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washed with small portions of chloroform to remove the dithizone completely. A chloroform trap must be maintained continuously. LEADESTIMATION BY TITRIMETRIC EXTRACTION. The aqueous lead nitrate solution is now ready t o be prepared for the titrimetric extraction in which the lead content is determined. Two to three drops of phenol red, standardized 0.02 per cent indicator solution, and 2 ml. of 10 per cent potassium cyanide solution are added and the pH of the resulting solution is adjusted to 7.5 with 5 per cent nitric acid. (The residual mcresol purple does not interfere.) The solution is then extracted with standardized dithinone solution 2 as described previously (1). As in the original method, combined reagent blanks, consistingof the same amounts of all the reagents used in the analysis, are carried along simultaneously with the sample and in identical apparatus in order to correct for lead added from this source.
VOL. 7, NO. 4
Experimental Results The data in Table I are representative of the results obtained. It is evident that the separation of bismuth does not affect the accuracy with which lead is recovered by this method.
Literature Cited (1) Wilkins, E. S., Jr., Willoughby, C. E., Kraemer, E. O., and Smith, F.L., 2nd, IND.ENG.CHEM.,Anal. Ed., 7, 33 (1935). RECEIVED June 6, 1935. This work was supported by The Elizabeth Storck Kraemer Memorial Fund created by Pierre S. and Lammot du Pont.
Permanent Aqueous Microscopic Mounts H. R. SMITH, National Canners Association, Washington, D. C.
R
ESEARCH and instruction in many branches of the biological sciences call for microscopic examination of specimens of materials, many of which have to be in aqueous mountings. The lack of a quick and easy means for preparing permanent mounts of such specimens has made it impractical to assemble sets of standard slides for comparison, exhibition, and instruction, and has also delayed work in the field of microscopic study of slow reactions in situ. The need has been for a suitable sealing material to prevent evaporation. The desired preparation must adhere tenaciously to smooth glass surfaces, flow freely into place, not dissolve or diffuse in water, not dry out or crack on long standing, and be firm enough to hold the mount in place. Many products were tried, such as Canada balsam, shellac, rubber, cement, beeswax, Halowax, asphaltum, glycerolgelatin compound, etc., but each one failed to have all the necessary properties. Finally the suggestion of Parker (1) led to the right combination. Wool fat (lanolin) is a sticky animal wax, but being rather soft it has to be hardened somewhat. The sealing material recommended is made by heating anhydrous wool fat (adeps lanae, U. S. P.) with not more than 20 per cent of rosin, until the constituents are blended. (A few minutes over a Bunsen flame are sufficient.) This mixture is firm a t ordinary temperatures but on being heated it becomes liquid. It contains no volatile solvent and quickly congeals to hold the mount in position. Any microscopic mount may be preserved by sealing the edge of the cover slip with the melted wax. The space under the slip should be filled with liquid, although a few air bubbles do not interfere with the permanence of the seal. The slide and cover slip should be dry a t the points where the wax is to be applied. The melted wax may be applied with a thin glass rod or small brush, care being taken to have the wax come on top of the slip all the way around. Unless the slide is intended for the study of progressive reactions of reagents, growth of microorganisms, etc., the water used in their preparation should contain about 0.2 per cent by volume of 40 per cent formaldehyde solution. A number of uses for such permanent mounts have come to the author’s attention. A series of slides showing successive stages of a biological process may be prepared for classroom instruction, and once prepared they are available a t all times. Preparation of such slides may be made part of students’ laboratory work, and excellence of technic encouraged by making the best slides a part of the permanent collection. Special or abnormal tissues may be kept for reference and further study. Observations by one investigator concerning
a particular tissue may be reviewed by another investigator in some distant laboratory more understandingly if the comments of both are directed toward the same tissue on a permanent mount which is sent from one to the other. Research workers may be able to follow with the microscope the slow reaction of a reagent on a biological tissue, diffusion through the walls of unbroken cells, the growth and multiplication of microorganisms, or the growth of crystals. The same microscopic structure may be examined a t intervals over a period of days, weeks, or months. An application of the use of permanent slides for instruction purposes was developed by Wildman (a) in connection with the Howard mold-count method for tomato products. He found it necessary to devise two additional features: (1) the thickening of the sample so that the filaments would not change position on the slide; and ( 2 ) the designation of specific fields on the slide for examination. The sample was thickened by stirriqg into the hot sample about one-third its volume of hot 3 per cent agar solution. The particular fields to be examined were designated by a pattern made by punching in thin transparent colored Cellophane 25 holes, each exactly the diameter of the microscopic field. This pattern was first mounted in balsam on the slide beneath a cover slip and the prepared sample was spread on top beneath a second cover slip and sealed. Wildman used balsam for sealing the mount, but this is difficult to apply, slow to harden, and the seal is not always permanent. The wax described above has been used with entire satisfaction in the preparation of a number of such slides for the Howard mold-count method. An additional operation has been included in the examination of these slides which increases their usefulness for instruction purposes : A speciaI reporting sheet having 25 circles each about 1 inch in diameter, arranged in the same order as the holes in the pattern, is used to record the observations of each observer. The analyst sketches in each circle a representation of the relative position and size of each piece of mold he finds in the corresponding microscopic field. After each analyst has recorded his observations on a separate reporting sheet, the results may be reviewed and further examination made of particular filaments.
Literature Cited (1) Parker, B. W., Science, 80, 456 (1934). (2) Wildman, J. D., Ibid., 77, 263 (1933).
RECEIVED June 26, 1935.
