HARDWICKE'S SCIENCE-GOSSIP. 



257 



MICROSCOPY. 



The PRErARATioN OF Dammar Varnish for 

 Microscopic Purposes. — So far as I know, none 

 of the receipts given in books for the preparation of 

 dammar varnish for microscopical purposes enable 

 the amateur to prepare a satisfactory article. 

 Dammar is not entirely soluble in ether, benzole, or 

 turpentine, at ordinary temperatures. If heat be used, 

 the solution is more complete, but, sooner or later, 

 the product will become milky, and then it will be 

 found impossible to clarify it. To obtain a perfectly 

 limpid solution, permanently remaining so, proceed 

 as follows : to 4 drachms of crushed Indian dammar 

 add 8 liquid drachms of pure benzole, and allow the 

 jesin to dissolve at the ordinary temperature. After 

 a day or two, an insoluble residue will be found at 

 the bottom of the vessel. Carefully decant the 

 supernatant clear liquid, and add to it 80 minims 

 ■(i^ drachm) of spirits of turpentine. The preparation 

 is then complete. The object of adding turpentine to 

 the solution is to ensure toughness in the dried film. 

 The above proportion is sufficient for the purpose. 

 Without the turpentine the dried film would be brittle. 

 I do not think that any advantage is derived from the 

 addition of mastic to the preparation, A very good 

 article as a substitute for Canada balsam may be 

 made with Reeves's or Winsor and Newton's copal 

 varnish, by evaporating it gently over a spirit-lamp 

 in a suitable vessel until nearly all the solvent 

 (turpentine) is driven off, and the residue becomes 

 viscid. To this, while warm, pure benzole may be 

 added, until it is considered to be sufficiently liquid. 

 It may then be used like fluid balsam, with or without 

 heat, and is probably just as good. In both the above 

 preparations it is necessary to use pure benzole, such 

 as is procurable from the vendors of chemical drugs. 

 —C. y. M. 



Preparation of Diatoms, — In the Journal de 

 Micrographie," * Monsieur L. J. Brun, Professeur a 

 I'Universite de Geneve, publishes a new process for 

 •cleaning diatoms, of which we give a translation : " I 

 have devised the following process for the destruction 

 •of the endochrome and the preparation of diatoms. 

 It is so convenient, and gives such good results, that 

 I feel it a duty to devote a few lines to making it 

 known. To a magma f of fresh diatoms still moist, 

 add some crystals of permanganate of potash 

 dissolved in a small quantity of water (i part of the 

 salt to 10 of water). If the diatoms are dry, pure, or 

 mixed with earth or organic matter, moisten with a 

 small quantity of a concentrated solution of the salt, 

 and in which there are still some crystals in excess. 

 The reaction of the permanganate must be continued 

 for about twelve hours. It is sometimes desirable to 



* "Jour, de Micr.," edited by Dr. Pelleton. Paris, No. 9, 

 1882. t A crude reuduum. 



remove the mixture to the bottom of a lOO-gramme 

 phial and then place it in a warm oven or in the sun. 

 The phial must afterwards be half filled with water 

 and a little calcined magnesia (about 0*50 centage) 

 added. This must be allowed to act for two or three 

 hours, occasionally shaking the contents. Now add 

 one gramme every minute for ten minutes of pure 

 hydrochloric acid. When the contents of the phial 

 are bleached the process is complete. In order to 

 facilitate the reaction, plunge the phial into hot or 

 boiling water. The usual washing and decantation 

 may now be employed ; for the washings absolutely 

 pure distilled water is imperatively necessary. In 

 this process we have, first, the energetic oxidation of 

 the endochrome by the permanganate and magnesia, 

 afterwards the liberation of oxygen gas which acts as 

 an oxydiser (comburant), and then the disengagement 

 of the chlorine which acts as a bleacher. Doubtless 

 it is to these multipHed and successive reactions upon 

 the valves, both externally and internally, that the 

 perfect cleaning of the silica is due. By this treat- 

 ment the delicate species are not corroded, especially 

 if before the addition of the acid a sufficient quantity 

 of water is added. The surface of the valves having 

 lost all its ' coleacterine ' appears with all its 

 brilliancy, and the minutest details, the striae or 

 puncta are clearly distinguished." I have for several 

 years successively tried all the physical and chemical 

 processes that have been pubhshed, and I am able 

 to say that I have found none of them in which the 

 results are so perfect and so uniform. I have not yet 

 tried Professor Brun's method, but, theoretically, it 

 appears to be a good one. I fear, however, that it 

 will not prove so effective when much vegetable or 

 animal matter is present as the old sulphuric acid and 

 chloride of potash process. — F. K. 



Talc— Reading Saville Kent's recent work, I 

 came upon the following passage. After giving 

 objections to the use of thin glass as a cover for 

 microscopic objects under high powers, that author 

 says : " This substance, represented by ordinary talc, 

 as extensively used for gaselier shades, may with a 

 little practice be split into laminae of such extreme 

 tenuity that they may be blown away with the 

 slightest breath, while for perfect evenness and trans- 

 parency they will compare favourably with the finest 

 manufactured glass. With the employment of these 

 talc-films the investigation of Infusoria with the j'g, -}^, 

 or even the 3'fj inch objectives becomes a comparatively 

 easy task. The material in question possesses the 

 further considerable advantages of bending readily, 

 and permitting the object glass to be brought close 

 down on the more remote objects in the microscopic 

 field, while it may be cut with the scissors to any 

 required size or shape." (Man, Infus. i. p. 115.)— C,^. 



" Studies in Microscopical Science," — Edited 

 by A, C, Cole. Nos, 19, 20, and 21, of this im- 

 portant and helpful serial are devoted severally to 



