70 The Scottish Naturalist. 



Another use made of seaweeds is their employment for orna- 

 mental purposes, owing to the ease and success with which they 

 can be laid out on paper and preserved by drying ; and still other 

 uses might be mentioned, such as the employment of the stems of 

 large Laminarice, to make handles for knives. 



The Algae of the freshwater lochs and streams are even more 

 numerous in species than the seaweeds ; but they are, for the most 

 part, so small individually that they are far less conspicuous, and 

 might often be wholly overlooked by the nonobservant. Yet they 

 swarm in every place where water can lodge, and at times force 

 themselves upon the notice of civic authorities by appearing 

 in the water supplies of towns. There have frequently been 

 serious consequences from the excessive multiplication and sub- 

 sequent putrefaction of certain Algae in the reservoirs and pipes, 

 more especially on the continent of Europe. But in moderate 

 quantity the freshwater Algae are useful in the sources of town 

 supplies, as they aid efficiently in the removal of organic impurities, 

 and in supplying oxygen to the water. 



The most important of the Algae, from an economic point of 

 view, are the diatoms, so well-known to microscopists lor their 

 wonderful beauty and variety, and so much esteemed as tests of 

 the excellence of the lenses of microscopes, on account of the ex- 

 treme delicacy of their markings, which only the best and most 

 powerful lenses can show clearly. The flinty covering of the 

 diatoms is almost indestructible by the agencies to which they are 

 exposed after death ; hence they remain unchanged at the bottom 

 of ponds and lakes. Despite the minuteness of these plants, of 

 many kinds of which upwards of 1,000,000,000 could lie in one 

 cubic inch, in course of time beds of many feet in thickness be- 

 come deposited, and assume the form either of fine-grained rocks, 

 or of gray friable deposits, often under peat this latter form re- 

 ceiving the name of Kiesel-gnhr. The rock is often used for 

 making polishing-stones, and the powdery deposits are also used 

 in this way ; but the chief use of Kiesel-guhr is in the preparation 

 of dynamite. A very valuable bed of it is worked in the vicinity 

 of Loch Kinnord in Aberdeenshire. 



The Lichens afford fewer plants of value to mankind than do 

 the Algae. In their common forms, on stones and on tree trunks, 

 they are familiar to everyone ; varying in colour, but usually being 

 some shade of grey or brown, less often black, yellow, orange, or 



