102 INTRODUCTION TO CllYPTOGAMIC BOTANY, 



ations, however, contains many undoubted Diatomacece, and 

 some indications of these organisms appear in flint and opal,* 

 as also of Desmidiacew.-f 



84. The economical purposes to which Alga3 are applied are 

 various ; but they are not now of so much importance as they 

 were before modern improvements in chemistry taught a 

 cheaper mode of obtaining carbonate of soda from common 

 salt. The preparation of kelp, which gave employment to so 

 many men, and which was a source of such large emolu- 

 ment on many of our rocky coasts, is now almost obsolete, 

 and there is no prospect of its being revived. A small 

 quantity, however, is still prepared for the manufacture of 

 iodine, which was first discovered in the lees of kelp, and 

 is so important in medicine. The vegetable j^thiops is pre- 

 pared by burning Fucus vesiculosus in a covered crucible. 

 It is given in doses of from ten grains to two drams, and is 

 said to be more efficacious than burnt sponge in scrofulous 

 disorders. Its virtue is doubtless due to the small quantity 

 of iodine it may contain, to which also is attributable the 

 benefit of sea water in similar disorders. Calcified sea-weed 

 is especially useful in the cure of fibrous polypus of the 

 uterus, for which the Kreuznach waters have long been cele- 

 brated, in consequence of their containing the same principle, 

 (Med. Times, Aug. 18, 1855.) The principal use for which the 

 large masses of seaweed which are thrown upon the coast are 

 now employed, is in the preparation of manure, and it is to this 

 that the fertility of many a district is due, as, for instance, the Isle 

 of Thanet, which has for years sent to London the best English 

 corn which appears in the market.:): A few species, such as 

 Alaria escidenta, Rliodymenia ixdmata, Tridcea edidis, &c., 

 are regularly sold in the Scotch markets; and I can myself bear 

 witness on more than one occasion, at so late a period as 1823, 

 to being able to procure no other food amongst the Western 

 Islands. Chondrus ciyspus and other Rhodosperms have long 

 been sold by chemists for the preparation of a sort of blanc- 



* Ehrenberg iu Poggendorff Annalen d. Phys., 1836. Ann. d. Sc. 

 Nat., Ser. 2, vol. 7, p. 27. (Zool.) 

 t Turpiu, 1. c, p. 129. 

 I Lewis, Hist, of Tenet,l 723, p. 13. 



