2l8 



HARD WICKE 'S S CIENCE - G O SSIF. 



whicli is principally composed of round or prismatic 

 granules. 



If the diatoms are to be mounted in balsam, I prefer 

 that they should be on the slide ; if for dry mounting, 

 and for examination by high power (anything beyond 

 an -g-), they ought to be mounted on the cover. 



We will suppose that the gathering consists of 

 diatoms requiring to be mounted dry. We must 

 make a cell of asphalte varnish on the slide, taking 

 care that it is of sufficient depth to prevent the 

 fracture of the diatoms when the cover is pressed 

 down. 



Asphalt varnish should always be thinned with 

 benzine, as it evaporates quickly, and the cell can 

 be made hard in a short time. Cells made of turpen- 

 tine and asphalt are objectionable for two reasons : 

 the first is, that the cell never becomes thoroughly 

 hard, and in consequence a deposit of oily globules 

 takes place on the cover, and the slide is spoilt ; 

 secondly, the coloured varnishes used for finishing off 

 (unless made of sealing-wax) are almost certain to 

 nm in. If the diatoms are large and heavy, a minute 

 quantity of gum must be used to make them adhere, 

 otherwise they are in danger of being detached. 

 Mounting in Canada balsam generally gives the tyro 

 no end of trouble, particularly if he tries to harden it ; 

 air-bubbles often appearing in considerable numbers. 

 These are caused by the balsam having been mixed 

 with turpentine. Pure balsam may be heated until 

 it is brittle without the formation of bubbles. A 

 little turpentine should be dropped on the diatoms, 

 which should be allowed to permeate them before 

 putting on the balsam. When this has penetrated, 

 the slide should be heated until the balsam, when 

 cold, is too hard to be indented with the nail. The 

 slide should be again heated, and the cover applied 

 to the edge of the balsam, but must not be allowed 

 to fall suddenly upon it. I prefer to hold the cover 

 in a slanting position, until the balsam is sufficiently 

 hard to retain it. I then reverse the slide (the cover 

 downwards) over the lamp, and as the balsam melts, 

 the cover is gradually drawn up by capillary attrac- 

 tion, and the superfluous balsam may be gently 

 pressed out, and when the slide is cold, carefully re- 

 moved with a knife, and the slide cleaned with spirits 

 of wine and ammonia, or benzole. The advantages 

 of hard balsam are twofold : first, the covers are not 

 likely to be displaced, particularly when they are sent 

 to hot climates ; secondly, the refractive index of the 

 balsam is altered, and less nearly approaches that of 

 the diatom-valve ; consequently markings that would 

 be invisible in soft balsam or damar are distinctly 

 seen in the hard balsam. Unfortunately hard balsam 

 cannot always be used — e.g. , when it is desirable to 

 mount the frustule, or the valves are very convex. In 

 these cases the heat drives out the fluid balsam, and 

 as it thickens on cooling, it cannot re-enter, and a 

 pseudo-bubble or vacuole makes its appearance. To 

 avoid this I do not harden the balsam more than will 



allow the nail to easily indent it. This is best done 

 by subjecting the slide to a gentle heat (about 80° or 

 90°) for several hours after the cover is placed upon 

 it. As much of the superfluous balsam as possible 

 should be removed, taking care not to shift the cover: 

 it should be allowed to remain for several days (if 

 possible exposed to the above heat). To avoid 

 accident, a ring of gum and whiting should be spun 

 round the edge of the cover : this, when dry, will 

 allow of a finish with asphalt or coloured varnish. 

 Before mounting in slightly hardened balsam, the 

 diatoms should be attached to the slide by a very 

 weak solution of gum, to keep them in position if the 

 slide should be placed in a racked cabinet. In no 

 case should the cover be placed on the balsam or 

 damar when quite soft : it never hardens afterwards, 

 excepting at the edges ; if it did, the slide would be 

 much disfigured by vacuoles making their appearance. 

 As balsam or damar owes its fluidity to the presence 

 of turpentine (natural in new balsam) or benzole, it 

 must inevitably occupy less space as it becomes 

 harder, which can only take place through the escape 

 of the spirit. 



The desideratum of the diatomist is, or ought to be, 

 gatherings containing only one species, or at most 

 tv/o or three. But as these are not of frequent 

 occurrence, the plan of selecting or picking one and 

 transferring to another slide has been adopted, parti- 

 cularly by professional mounters, some of whom have 

 acquired considerable skill in arranging these tiny 

 forms in various designs. To do this requires a great 

 deal of practice and time, and no particular advantage 

 is gained. All that the student need attempt is to 

 pick out as many as he can of one species, place them 

 close together in the centre of the slide, and in various 

 positions. I trust the following hints and practice 

 will enable the learner to prepare slides in this 

 manner. 



Selecting diatoms can only be done by the aid of a 

 microscope, and the question is, whether the simple 

 or compound instrument is the better for that pur- 

 pose. Individually, I prefer the simple form ; the 

 powers I use are ^, |, and i inch. The ^'^ I 

 use to search over the drop, and when I see a 

 form I wish to select, I use the | or I inch to push 

 it on the clean part of the slide. I usually place 

 a drop of water near the drop of material : the 

 advantage of this is, the diatom may be freed from 

 any extraneous matter hanging to it. If it is intended 

 to remove it to another slide, it must be pushed out 

 of the drop and allowed to dry, when it can be 

 picked off and transferred to another drojD of water, 

 which should be about half an inch from the centre 

 of the slide, or, if mounted on the cover, to a small 

 drop near its edge. In this drop other specimens 

 must be placed, and when the desired number are 

 selected, pushed into the centre of the slide or cover. 

 If the specimen appears to be new or very rare, it is 

 better not to risk the removal to a fresh slide ; tlie 



