ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 635 



small hole in the lower part of this tube. The impetus of the air- 

 bubbles is useful in certain cases of injection. But if the oscillations 

 ■caused by them are not required, then A should be only used as an air- 

 reservoir, the elasticity of the air acting as a compensator. The cock m 

 serves for drawing off the mercury which has flowed into A. The 

 pressure attainable ranges from 0-500 mm. of mercury. 



(5) Mounting', including- Slides, Preservative Fluids, etc. 



Mounting Media of High Refractive Index.* — H. van Heurck, who 

 at various times since 1883 has introduced to the notice of Microscopists 

 mounting media of high refractive index, makes a further communication 

 on this subject. With regard to Styrax he finds that the commercial 

 variety can be quickly purified by first baking to get rid of as much 

 moisture as p'ossible, and then dissolving in boiling absolute alcohol. 

 After filtering, the liquid thus obtained is evaporated to dryness and the 

 yellowish-brown mass dissolved in benzine. This solution should be 

 filtered anew. The author mentions that L. van Italic obtains a light- 

 coloured styrax by treating the raw commercial article with petroleum 

 ether. The dissolved portion is evaporated, and the extract thus 

 obtained is of a quite light colour. 



By dissolving equal parts by weight of piperine and styrax or liquid 

 amber a medium with refractive index of 1 • 63 is obtained. This has 

 remained perfectly unchanged for about five years. By mixing 6 parts 

 of the foregoing with 1 part of piperine a still higher index is obtain- 

 able. This medium is liable to deposit crystals, but if re-melted and a 

 little liquid amber added the crystals do not re-form. 



If real benzoin of Siam be dissolved in chloroform it gives a medium 

 with index about 1*60. It is yellowish, and sets quite hard. 



By melting together in a porcelain capsule 3 parts of piperine and 

 2 parts of bromide of antimony, a medium with a refractive index about 

 1 • 70 is obtained. Too much heat should not be employed, as the 

 mixture may become brown. The successful result is a yellowish sub- 

 stance which is very durable. 



Monobromide of naphthaline and iodide of methyl have refractive 

 indices respectively of 1-658 and 1-743. Both of these, while having 

 ■excellent features as mounting media for diatoms, are subject to in- 

 conveniences which render them difficult for practical purposes. 



The double iodide of mercury has a refractive index of 1 • 654, and 

 though showing up the details of diatoms admirably, is not very con- 

 venient in practice. It is prepared by adding the red iodide of mercury 

 to a saturated solution of potassium iodide until no more is taken up. 



The arsenical medium of H. L. Smith, though most excellent, is 

 liable to become opaque from the deposit of sulphur. The cause of this 

 is due to insufficient boiling. If the heating be prolonged until all the 

 bromide of arsenic has been driven off, the mounts will be found 

 ■eventually to be perfectly stable. After the slide has cooled down, the 

 coverslip should be ringed round with paraffin, and this afterwards 

 varnished over. 



The author opines that Canada balsam mounts are of no value, and 



* Mem. Soc. Belg. Micr., xxviii. (1907) pp. 56-63. 



