OF MICROSCOPIC OBJECTS. 89 



of hot water until liquefied, when it must be used like Canada 

 balsam, except that it requires less heat. A ring of asphaltum 

 varnish round the thin glass cover completes the mounting." 



The Infusoria (see Chapter III.) are sometimes preserved in 

 liquid ; but present many difficulties to the student. Different 

 kinds require different treatment, and consequently it is well, 

 when practicable, to mount similar objects in two or more liquids. 

 Some are best preserved in a strong solution of chloride of cal- 

 cium, others in Thwaites's liquid, whilst a few keep their colour 

 most perfectly when in glycerine alone. Many of them, however, 

 are so very transparent that they present but faint objects for 

 ordinary observance. The Desmidiacese require somewhat simi- 

 lar treatment, and may be mentioned here. The solution of 

 chloride of calcium has been strongly recommended ; but no pre- 

 servative liquid seems to be without some action upon them. 

 Both of the above classes of objects should be mounted in shal- 

 low cells, so as to allow as high a microscopic power as possible 

 to be used with them. 



Many of the ZOOPHYTES which are obtained on our sea-coasts 

 are well preserved by mounting in cells, in the manner before 

 mentioned, with Goadby's fluid, or distilled water with one of the 

 additions noticed amongst the preservative liquids. For examina- 

 tion by polarized light, however, they are usually mounted in bal- 

 sam (see Chapter III.), whilst those in cells present a more natural 

 appearance as to position, &c., for common study. 



As to the use of preservative liquids with the Diatomacese there 

 are various opinions. Some experienced microscopists say that 

 there is little or no satisfaction in mounting them in this way. 

 Dr. Carpenter, however, explains this difference by his instruc- 

 tions as to what method should be used when certain ends are 

 desired. He says : " If they can be obtained quite fresh, and it 

 be desired that they should exhibit as closely as possible the 

 appearance presented by the living plants, they should be put up 

 in distilled water within cement-cells ; but if they are not thus 

 mounted within a short time after they have been gathered, about 

 a sixth part of alcohol should be added to the water. If it be 

 desired to exhibit the stipitate forms in their natural parasitism 

 upon other aquatic plants, the entire mass may be mounted in 



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