ZOOLOdY AND BOTANY, MICROSCOPY, ETC. 513 



diatoms contaiuing a small amount of organic matter ; such are the 

 earths of Java (consisting mainly of Melosira orichalcea), Lapland, the 

 mountain-meal of Switzerland and Finland, and the fossil-meal of 

 Mount Amiata, in Italy, which contains about 80 species. A mixture of 

 marine algse (Fucus, various species, RTiytiphlcea, Cladophora, Cera- 

 mium), of Corallines and Sertularians, is known to druggists under the 

 name of Corsican moss, in which as many as 120 species of diatoms 

 have been found. They also occur as the Carrigeen moss {Ghondrua 

 crispus) used in pharmacy, and on other marine plants, whose ashes 

 have been from time to time used as medicine (e. g. Ulva lactuca, Fucus 

 vesiculosus). In this connection, however, they must be regarded as 

 indifferent substances, producing no appreciable effect when thus taken, 

 as 8-4 per cent, of them is composed of silica, and only 12 per cent, of 

 water and organic matters ; the latter constituents being the protoplasm 

 and the accompanying sap and the endochrome, which are all, with 

 the exception of the phycoxanthin of the latter body, already found in 

 the accompanying Algse, They have a further interest, in the part 

 which they play when living in fresh waters, in purifying them from 

 the excess of carbonic acid and waste azotized matters, and in giving 

 out at the same time more oxygen than any other aquatic plants. 



In their relation to geology, diatoms furnish proofs of the aquatic 

 origin of the strata in which they occur, of their age, and of the salt- 

 ness or freshness of the water in which they were deposited, and of 

 whether near the coast or in deep water, by the difference between 

 the species which are associated with each of these conditions. 



Diatoms are connected with the useful arts through agriculture, 

 to which they are of assistance under the form of guano. Water rich 

 in diatoms is more fertilizing than that which is not so, as the latter 

 generally contains an undue amount of mineral salts. 



Diatoms are used — as tripoli — to polish metals, stones, and jewels, 

 and form a constituent of dynamite. Their most important applica- 

 tion to the arts here mentioned, is that of serving as tests for the 

 Microscope. 



MICROSCOPY. 



a. Instruments, Accessories, &c. 



Houston's Botanical Dissecting Microscope. — This instrument 

 (Fig. 116), designed by Mr. D. Houston, aiithor of 'Practical Botany,' 

 is intended to provide working botanical students with a dissecting 

 Microscope at a very low cost. 



The box measures, when closed, 9 inches long, 4 inches wide, and 

 2 inches deep, and is so constructed that, by using a divided sliding 

 lid (which acts as a support for the dissecting stage), a rest for 

 the wrists is secured while the hands are employed in dissecting. 

 The duplex lens, which gives three powers, magnifying 4, 6, and 10 

 diameters, is screwed to the end of a brass focussing tube, which 

 moves upon a brass pillar attached to a sliding bar at the bottom 



