SECT. II. COMPOSITE ORGANS. 133 



The specific gravity of the sap of the Sycamore* 

 tree extracted close to the ground was 1 '004 ; at 

 the height of seven feet, 1-008; and at the height 

 of twelve feet, 1-012. The same proportion was 

 also observed in the sap of the Birch ; but after the 

 sap had flowed for some days from the same incision 

 the specific gravity was reduced to 1'002 ; which 

 shows that the matter carried off from the alburnum 

 had diminished at least in the vicinity of the in- 

 cision. The third argument is founded on the 

 fact that the specific gravity of the alburnum is 

 greater in the winter than it is either in the pre- 

 ceding or following summer. Two equal portions 

 were taken from two poles that had sprung from 

 the same shoot, the one having been felled in De- 

 cember, and the other in the May following. They 

 were kept for the space of seven weeks near a warm 

 fire, at the end of which period they both ap- 

 peared to be perfectly dry. The specific gravity of 

 the winter felled wood was 0*679 while that of 

 the summer felled wood was only 0-609. Still it 

 was possible that the apparent difference of specific 

 gravity might have proceeded merely from a greater 

 degree of contraction in the winter felled wood ; 

 in consequence of which doubt it was thought that 

 the comparison of their respective quantities of ex- 

 tractive matter would be the best means of avoiding 

 all mistake. Accordingly six ounces of boiling 

 water were poured on 100O grains of each, reduced 

 to small fragments ; and at the end of twenty-four 



