298 



SCIENCE-GOSSIP. 



preservative fluid used, and then to protect 

 this cement with a reliable varnish. No single 

 cement, therefore, can possibly do for all mounts. 

 Three kinds of cements are available : first, those 

 gums and balsams soluble in the terbene and the 

 mineral naphtha series of solvents (turpentine, 

 benzol, etc,) ; secondly, those gums and lacs soluble 

 in alcohol and wood naphtha ; thirdly, oil 

 varnishes, such as gold size. Each of these three 

 kinds of cement will have to be used according to 

 the nature of the preservative fluid. 



Canada balsam and similar mounts can very 

 readily be protected by two or three rings of a 

 solution of shellac in alcohol. Shellac is a hard 

 and stable substance, unalterable in air of ordinary 

 temperature, and will protect the balsam from 

 contact with air, and, therefore, from oxidation and 

 disintegration. Pure shellac, however, is some- 

 what brittle, but it can be made tough by adding 

 a little castor oil (twenty drops to the ounce of 

 shellac varnish) or Venice turpentine to thesolution. 

 Venice turpentine is the resinous exudation of the 

 larch tree, and dissolves in alcohol. 



Watery fluid mounts are best secured, first with 

 a coat of a cement made by mixing two-thirds 

 volume of a solution of gum-damar in benzol with 

 one-third volume of best gold size. Gum-damar 

 by itself is a bad and brittle cement, but it has the 

 exceedingly valuable property of sticking firmly 

 to glass, even when moist with water or glycerine. 

 I know of no other substance which will do that so 

 well. When the first coat of the damar-gold size 

 is dry (twenty-four hours) it must be protected by 

 three or four successive layers of pure gold size. 

 Gold size consists mainly of boiled linseed oil 

 combined with a resin dissolved in turpentine. 

 When spread in a thin layer this solvent evaporates, 

 and the oil then hardens, not by evaporation 

 but by oxidation, into a tough, stable substance 

 named linoxine. In order to prevent the complete 

 oxidation of the gold size, I prefer to cover it 

 finally with a protecting layer of some alcoholic 

 cement, such as shellac and Venice turpentine, or 

 Ward's Brown Cement, the basis of which is 

 shellac. It will be observed that in this case I 

 use three separate cements one over the other, 

 having different qualities, and each sealing or 

 protecting the previous layers. In ringing the 

 slide each coat is made to slightly overlap the 

 previous one. 



Glycerine is notoriously the most difficult 

 substance to seal securely and permanently. It 

 can be done by closing first with a ring of the 

 above-mentioned damar-gold size cement, then, 

 after washing away every trace of glycerine on 

 the slide, making two or three rings of shellac 

 varnish, then three or four rings of pure gold size, 

 and finally a coat of Ward's Brown Cement. 

 Glycerine mounts closed in this way will not leak, 



In very deep cells containing much fluid a small 

 air-bubble should be left ; this acts as a safety- 

 valve, otherwise the expansion of the fluid in hot 

 weather is liable to force off the cover-glass, how- 

 ever well it may have been sealed. 



Dr. Keegan wishes to banish gold size from our 

 cabinets ; yet, when of the best quality, this is a 

 first-class oil varnish, and one of the few micro- 

 cements which have well stood the test of time. I 

 have a slide, and know of other similar ones, a 

 thick injected anatomical preparation mounted in 

 in a deep cell with watery fluid, prepared by 

 H. Hett, about 1854, closed with gold size, which 

 is now as good and sound as when it was made 

 forty-five years ago. That is a good record for 

 gold size. 



Asphalt varnish, zinc white and similar sub- 

 stances are of little practical use, except as 

 ornamental cements. A complete treatise on the 

 chemistry of micro-cements and varnishes, and 

 how to use them, is very much needed, and 

 should be written by a thoroughly competent 

 chemist, well acquainted with the various gums, 

 resins and lacs, their chemical composition, their 

 qualities, their solvents and diluents, and the 

 changes they undergo under various conditions. 

 Thousands of slides, some of my own included, 

 prepared at great cost of time and labour, have 

 been ruined and lost by using unsuitable cements 

 in closing the mounts. Hardly any trustworthy 

 information can be found on this subject in any 

 book dealing with microscopic mounting and 

 manipulation. I trust, therefore, that a competent 

 man, after a thorough investigation of the subject, 

 will do for microscopists what Prof. A. H. Church 

 has done so well for painters in his " Chemistry of 

 Paints and Painting." 



Gt. Castle Street, London, W. 



Volcanic Ash in the Yoredale Beds. — On 

 January 4th a paper was read before the Geo- 

 logical Society, by Mr. H. H. Arnold Bemrose, 

 M.A., F.G.S., on the sections exhibited in the 

 cuttings on the Ashbourne and Buxton Branch 

 of the London and North-Western Railway. The 

 southern part of the new railway from Ashbourne 

 through Tissington and Crake Low to Buxton 

 exhibits several sections in Trias, Boulder-clay 

 and Yoredale Beds. Interstratified with the latter 

 is a thick bed of volcanic ash, with thinner inter- 

 calations of tuff. Within a mile of Tissington, ash 

 is exhibited four times in the cuttings, and accord- 

 ing to the view of the author, substantiated by 

 Dr. Wheelton Hind, it is the same bed repeated 

 by domes and basins, which are sometimes faulted. 

 The ash-bed is 144 feet thick where fully exposed. 

 The ejected blocks in the ash vary from several 

 inches up to one foot in diameter, and are similar to 

 the blocks in the vent at Kniveton. The small 

 lapilli scattered through the limestones are gene- 

 rally converted into calcite or dolomite with oxide 

 of iron. — Edward A. Martin, 69, Bcnsham Manor 

 Road, Thornton Heath. 



