28 OF THE FREE DIATOMACE^. 



tap, &, leading into the funnel. The amount of disturbance, 

 also, to which the Diatomacese are subjected can be altered 

 at will, by having mouth-pieces of different sizes made for the 

 funnel tube, h, so that the apparatus is completely under 

 the control of the operator, and any degree of washing can 

 be given to the contents of the cylinder— the column of 

 water issuing from the funnel and the opening of the 

 mouth-piece below being made to bear a certain relation to 

 each other. 



The Diatomaceae being now cleansed from all impurities, 

 the next step is to put them away in the herbarium. If 

 the quantity be sufficient, it is a good plan to lay them 

 (using a camel's hair pencil for the purpose) on stout paper 

 or cardboard, and there let them dry. Should they be 

 wanted for examination under the microscope, it is easy to 

 take up a portion, about che size of a pin's head, and lay it, 

 on a glass slide. If, however, the original supply is small, 

 the whole may be placed at once on a glass slide, protected 

 by a thin glass covering. The advantage of this method is, 

 that the object is always ready for microscopical observation 

 and comparison, and the same preparation may be examined 

 any number of times. Care, however, must be taken, 

 while laying on the drop, that the individuals are not too 

 crowded together, but that each one stands out distinctly ; 

 otherwise the passage of light is interfered with, and a dark 

 shapeless mass is all that meets the observer's eye. 



The scientific value of each preparation is increased by 

 having at hand some specimens in a ' crude ' state, that is 

 to say, with the endochrome in situ, and not removed by 

 an acid. The difference between these two states is this: 

 when the valves have been submitted to the action of a 

 strong acid, the delicate sculpturing of the silicious coat 

 comes out, under proper illumination, in a marvellous 

 way ; but the endochrome has entirely disappeared ; the 

 plant is, in fact, a mere shell. In the other case, when 

 the individuals are not submitted to an acid, and are 

 examined in a fluid, the markings are scarcely visible ; 



