2 PKOCEEDIXGS OF THE XATIOlSrAL MUSEUM. vol. 60. 



net is drawn through the water slowly, preferably back of a rowboat, 

 until the desired quantity of plankton diatoms has been secured. 

 This is washed into a jar, partly filled with water, bj'^ reversing the 

 net, A small quantity of commercial foi'malin is then added to the 

 jar to kill all the organisms and allow them to settle. The time re- 

 quired for settling depends upon the height of the water in the jar, 

 but should be given an hour in a jar containing 10 to 12 inches of 

 water, as the diatoms, which are characteristically delicate in planli- 

 ton forms, settle slowly. When settling is complete the excess of 

 water is very carefully poured away and the diatom sediment is 

 bottled. 



The method used for cleaning and preparing the more robust 

 diatoms collected from the bottom of lakes, rivers, or the ocean, is 

 not well adapted to these delicate plankton species and special 

 methods are therefore necessary to prepare them for microscopic 

 study. In general the material in the bottom of the bottle can be 

 microscopically examined under a cover glass without further prep- 

 aration; but as the markings upon many of the species are difficult 

 to see in water, it is preferable to use a mounting medium of high 

 refractive index. Several methods can be suggested. By trans- 

 ferring some of the material from the collecting bottle to a small 

 homeopathic vial and gradually replacing it by alcohol, one can then 

 mount the material in so-called gum thus, also known as frankincense. 

 This is obtainable at most druggists. It is dissolved in pure alcohol 

 to the consistency of thick syrup. After the diatoms have been 

 transferred to a glass slide, and a drop of gum thus placed upon them 

 and covered with a cover glass, the mount is ready for examination 

 and has the advantage of being practically permanent, at least for 

 several years. This mounting medium will bring out the markings 

 of most of the plankton forms. There is, however, a better medium 

 which, because of its very much higher refractive index, shows the 

 more delicate structure which is hard to see in gum thus. This 

 medium, a solution of barium mercuric iodide, is made as follows : 



To a saturated aqueous solution to barium iodide is added red 

 mercuric iodide until a slight excess of the latter remains undis- 

 solved. A drop or two of the saturated solution of barium iodide 

 is then added, in order to effect the solution of this excess of the 

 mercuric iodide. The perfectly clear yellow solution is now ready 

 for use and can be kept indefinitely in a well-stoppered bottle. A 

 minute quantity of the diatom sediment is placed by a pipette on the 

 slide. This is taken from the collecting bottle where the diatoms 

 are in weak formalin. The excess of water is drawn away from the 

 material on the slide by means of a triangular piece of thin blotting 

 paper or filter paper. A drop or two of the barium mercuric iodide 

 is then added, the diatoms gently stirred in with the needle, and 



