PKEPAEATION OF DIATOMS. 791 



as the -willo-w, from its softness, is peculiarly difiScult to turn. It is 

 also of consequence to remark that both sides be turned so as that the 

 lap, when dry, is quite parallel. This lap is most conveniently adapted 

 to the common face chuck of a lathe with a conical screw, so that either 

 surface may be used. This is made evident when we state that this 

 polisher is always used moist, and, to keep both surfaces parallel, must 

 be entirely plunged in water before using, as both surfaces must be 

 equally moist, otherwise the dry will be concave, and the moist sur- 

 face convex. The polishing substance used with this lap is putty 

 powder (oxide of tin), which ought to be well washed to free it from 

 grit. The calcareous fossils being finely ground, are speedily polished 

 by this method. To polish softer substances a piece of eloth may be 

 spread over the wooden lap, and finely levigated chalk used as a 

 polishing medium. 



In aU instances slides should be labelled with the name, locality, 

 and date, and they should be numbered and catalogued, so that they 

 may be easily referred to when put up in cases, such as that shown in 

 fig. 956, or in cabinets.* 



The Diatomacese being either free, or attached to Algse, etc., dif- 

 ferent modes must be resorted to for collecting them. Those which 

 are attached require only (either at the time or after being dried) to 

 be rinsed gently in fresh water to get rid of the sand or mud, and salt 

 if any, and then placed in a small saucer in boiling water, with a few 

 drops of nitric or muriatic acid. The cuticle being corroded, the 

 ' Diatoms fall to the bottom, the fioating Algse are taken out with a 

 glass rod, and the residue washed. This step is merely preparatory 

 to that of burning or boilmg the objects. If the Diatoms be free, 

 they should, as far as possible, be gathered free from sand or mud, 

 by skimming the surface of the pond or pool with an iron spoon ; but 

 as much mud and sand may still be mixed with them, they ought to 

 be afterwards placed in a saucer in a little water, and exposed to the 

 sun for a day*or two. A tumbler or hand-glass wiU prevent too much 

 evaporation. Diatoms, if recently gathered and alive, wiU come to the 

 surface of the sediment, or water, or both, and this affords an easy 

 mode of separating certain species. They may now be skimmed off 

 with a small spoon, or, what is preferable, a camel's hair pencil, and 

 removed to clean water ; and this process is to be repeated till the mud 

 is got rid of entirely. As for preparing the specimens, they may be 

 either burned, or boiled in nitric acid. Tor the isolated Diatoms,+ as 

 Navicula, Pleurosigma, Cocconeis, etc., boiling is preferable; but for the 



* In making sections of minute objects, sucli as Diatoms, they are mixed with, 

 plaster of Paris and mucilage, and tlien tlie wliole is sliced by means of a sharp razor. 

 Small pieces of wood are sometimes put into a slit in a cork, and then the whole sliced. 



t By free Diatoms are meant those that are not parasitical. By isolated or solitary 

 Diatoms are meant those not connected nor cohering together into threads or plates, or by 

 a stipe, tube, or gelatine. 



