142 Further Observations 
Upon a superficial examination of the varieties of kelp mef 
with in the market, we soon become convinced that this sub- 
stance differs prodigiously with regard to its external qualities as 
well as chemical composition. An extended series of experiments 
lately made upon different varieties of kelp, gives me reason to 
state, that some sorts contain, besides carbonate of soda and the 
usual salts peculiar to the sea-weeds from which kelp is, ob- 
tained, a considerable portion of subcarbonate of potash. Others 
abound in sulphurets ; again others, are almost wholly composed 
of common salt, with scarcely a notable portion of mineral alkali ; 
whilst others again furnish from 2 to 34 per cent. of mineral al- 
kali. In certain varieties of kelp we find a large quantity of sul- 
phurets, and others again are almost free from sulphur. 
’ This difference of composition is unquestionably owing to the 
mode in which kelp is prepared, as well as the articles of which 
it is manufactured. 1am enabled to state from good authority, 
which I have received from different quarters, concerviing the 
manufacture of this article, that in some places a considerable 
quantity of fern, [Pteris aquilina Linn.,} and woods of all kind, 
are burnt, together with the leafy fuci, for the production of 
kelp; and that in other districts, a mixture of common salt, sul- 
phur, and the ashes of peat and brushwood, is added to facilitate 
the complete fusion of the ashes of the sea-weeds, to effect which, 
is said to require a considerable degree of practical knowledge 
of the kelp manufacture. 
Without entering into a further detail concerning other abuses 
committed in the manufacture and sale of kelp, some of which 
are practised with a view of giving to British kelp the appearance 
of Sicilian or Teneriffe barilla, I shall confine myself, on the pre- 
sent occasion, merely to point out the characters by which the 
variety of kelp best suited for the preparation of iode, may be 
discriminated, from those sorts which contain almost none of this 
substance, or from which at least it cannot be obtained with- 
out much trouble and expense; and I shall likewise state the 
processes and practical proceedings which I have found to an-- 
swer best to obtain this substance. 
Kelp from which iode may readily be procured in the manner 
to be mentioned presently, is of a deep blueish-gray.colour. It 
readily attracts moisture when exposed to a damp atmosphere, and 
then becomes intensely black. It is of a moderate hardness, and 
may easily be pulverized. Its recent fracture is minute cellular, 
porous, and earthy age crystalline]. It emits a faint sulphureous 
odour when moistened with water or dilute acids. It possesses 
a saline alkaline taste. It affords from 2! to 34 per cent. of 
carbonate of soda, and when kept in a dry place it becomes co~ 
vered, 
