232 THE AMERICAN MONTHLY [Aug. 



water carefully poured back into the bottle, retaining 

 in the cup about a half inch of water. The cup was then 

 shaken alternately at right angles to cause the sediment 

 to gather together, and was then finally twirled with a 

 gentle circular motion, which massed the diatoms into a 

 visible flocculent ball-like mass. At this stage more 

 water can be poured off without disturbing the flocculent 

 ball, when a bulb pippete will remove the diatoms with 

 the least amount of liquid in the tube. 



A drop from the tube is now deposited at the center of 

 a glass slip. In the meantime, there should have been 

 rubbed up a small quantity of India ink A drop of the 

 India ink is added to the living diatoms, and a clean 

 cover glass of at least three-fourth inch diameter is 

 placed over the liquid. The slide is now placed on the 

 stage of the microscope, and if inspection is made by 

 means of half inch lens, it will be apparent at once, that 

 all the living diatoms of the larger Navicula species are 

 surrounded with either a narrow or broad peripheral 

 hyaline or transparent zone. This is the evidence of the 

 protoplasmic epidermis, and that the transparent and 

 empty shells in most cases are not surrounded by the 

 gelatinous coat. At other times the recently dead and 

 partially transparent shells seem to be surrounded by 

 the gelatinous layer, which of course is inert. 



While the slide is under study two conditions may 

 happen i.e., the layer of India ink may prove to be too 

 dense, when nearly all transmitted light will be cut off, 

 and the field appears as an opaque surface, though the 

 diatoms will be partially visible as translucent spots. 

 The other is when the layer of India ink presents a uni- 

 form layer of a grayish tone. Then the slide is proper 

 for continued examination under a 1-6 lens or even 

 higher powers. When the proper density of pigment 

 obtains, the pseudociliar effect of the anterior, or advanc- 

 ing end of the protoplasmic mantle of the diatom is 



