PREPARATION, MOUNTING*, AND COLLECTION OF OBJECTS. 203 



b. Chromic Acid, which is one of the most generally useful of hardening 

 agents, is most conveniently kept in a 1 per cent solution, which may be diluted 

 with several times its volume of water, with or without the addition of spirit. 

 Although its hardening action may be effected by a strong solution in two or 

 three days, it is far better to prolong the process by using the menstruum weak, 

 especially when the substance is in mass; since, if its exterior be so hardened as 

 to prevent the penetration of the fluid, its interior will soften and decay. The 

 following is the mode of procedure most generally approved: The menstruum 

 having been prepared by mixing two parts of a l-6th per cent solution of 

 chromic acid and one part of methylated spirit, the material must be cut into 

 small pieces about half an inch square, and put into a wide-mouthed stoppered 

 bottle holding from 6 to 10 ozs. of the fluid; this fluid should be changed at the 

 end of 24 hours, and then every third day; and the material will be probably 

 found sufficiently hardened (which must be ascertained by trying whether a tol- 

 erably thin hand-section can be made with a razor) in the course of from 8 to 12 

 days. If not, the process must be continued, care being taken that it be not so 

 prolonged as to render the substance brittle. The hardening may afterwards be 

 completed by transferring the substance first into dilute and then into stronger 

 spirit; and this will get rid of the color given by the chromic acid, as well as of 

 other flocculent matter. The spirit must be changed as often as it becomes foul 

 and discolored; and when it remains bright and clear, the specimens will be ready 

 for cutting. 



c. Bichromate of Potass, in a 2 per cent watery solution, may be used where 

 very slow and prolonged hardening is required. With the addition of 1 per cent 

 of sulphate of soda, it constitutes Mutter's Fluid, which may be conveniently 

 used to harden large pieces that may be left in it for several weeks; no change 

 of the fluid being necessary after the first week. The hardened substance, after 

 being well washed, is to be treated with spirit, as in the preceding case. 



d. Picric or Carbazotic Acid is used for the same purposes as Chromic acid; 

 its hardening power is not so great, but it does not shrivel the tissues as much, 

 its action is more rapid, and it may be advantageously used where ' decalcifica- 

 tion ' is necessary ( 197). As it is but slightly soluble in water, a cold-water so- 

 lution must be saturated; and the quantity of liquid should be large in propor- 

 tion to that of the substance to be acted-on. Picric acid is used, in combination 

 with Carmine or Aniline-blue, as a staining material ( 202, 6). 



e. Kleineriberg's Fluid. The following method of preparing delicate and per- 

 ishable tissues is strongly recommended by Kleinenberg, who has had much 

 experience of it in his investigations on the anatomy of the lower Invertebrata: 

 To a saturated solution of picric acid in distilled water, add 2 per cent of con- 

 centrated sulphuric acid; all the picric acid which is precipitated must be re- 

 moved by filtration. One part of the filtrate is to be diluted with 3 parts of water; 

 and, finally, as much pure kreosote must be added as will mix. The object to 

 be preserved must remain in this liquid for 3, 4, or more hours; and is then to 

 be transferred for 5 or 6 hours into 70 per cent alcohol, and thence removed into 

 90 per cent alcohol, which should be changed until it ceases to acquire a yellow 

 tint. 



/. Osmic Acid This agent is one of peculiar value to the Microscopist whose 

 studies lie among the lower forms of Animal and Vegetable life; as its applica- 

 tion immediately kills them, without producing any retraction or shrinking of 

 their parts, and .only not preserves their tissues, but brings out differences in those 

 which might otherwise escape observation. It is sold in the solid state in sealed 

 tubes; and is most conveniently kept as a 1 per cent solution in distilled water. 

 The solution should be preserved in well-stoppered bottles secluded from the 

 light; and should be used with great caution, as it gives forth a pungent vapor 

 which is very irritating to the eyes and nostrils. It is recommended by 

 Dr. Pelletan, 1 M. Certes, 2 and M. Raphael Blanchard, 3 for fixing and preserving 

 Animalcules (Infusoria and Rotifer a), Desmidiece, Diatomacece, Bacteria, and Vi- 

 briones, etc.; by Dr. Vignal 4 for Noctiluca ; by Mr. T. Jeffrey Parker 5 for Ento- 

 mostraca and other small Crustacea ; and it has been successfully used also in 

 the preparation of Insect structures. To the Histologist its special value lies in 



1 " Journ. of Roy. Microsc. Soc.," Vol. i. (1878), p. 189. 



2 Ibid., Vol. ii. (1879), p. 331, and 'Comptes Rendus,' 1879, p. 433. 



3 Ibid , Vol. ii. (1879), p. 463. 



4 Robin's "Archives de Physiologic," Tom. xiv. (1878), p. 586. 

 6 "Journ. of Roy. Microsc.* Soc.," Vol. ii. (1879), p. 381. 



