THE SHORES OF BRITAIN. 43 
equal state of fusion. It is then allowed to cool, 
‘ and having been taken out and broken to pieces, 
it is carried to the storehouse to be shipped for 
market. The general yield of this alkali is one- 
fifth of the weight of the ashes from weeds pro- 
miscuously collected; but from one species, the 
Sea-wrack, or Black-tang (Fucus vesiculosus), one of 
the most abundant on our coast, the ashes yield 
half their weight of alkali. The Sea-wrack is of a 
dark-green hue, bearing long, flat, and narrow 
fronds, resembling leaves, divided into branches, 
and having a midrib running through the centre; 
the leaf-like branches terminate in large yellow 
oval receptacles, containing many seeds, enveloped 
in a thick mucus. But its chief peculiarity is, that 
the substance of the frond swells at irregular in- 
tervals into oval air-cells, always arranged in pairs, 
one on each side of the midrib. The Dutch use 
this sort, and another called Black-wrack (/. ser- 
ratus), to pack their lobsters; the latter, how- 
ever, is preferred, on account of its containing less 
mucus, and therefore being less liable to ferment- 
ation. 
Scarcely inferior in its alkaline properties to the 
Sea-wrack is the Knotted-wrack (/. nodosus). The 
fronds look like slender stems, swelling at intervals 
into oval bulbs or air-vessels. Boys amuse them- 
selves occasionally by cutting off these nodules in 
a diagonal direction, to make them into whistles. 
They are too tough to be burst by the pressure of 
the fingers, like those of the Sea-wrack; but if 
stamped on, or put into the fire, they explode 
