fig5 fig8 a illustrates the case where the slit plate 25 is bolted to the drill plate 24 and fig8 b illustrates the case where the drill plate 24 and the slit plate 25 are integrally constructed fig9 a illustrates the case where the drill plate 26 has a widthwise groove 26a while the slit plate 27 is provided with a widthwise extended projection 27a to be inserted into the groove 26a and owing to the engagement of the groove 26a and the projection 27a the drill plate 26 and the slit plate 27 are fixedly connected with each other reference numeral 27b designates a restraining element fitting part then fig9 b illustrates the case where the widthwise extended restraining element 28 is held in the widthwise groove 29a of the drill plate 29 by the widthwise extended projection 28a provided on the restraining element 28 a fitting part 28b of the restraining element 28 forms the slit 30 proceeding now to fig1 this is the case where the widthwise extended restraining element 31 has the widthwise groove 31a and into this groove 31a the widthwise extended projection 32a of.