336 MICROSCOPIC ANIMALS AND PLANTS [Ch. X 



or rusty looking substance on plants, rocks, etc., practically always 

 contain diatoms, and sometimes is made up mostly of them. It is most 

 interesting to study the diatoms alive and watch them ghde around 

 in the water. The shells of the diatoms have been favorite objects 

 of study for a long time. They are often beautifully marked. Being 

 sihcious, they resist acids, and the hving substance in and around them 

 can be destroyed without hurting the shells. This may be done by 

 placing the material containing a large number of diatoms in a test 

 tube and when the diatoms have settled pour off a part of the liquid 

 or draw it out with the pipette (fig. 200 A), and add an equal amount 

 of nitric acid. Boil for a few minutes, let the diatoms settle, pour 

 off or draw off most of the liquid, and add more nitric acid and boil 

 again. Finally, add water and gradually wash the diatom shells by 

 drawing ofE the water and adding fresh. The shells should be clean 

 and almost colorless and show their markings well. One can take a 

 sample and see if the cleaning is sufficient. (For full and elaborate 

 directions see Boyer's Diatomaceae of Philadelphia and Vicinity, 

 p. 122-123). 



§ 523. Arranging minute objects. — Minute objects like diatoms 

 or the scales of insects may be arranged in geometrical figures or in 

 some fanciful way, either for ornament or more satisfactory study. 

 To do this the cover-glass is placed over the guide. This guide for 

 geometrical figures may be a net-micrometer or a series of concen- 

 tric circles. In order that the objects may remain in place, however, 

 they must be fastened to the cover-glass. As an adhesive substance, 

 mucilage or liquid gelatin (§578), thinned with an equal volume of 

 50 % acetic acid, answers well. A very thin coating of this is spread 

 on the cover with a needle, or in some other way, and allowed to dry. 

 The objects are then placed on the gelatinized side of the cover and 

 caref uUy got into position with a mechanical finger, made by fastening 

 a cat's whisker in a needle holder. For most of these objects a simple 

 microscope with stand (fig. 201-202) will be found of great advantage. 

 After the objects are arranged, one breathes very gently on the 

 cover-glass to soften the mucilage or gelatin. It is then allowed to 

 dry, and if a suitable amount of gelatin has been used and it has been 

 properly moistened, the objects will be found firmly aijchored. In ■ 



