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CHAPTER IX.
OF THE SAP, AND INSENSIBLE PERSPIRATION.
The sap of trees, as has been mentioned in the last chapter, may be obtained by wounding a stem or branch in spring, just before the buds open, or in the end of autumn, though less copiously, after a slight frost; yet not during the frost. In the Palm-trees of hot countries, it is said to flow from a wound at any time of the year. It has always been observed to flow from the young wood or alburnum of our trees, not from the bark; which agrees with Mr. Knight's theory.
A common branch of the Vine cut through will yield about a pint of this fluid in the course of twenty-four hours. The Birch, Betula alba, affords plenty of sap; some other trees yield but a small quantity. It flows equally
