Preservatives 



FLUID WITOLEMOUNTS — AQUEOUS TYPE 



23 



shelf, and a heavy weight placed on top 

 until the cement is cooled. 



Whether gold size or marine glue be em- 

 ployed, care must be taken to remove 

 those portions which have been extruded 

 into the interior of the cell. These cements 

 swell up and become white in the presence 

 of water, and even a trace of remaining 

 cement will give an unfinished appearance 

 to the slide. The excess cement may be 

 removed by scraping with a scalpel, and a 

 final cleaning may be given with a 10% 

 solution of potassium, or sodium, hydrox- 

 ide, which is wiped over the inside of the 

 cell with a piece of cotton held in a pair of 

 forceps. The cell is then thoroughly 

 washed and laid on one side to dry. It 

 is best to cement cells onto slides in ad- 

 vance of requirements and thus secure an 

 adequate reserve. 



Selection of a Preservative Medium 



The introduction of formaldehyde to 

 microscopic technique was welcomed as 

 the beginning of the millennium, and 

 almost all of the older aqueous media 

 were thrown overboard by mounters. 

 This is to be regretted since formaldehyde 

 is by no means a perfect medium, particu- 

 larly for the preservation of small inverte- 

 brates and single-celled plants, so that 

 attention should be given to the list of 

 aqueous preservative media in Chapter 

 17 (P 11.1). These media are mostly 

 variations on the fluid of Goadby which 

 was a mixture of sodium chloride and am- 

 monium alum — designed to approximate 

 an isotonic solution — containing a very 

 small quantity of mercuric chloride as a 

 preservative. Some of these solutions (cf. 

 Kronecker 1907) had an alkaU added to 

 preserve the green color of small algae. 

 Another excellent preservative of green 

 material is the solution of Ripart and 

 Petit which is given in Chapter 18 (F 

 3000.0010 Ripart and Petit 1884) because 

 it serves the dual purpose of fixation and 

 preservation. A very similar formula was 

 pubhshed by Woods in 1929 (Chapter 17, 

 P 11.1) as a preservative for green algae. 



Simple solutions of various reagents 

 may also be employed. The best of these is 

 formaldehyde, which for purposes of 

 mounting should never be neutraUzed 



since, once subjected to this treatment, it 

 is liable to develop precipitates. The 

 ordinary 1 to 10 dilution of 40% formalde- 

 hyde, which is commonly employed for 

 the preservation of gross biological speci- 

 niens, is far too strong for microscope 

 mounting of the type being discussed. 

 It must be remembered that these strong 

 solutions become greatly diluted from the 

 water contained in the specimens placed in 

 them, whereas in the case of a microscope 

 mount the material will have already been 

 impregnated with formaldehyde before 

 being mounted. A dilution of 1 to 100 of 

 the commercial 40% formaldehyde is 

 adequate as a mounting fluid. Camphor 

 water and chloroform water, which are 

 merely saturated solutions of these rea- 

 gents in distilled water, are also excellent 

 preservatives for the more delicate Pro- 

 tozoa and Algae. 



It must be emphasized that if glycerol 

 is added to these media, the material will 

 have to be handled by the special methods 

 necessary for making glycerol mounts, 

 which are described in the next chapter. 



Selection of a Coverslip Cement 



Though the cell is best attached to the 

 slide with a thermoplastic cement, it 

 must be obvious that a liquid cement must 

 be used to attach the coversHp to a cell 

 containing a fluid mounting medium. 

 Numerous formulas have been developed 

 for this purpose, and the author most 

 warmly recommends either gold size, or 

 the cements given under Behrens 1883 or 

 Carany 1937 in Chapter 28 (V 12.1). 

 These last two cements are quick drying, 

 which is desirable, since at least three 

 successive coats must be used properly to 

 seal an aqueous wholemount. The first 

 of these coats is designed to block off the 

 water, and to provide a temporary support 

 for a second layer of waterproof cement 

 which would not adhere to the moist 

 glass. This second coat of waterproof 

 cement should always be an asphalt 

 varnish (formulas are given in Chapter 

 28, V 11.2) and, if the black color is ob- 

 jected to, any colored varnish may be 

 coated over the asphalt to provide a more 

 finished appearance to the mount. 



