.S( Hi. NCR. GOSSIP. 



359 



DKSMIDS. 



liV 1)K. (i. 11. HflVAN, I'.K.S. 

 (CoHtintied from page 336.) 



I\/T CH'N riN(i. — A ihiii cciui.-iit cell is necessary to 

 prevent the varnish used in sealing up the pre- 



p«ratiun from runiiinj; in. I have used gold-size, zinc 

 cement, brown cement, and sealing-wax varnish for the 

 cells, and gold size and hrown cement lor closing lliem 

 up. As to the efficacy of dilferent cements it may be 

 po,ssible to express some opinion when my slides have 

 been mounted twenty years. Thi- well-known slides 

 mounted by Mr. Joshua of Cirencester, at about that 

 time ago, were sealed with zinc cement, and very few 

 of them now survive. Some of them were sent to me 

 about a year ago by Mr. Watkins, of Painswick, of 

 which I managed to save four by the application of 

 fresh glycerine and gold-size. In most of these once 

 beautiful slides, the zinc cement has chipped away 

 and the mounting medium has escaped, or evaporated. 

 Nevertheless, I have mounted most of my desmids in 

 zinc cement cells, trusting to the use of gold-size in 

 sealing them, as affording some protection ; and some 

 slides of Cosmarium and of the diatom Mcridioii 

 lirailare mounted in cainphor water in this way, in 

 1890 and 1891, are still as good as new. Gold size 

 hardly gives cells of suflicient consistency. My pre- 

 sent idea is to place a thin ^ing of gold size on the 

 slide and over it to spin a ring of zinc cement, the 

 latter giving the cell the required degree of thickness, 

 and the former by its adhesion preventing the zinc 

 cement froin chipping off. 



The cell in which the desmids are mounted 

 should not be deeper than is absolutely necessary. 

 In order, therefore, to get a sufficient nuniber of 

 desmids into the slide, it is best to push the pen- 

 filler right down into the sediment of desmids at the 

 bottom of the specimen tube or cork float contain- 

 ing them, so as to take up a thick mass of desmids 

 with little glycerine. In mounting such objects the 

 text-books tell us not to use more preservative fluid 

 than will just fill the cell ; but they do not say how tliis 

 is to be done. The only way is to make the fluid 

 form a convex drop, without extending quite to the 

 edges of the cell. The drop must stand higher than 

 the cell-wall, and, if a cover-glass is lowered hori- 

 zontally, the fluid will spread out and fill the cell. 

 The amount of fluid must be estimated by holding up 

 the slide and looking at it edgewise ; the part of the 

 drop projecting above the cell should just be suffi- 

 cient to fill the uncovered portions of the cell. If 

 too much glycerine has been used, the superfluous 

 liquid must be soaked up with blotting-paper ; but 

 in doing this a large proportion of the desmids are 

 apt to be drawn out from undfer the cover. If there 

 is too little fluid an air-bubble will be left in the 

 cell. If this bubble is small it may be worth while 

 to try whether its presence renders the slides less 



liable to leakage by allowing for expansion and con- 

 tractitm with variatinns of temperature. 1 have sealed 

 up several desinid .slides containing small air-bubbles, 

 but the bubbles have generally disappeared in a 

 short time, fortunately without causing the varnish 

 to run in. 



Before lowerint; the cover-glass, it may be well to 

 stir the drop of fluid with a needle to spread the 

 desmids. If the gathering is not exceptionally clean, 

 any particles of dirt that are visible to the naked eye 

 may at the same lime be picked out with the mounting- 

 needle, while any dust-fibres which may have crept 

 into the material in the process of cleaning may be 

 similarly removed under the microscope. If the 

 cleaning process has failed to remove all the flocculent 

 matter or vegetable lii'bi is. better results can some- 

 times be got by putting too large a drop of fluid on the 

 slide, and then draw ing pan of it off, which will remove 

 more of the dirt than of the desmids, often producing a 

 much cleaner slide. If saiid'M the chief impurity this 

 plan will only increase the proportion of it left on the 

 slide and spoil its appearance. In most cases a little 

 care in picking out dirt makes an immense difference 

 in the quality of the slides. 



For sealing the cells I have used brown cement or 

 gold-size : generally the latter. Most writers recom- 

 mend Ijrown cement, or some other spirit varnish 

 containing shellac for sealing glycerine, in preference 

 to gold-size, on the ground that the latter does not 

 close the cell securely if the edges are moist with 

 superfluous glycerine. I have found no such diffi- 

 culty ; on the contrary, gold-size seems to me full) 

 as efficacious as the cements with alcoholic solvents. 

 When a second and thicker layer of gold-size has 

 been added it may take weeks to harden, and may 

 be easily indented with the finger after months ; but 

 it is probably the more reliable, because it dries s.i 

 slowly. The brown cement dries more quickly, and 

 has stood the test equally well up to the present. 



Desmids, being as a rule far larger than the average 

 run of diatoms, are easily selected either by using a 

 bristle to move them from a drop of fluid into the 

 centre of the slide, or by drawing them up in a finely 

 pointed glass tube, and depositing them on a clean 

 slide. Possibly even a split hair from a shaving- 

 brush, though far too clumsy for manipulating diatoms, 

 might prove useful with the larger desmids. The 

 chief difficulty is that there appears to be no way ot 

 fixing the desmids when selected, and there is a risk 

 of their floating from under the cover— or, at least, to 

 the edges of the cell when the cover is applied. T" 

 obviate this, Mr. White, of Litcham, has used sunk 

 cells, in which the specimens naturally gravitate 

 towards the centre. 



