The manufactures in which kelp is employed are prin cipally soap-making, bleaching, and glass-making. The soap maker and bleacher use the soluble parts alone ; and these are only so far valuable to them, as they contain un combined alkali, or can be made to yield it. After the soda is extracted, an additional quantity of this alkali may be procured from the sulphates. For this purpose the spent lees are mixed with saw-dust, or waste tanner's bark, and a portion of lime is added ; these are exposed to an intense heat, with the free access of air, in a reverberatory furnace. —The effect of this process is to decompose the sulphuric acid, in consequence of which the alkali is set free ; and though this alkali should consist partly of potassa, yet it is the soda that is obtained free, as the potassa evolved de taches it from the muriate of soda, by combining with the acid of this salt. The comhustion of the carbonaceous matter employed produces a quantity of carbonic acid, by uniting with the oxygen of the sulphuric, and this carbonic acid combines with more or less of the alkali, set free from the more powerful acids. The sulphur obtained by the decomposition of the sulphuric acid, unites with the lime to form a sulphuret. The muriate of potassa, formed in this process, is obtained by evaporation, and purchased by the alum manufacturers. The remaining lees contain io dine, which may be separated copiously, by the processes described under our article IODINE, to which the reader is referred for a full account of that singular substance.
As the soluble matter is thus valuable, and it is of some importance to be able to determine the total quantity of it contained in a given portion of any particular kelp, Dr.
fe proposes, as the readiest method of doing this, to ex tract it from a given weight of the kelp, by means of a de terminate pi opurtion of water, and then to use the common means for determining the specific gravity of the solution. To assist this process, he gives a table of the specific gra vities of solutions of different degrees of strength, varying from 1 to 20 per cent.
Two ounces of finely ground kelp may be put into a tall narrow glass ; on this six ounces or water may be poured, and the height of the liquid marked on the glass. It may then be boiled gently for an hour, the whole returned into the glass, and a quantity of water added, till it rises to the mark. The specific gravity of a clear portion of this may then be ascertained, and, as six times the weight of water was employed, the proportion per cent, is to be multiplied by six.
When the whole quantity of soluble matter is ascertain ed, and also the quantity of uncombined alkali, or alkaline carbonate, it may be of use to determine the proportion of sulphuric acid, by treating it with muriate of barytes, and weighing the precipitate afforded. This will show what
proportion of alkali may be obtained by the application of heat to the soaper's lees, mixed up with saw-dust, or other carbonaceous matter.
The insoluble residue of kelp is used by the glass-ma kers for preparing the coarser kinds of glass. When ob tained from the soap-boiler, it contains a quantity of lime, which he has added to it, and this earth has a considerable share in the formation of the glass. This residue also forms a valuable manure.
In the manufacture of crown glass, kelp is employed in the entire state, being subjected to the process of commi nution, but to no chemical separation of its ingredients, ex cept that the carbon and sulphur are driven off by the ap plication of a strong heat, and this is done after it is mixed with the sand, which is to form the glass along with it.
The kelp procured from the Fucua Vceiculosua is ifs largest quantity, and is generally in most esteem for its quality. The Fucua Nodosus affords a kelp of equal va lue, but in smaller quantity. The Fucua Serratua is in both these particulars less productive ; and the Fucua Di gitatus still less. The last forms the principal part of the drift ware, which is regarded as affording kelp of an infe rior quality, though to this there are occasional excep tions.
Kelp procured from foci which have been exposed to rain during the process of drying is found inferior in its quality to that which has escaped such exposure.
Fuci cut when two or three years old yield more kelp in proportion to its bulk, than when suffered to stand lon ger.
The alkali, free or carbonated, contained in kelp, is the product of the burning process ; none being obtained from sea-wced by subjecting it to the action of boiling water. It is found, however, that the access of air is not necessary to the evolution of the soda, and that it is equally procured by strong heat applied to the weed, in close as in open ves sels. The heat acts by the destruction of the sulphates, which is effected by the union of the carbonaceous matter of the weed with the oxygen of the acid, and has therefore exactly the same rationale with the process already men tioned, to which soapet 's lees are subjected, by the artificial mixture with carbonaceous matter.
From some ingenious experiments of Dr. Fyfe, one valu able practical result was obtained ; that the quantity of al kali in kelp may be increased, by sprinkling sea-water on the weed while it is drying. During hot weather, when it is nearly sufficiently dry, this might be done without much risk of a retardation to the manufacture, by the occurrence of the rains, to which our uncertain climate is so liable. Perhaps a similar sprinkling might be employed during the burning of the weed, especially when at any time the heat is too powerful, and the combustion too rapid.