HARDWICKE'S SCIENCE-GOSSIP. 



271 



ticular species where I had already collected it, it 

 had disappeared, and another species had succeeded 

 it. But mostly the species were able to develop 

 simultaneously and in great abundance without in- 

 juring one another. It is in general during spring, 

 and when all the vital conditions are best fulfilled, 

 that the species found are most separated. Later on, 

 in summer, we often find in the same place and at 

 the same time, as many as twenty, thirty, or even 

 forty different species. 



Determination of the Species collected: First Exami- 

 nation. — In order to determine a diatom it is neces- 

 sary that the observer takes the water just as he has 

 collected it, and in the normal state, in a thin film of 

 water under the covering-glass of the microscope. 

 A magnifying power of + 200 or + 300 linear is 

 sufficient. All the soft and mucilaginous part, the 

 membranous envelopes, or even the filaments, the 

 points of endochrome attachment, &c, are visible, 

 and furnish important characters. It is well, for 

 this examination of living diatoms, to change their 

 position by pressing lightly and by tapping (with 

 a fine point of wood or of a pen) on the glass 

 covering the drop which contains them. We, in this 

 way, are able to appreciate clearly their exact form 

 and the relief of the different faces. 



Second Examination. — We next heat a few to a 

 dull red heat on a plate of iron, porcelain, or platinum. 

 The organic matter which much obstructs the obser- 

 vation of the valves, is carbonised and burnt. There 

 then remains the silicious envelope, which is termed 

 the frustule. It is only after this operation that the 

 beautiful striations appear in full relief, and the varied 

 designs which afford us so many useful specific 

 characteristics. It requires for this a power of about 

 + 400 or 600 (rarely 1000). 



Method of making Microscopic preparations. — Those 

 who wish to make a herbarium of diatoms, or in 

 other words, a collection of preparations all ready 

 for microscopic examination, and preserving indefi- 

 nitely their distinct characteristics, should proceed as 

 follows : — 



(a) Quick Method. — Of all the methods employed, 

 the following is the most rapid : it is necessary a: 

 once to separate with the greatest care the diatoms 

 from the slime or organic debris with which they are 

 encumbered. This is accomplished with a powerful 

 lens and a very fine bristle. 



They are next dried (after the addition of some 

 drops of nitric acid) in a small porcelain capsule, or 

 better, of platinum, afterwards they are heated to 

 about a temperature slightly below dull red heat, and 

 this is maintained for from five to ten minutes, till 

 all the organic matter is completely burnt. As this 

 incineration sometimes proceeds with difficulty, we 

 can accelerate it much by leaving it to cool, adding 

 a few drops of nitric acid, and then drying slowly and 

 heating again two or three times in a very airy place, 

 in order that the acid and corrosive vapours should 



not injure the operator nor attack the microscope. 

 The residuum is in general light yellow ; sometimes 

 the colour is reddish-brown, in consequence of the 

 presence of peroxide of iron proceeding from the 

 endochrome and the gelatinous envelopes. It is next 

 sprinkled with hydrochloric acid, then heated (but 

 not to boiling-point), and the whole set in a lipped 

 glass, and then filled with water. The first decanting 

 separates the sand which settles rapidly. As soon 

 as the diatoms are deposited they form a light 

 stratum, white and powdery, they are then washed 

 with boiling water by decanting, next with very pure 

 distilled water. The purity of the distilled water 

 may be tested by evaporating a few drops on a per- 

 fectly clean plate of glass ; it ought not to leave any 

 trace of deposit. A little water is left in order to 

 deposit the diatoms clean, and they spread about on 

 the small plate of glass called the cover-glass, and 

 are left there to dry. 



For the preparations called dry, we make on the 

 surface of the carrying-glass a circle of bitumen 

 (cellule) that has been heated, and then place the 

 covering-glass over when the bitumen is very dry ; 

 or else during the evaporation of the essence of the 

 bitumen the internal surface of the cover will be 

 covered with little oily globules, much obscuring 

 observation. It is necessary to have slender covers, 

 averaging one-tenth, or at the most two-tenths of a 

 millimetre thick. 



The adhesion of the cover to the dry bitumen of 

 the cellule is produced by heating to almost dull red 

 heat a piece of iron, and passing it over the entire 

 edge of the cover ; it must be pressed lightly. The 

 eye can easily follow the softening of the bitumen 

 and its immediate and certain all-round adhesion to 

 the edge of the cover. 



For the preparations in balsam it is necessary 

 (once the diatoms on the cover are perfectly dry) to 

 soak them with a little essence of terebinthine, and 

 add a drop of half viscid Canada balsam ; then apply 

 the cover on the slide which has been heated with 

 care in a spirit-lamp until the balsam is just about to 

 boil. At this moment quickly remove the flame. 

 The balsam is then sufficiently dry to adhere strongly. 

 The essence of terebinthine is for the purpose of 

 removing (by the tension of its vapour) the bubbles 

 of air which always remain in the interior of silicious 

 valves. 



This method gives very pure preparations and of 

 great beauty, but it is necessary to be careful in 

 avoiding a heat too strong, for there are some diatoms 

 whose silicious valves are so thin that even a dull 

 red heat softens and deforms them. Such are, for 1 

 example, the valves of Amphipleura pellucida, those 

 of Navicula pelliculosa, ocnlata, levissima, Bacillum, 

 and appendicnlata ; those of Synedra gracilis and 

 tenera, and those of Nitzschia Pecten, palea, and 

 parvula, &c. 



If then the first examination under the microscope 



