302 Transactions. — Geology. 



ner : On the reverse side of the slide you are going to use 

 describe a circle of ink, either with or without the turntable, 

 in the centre of the glass. This ink-circle is to serve as a 

 guide for you to place your diatoms. Dry it. Then, on the 

 proper surface of the slide put a drop of very weak gum-water 

 so as to cover the space occupied by the ink-circle on the 

 other side. This gum- water must be very weak, and should be 

 filtered ; and must be fresh, or else it gets full of fungus. 



I adopt a solution of arabin instead, to avoid the nuisance 

 of having to freshly prepare the gum-water each time. The 

 method of preparing arabin is given in the " Microtomists' 

 Vade-mecum" (A. B. Lee). Arabin is the pure gum-extract 

 of gum acacia or arabica, and is prepared by pouring a 

 small quantity of thick gum-and-water into a large quan- 

 tity of alcohol or spirits of wine. The arabin is insoluble 

 in spirit, and separates as a thick, white, flocculent, opaque 

 mass. It curdles the more as you add more spirit to it. 

 It is then collected by filtering and drying ; it is then washed 

 in absolute alcohol and dried again : the result is a fine, pure 

 white powder, freely soluble in water. I prepare it by adding 

 a solution of corrosive sublimate to it, and make a strong solu- 

 tion, from which to make from time to time thinner solutions 

 for use. Practice will quickly teach the proj^er strength. 

 Well, having put a thin coat of arabin on the slide and dried 

 it, we proceed to pick off our diatoms. 



The best thing that I have found for this purpose is a cat's 

 whisker fastened on a thin handle so as to leave about ^in. 

 of whisker projecting. It is useful to have two or three of 

 these, mounted with various thicknesses, for some diatoms 

 come off easier with one than with the other. 



Use a low power to examine your diatoms, and when you. 

 find one you want get it in the centre of the field and pick it 

 up. With a little practice you will soon find that the diatom 

 adheres very readily to the bristle or whisker. Now steadily 

 transfer it to the centre of the dried gum. In the same way 

 take oft' a few others. Now, if you wish you can arrange 

 them in order in rows, or any design you please. Take the 

 slide and breathe on it. This n:ielts the gum or arabin, which 

 runs into the diatoms. While the gum is wet you can push the 

 diatom into any position you like, but it dries very rapidly, 

 and then the least touch will break the diatom. If not satis- 

 fied breathe heavily again on the slide. By degrees you will 

 arrange them as you please. 



I have recommended mounting on the slide, but this is for 

 beginners : it is much easier. Mounting on the cover-glass 

 gives the best results, and should be the plan adopted. 



However, next comes the mounting-medium. Diatoms 

 may be mounted dry, or in some fluid medium such as Canada 



