Some algae cells are also easier to filter than others, reducing the energy and economic cost of "harvesting" the algae from cultures.

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But most Britons still instinctively filter themselves and others into social classes, with attendant suspicions and snobberies.

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Others will provide telephone consultations first in order to filter those patients who need to be seen from those who don't.

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Mr. Savoni and others devised a way to push the water through the filter, a block of carbon encased in plastic, more swiftly.

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Though there is no reason to use Dr Knight's invention to replace existing optical fibres, it could, by varying the spacing of the air-filled tubes, make a handy filter allowing selected wavelengths (and the messages they carry) to enter or leave the waveguide while the others are contained.

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