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Plants are the best natural air filters.
In diffusion plants the milk of lime is added, in proper proportion, in the cells of the diffusion battery, and the chips or slices themselves act as a mechanical filter for the juice; while in the Sandwich Islands coral-sand filters have been employed for some years, in addition to the chips, to free the juice from impurities held in mechanical suspension.
In Germany very similar filters have also been used, pearl-quartz gravel taking the place of coral sand, which it closely resembles.