OF MICROSCOPIC OBJECTS. 33 



ingly small, so that the unaided eye can perceive nothing on a 

 prepared slide of these organisms but minute dust. Each sepa- 

 rate portion, which is usually seen when mounted, is termed a 

 " frustule," or " testule :" this consists of two similar parts, com- 

 posed of silica, between and sometimes around which is a mass 

 of viscid matter called the " endochrome." They are found in 

 almost every description of water, according to the variety : 

 some prefer sea-water, others fresh, arfcl many are seen nowhere 

 but in that which is a mixture of both, as the mouths of rivers, &c. 

 Ditches, ponds, cisterns, and indeed almost every reservoir, yield 

 abundance of these forms. They are not, however, confined to 

 " present " life ; but, owing to the almost indestructible nature of 

 their siliceous covering, they are found in a fossil state in certain 

 earths in great abundance, and are often termed " fossil Infusoria." 

 Upon these frustules are generally to be seen lines, or " mark- 

 ings," of different degrees of minuteness, the delicacy of which 

 often serves the purpose of testing the denning power of the 

 object-glasses. Some of the frustules are triangular, others cir- 

 cular, and, indeed, of almost every conceivable shape, many of 

 them presenting us with exquisitely beautiful designs. 



The markings, however, are seldom seen well, if at all, until the 

 frustules are properly prepared, the different methods of accom- 

 plishing which will be given a little further on. 



The collection of the fresh diatoms is so closely connected with 

 their preservation, that a few notes may be given upon it before 

 we pass on. For this purpose a number of small bottles must be 

 provided, which may be placed in a tin box, with a separate apart- 

 ment for each, so that all chance of breakage may be done away 

 with. The diatoms are generally of a light brown colour ; and 

 where they are observed in the water, the bottle may be so placed, 

 with the mouth closed by the finger, that when the finger is with- 

 drawn the water will rush in, carrying the diatoms also. If they 

 are seen upon plants, stones, or any other substance, they may 

 generally be detached and placed in the bottle. When there is 

 a green covering upon the surface of the water, a great quantity 

 of diatoms is usually found amongst it ; as also upon the surface 

 of the mud in those ponds where they abound. In these cases, a 

 broad flat spoon will be found very useful, and one is now made 



3 " 



