ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



619 



of a Petri's capsule. Some carbonate of magnesia is sprinkled over the 

 bottom underneath the filter-paper. The usual inorganic solution for 

 cultivating nitrite-formers is then poured in, care being taken that tho 

 fluid does not reach the topmost layer. The fluid should reach about 

 half up the clump of paper disks. The capsule is then sterilised, and 

 when cool inoculated. When the nitrification process is set up, the 

 fluid is tested for ammonia and nitrous acid. When all tho ammonia 

 has disappeared a few drops of sterilised 10 p.c. ammonium sulphate 

 are introduced. Colonies are just visible to the eye by the 10th to 15th 

 clay as yellowish points, which gradually become brown. 



(2) Preparing Objects. 



Methods for Use in the Study of Infusoria.* — A. W. Peters obtains 

 clean specimens of many kinds of Infusoria by the following " yarn- 

 siphon " method. From the culture-jar a quantity of 

 the liquid is removed with a pipette to a Stender dish. 

 The organisms are distributed by sucking up the liquid 

 into, and forcing it out of, the pipette a few times. A 

 few pieces of woollen yarn about 10 cm. long are then 

 laid parallel in a single strand, held in water and 

 pressed together until thoroughly wet. This yarn- 

 siphon is then placed with one end in the Stender dish, 

 the other hanging over into a receiving vessel. Ciliated 

 organisms soon pass over the siphon into the receiving- 

 vessel. From time to time fresh water is added to 

 replace that lost by siphoning. 



To concentrate the organisms in a small amount of 

 water, to remove the culture, or to change the medium, 

 the author devised an apparatus termed a " tube-filter." 

 One end of a short piece of wide glass tubing is closed 

 by a piece of filter-paper held in position by a rubber 

 band. The process essentially depends on the quality 

 and area of the filter-paper employed ; for rapid work 

 with about 50 ccm. of fluid a tube of about 3 cm. in 

 diameter and 6 cm. in length is used. The tube is held 

 in a vertical position on a ring-stand, and under it is placed a Stender 

 dish or other vessel containing the organisms. The tube is lowered 

 until its paper diaphragm comes within a few millimetres of the bottom 

 of the dish. In the tube is hung a filled glass siphon with the lower 

 end of the outer arm bent upwards to prevent its running empty. As 

 the water rises through the filter-paper and into the tube, it is removed 

 by the siphon. More culture-water with organisms or fluid desired as 

 medium may bo added from time to time. The process of upward 

 filtration leaves nearly all the organisms in the dish when tho tube is 

 removed. 



Another device, called the U-cell, serves much the same purpose as 

 the tube-filter, but on a smaller scale. To make this U-cell (fig. 123) 

 there are necessary two slides, some rubber bands, and coarse, tough 

 darning-cotton. A piece of the cotton one-and-a-half times the length of 

 the slides is saturated with water and then laid upon a slide in the form 



* Amer. Natural., ssxv. (1901) pp. 553-9 (2 figs.) 



Fig. 123. 



