ART. 15. COLLECTING AND PREPARING DIATOMS MANN, 3 



covered with a cover glass. If it is desired to preserve this mount, 

 any excess of the liquid is first removed from under the cover glass 

 by means of another triangular piece of blotting paper; the mount is 

 then sealed with a ring of hot paraffin, followed by one or two rings 

 of some good cement. 



It is also well to clean up a small portion of each sample of plank- 

 ton material by the acid methods to be described under the next head- 

 ing ; as there are always a number of forms in most plankton gather- 

 ings sufficiently robust to stand this, and these forms are much better 

 studied after such treatment than in the uncleaned condition of the 

 mounts prepared by the foregoing methods. This is due to the 

 removal of all organic matter from these denser diatoms and conse- 

 quently the better view that is obtained of their sculpture. By using 

 these two methods with plankton material, the student secures all 

 the advantages of each, and has no difficulty in identifying all the 

 species present. 



We now turn to the collection and preparing of diatoms inhabit- 

 ing the bottom of rivers, lakes, bays, etc. Aside from the places 

 where diatoms can be collected in shallow waters, where they are 

 seen by means of their color, as has been previously pointed out, the 

 collector needs some form of dredge to obtain samples in the deeper 

 water. The best dredge known to the writer is fortunately a very 

 simple one, and one involving a very small outlay of money. This is 

 made of about 15 inches of cast-iron drain pipe with a diameter of 

 about 4| inches. One end of this is closed, either by a shallow wooden 

 plug or by brazing a circular piece of metal to the end. At the other 

 end a stout wire bail is attached by means of two holes drilled 

 through the pipe about an inch apart and as close to the edge as 

 possible. The side of the rim opposite to this bail is sharpened on 

 the inner side with a round file, so as to make a cutting lip. 



The advantages of this dredge are several — its cheapness and the 

 ability to have one made even at remote places; its indestructibility 

 when used on stony bottoms; and the fact that it alwa^^s secures a 

 satisfactory amount of mMerial, about a liter of the surface of sand 

 or mud. It should be drawn very slowly over the bottom, either back 

 of a rowboat, or from the beach by casting the dredge out and 

 drawing it slowly ashore. It should be remembered that approxi- 

 mately five times the depth of the water should represent the length 

 of line when dredging, otherwise the dredge may be tilted at the 

 front end and fail to scrape up the bottom. When the dredge is 

 near the boat and is to be lifted it should be done gently, otherwise 

 its contents will be spilled. It should also be raised gently to the 

 boat, so that the rush of water does not sweep out the material. If 

 the bottom is muddy, the material must be put in a jar or bottle, 

 and the mud separated in the laboratory. If the bottom is sandy, the 

 3136— 22— Proc.N.]\I. Vol.60 37 



