MOUNTING IN BALSAM IN AQUEOUS LIQUIDS 481 



of over the spirit-lainp, there will be little risk of the formation of 

 air-bubbles. However large the section may be, care should be taken 

 that the balsam is well spread both over its surface and that of 

 its cover ; and by attending to the precaution of making it accumu- 

 late on one side by sloping the slide, and letting down the cover 

 so as to drive a wave before it to the opposite side, very large sections 

 may thus be mounted without a single air-bubble. (The Author has 

 thus mounted sections of Eozoon three inches square.) In mounting 

 minute balsam objects, such as diatoms, polycystince, sponge-spicules, 

 and the beautiful minute spines of ophiurida, no better plan can be 

 adopted than to arrange these objects carefully upon the cover, 

 either by ' scattering ' or ' arrangement,' and then to drop on to the 

 whole cover and its arranged objects as much balsam as the cover 

 will receive without overflow ; this should stand free from dust for 

 some hours, after which the partly hardened balsam may receive a 

 small drop of fresh balsam, and being placed upon the -slip in proper 

 position, may by the use of gentle heat be pressed finally into position. 

 When the chitinous textures of insects are to be thus mounted, they 

 must be first softened by steeping in oil of turpentine ; and a large 

 drop of balsam being placed on a warmed slide, the object taken up 

 in the forceps is to be plunged in it, and the cover (balsamed as before) 

 let down upon it. It is with objects of this class that the spring- 

 clip and the spring-press prove most useful in holding down the cover 

 until the balsam has hardened sufficiently to prevent its being lifted 

 by the elasticity of the object. Various objects (such as the palates 

 of gasteropods) which have been prepared by dissection in water or 

 weak spirit may be advantageously mounted in balsam ; for which 

 purpose they must be first dehydrated, and then transferred from 

 rectified spirit into turpentine or one of the other ' clearing agents ' 

 mentioned below. Sections of horns, hoofs, &c., which afford most 

 beautiful objects for the polariscope, are best mounted in natural 

 balsam, which has a remarkable power of increasing their trans- 

 parence. It is better to set aside in a warm place the slides which 

 have been thus mounted before attempting to clean off the super- 

 fluous balsam in order that the covers may be fixed by the gradual 

 hardening of what lies beneath them. 



Mounting Objects in Aqueous Liquids. By far the greater 

 number of preparations which are to be preserved in liquid, however, 

 should be mounted in a cell of some kind, which forms a well of 

 suitable depth, wherein the preservative liquid may be retained. 

 This is absolutely necessary in the case of all objects whose thickness 

 is such as to prevent the glass cover from coming into close approxi- 

 mation with the slide ; and it is desirable whenever that approxima- 

 tion is not such as to cause the cover to be drawn to the glass slide 

 by capillary attraction, or whenever the cover is sensibly kept apart 

 from the slide by the thickness of any portion of the object. Hence 

 it is only in the case of objects of the most extreme tenuity that 

 the cell can be advantageously dispensed with ; the danger of not 

 employing it, in many cases in w r hich there is no difficulty in 

 mounting the object without it, being that after a time the cement 

 is apt to run in beneath the cover, which process is pretty sure to 



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