416
ANALYTICAL CHEMISTRY DISCUSSION
The results obtained, clearly indicate the practicability of such a method for the determination of lysine and tryptophan. They further suggest that the method may be successfully used for the determination of other amino acids for which the microorganisms investigated show good response in conventional assay procedures. LITERATURE CITED
s., Shankman, s., Camien, N. M.9 Franu, Rockland, L. B., J. BWZ. Ckem., 156, 703 (1944).
(1) Dunn, M.
w., and
(2) Greenhut, I. T., Schweigert, B. 165, 325 (1946).
S.,and Elvehjem, C. A,, Ibid.,
(3) Hac, L. R., Snell, E. E., and Williams, R. J.,Ibid., 159,273 (1945). (4) Henderson, L. M., Brickson, W. L., and Snell, E. E.,Ibid., 172, 31 (1948).
(5) McMahan, J. R., and Snell, E. E., Ibid., 152, 83 (1944). ( 6 ) Reisen, W. H., Schweigert,B. S., and Elvehjem, C. A., Ibid., 165, 347 (1946).
(7) Schweigert, B. S., McIntire, J. M., Elvehjem, C. A., and Strong, F. M., Ibid., 155, 163 (1944) RECEIVEDMarch 5, 1948. Experiment Station.
Contribution 662 of Maqsachusetts Agricultural
Gravimetric Calibration of Micrometers .A. C . SHEAD, University of Oklahoma, Norman, Okla. One of the main sources of error in scientific work is undoubtedly connected with calibration of weights and measures. The best means of obviating these is to obtain values by means of the instruments employed, using widely differing methods of procedure. Under these circumstances, agreement in checks constitutes almost certainty of the correctness of results obtained. A n eyepiece micrometer was calibrated by comparing it with a stage micrometer evaluated by the manufacturer or other agency. GULAR micrometers are ordinarily calibrated by means of previously standardized stage micrometers, The details of this procedure can be found in practically all texts on chemical microscopy, such as that of Chamot and Mason ( 2 ) . An entirely different and independent method of calibrating eyepiece micrometers by a gravimetric method is desirable both for its own sake and as a check on the classical method of calibration. Results obtained in the chemical laboratories of the University of Oklahoma show that the gravimetric method of accomplishing this objective is entirely feasible and satisfactory. Briefly, this method consists in weighing a substantially perfect sphere of a pure noble metal, as the gold used in this study, on a sensitive microbalance. The volume of the gold spherical bead is then calculated from its known weight and density. From the volume, the diameter is calculated and applied as a primary standard to the unknown scale of the ocular micrometer undergoing calibration. APPARATUS
The apparatus employed consisted of a gold assay balance rated a t a sensitivity of 0.002 mg. The stage micrometer used in checking the results of the gravimetric calibration was divided into 0.01- and 0.10-mm. intervals. All measurements were made in accordance with established methods of procedure ( 2 , s ) . Pertinent representative data are summarized in Table I. SPECIAL PROCEDURES
The spheres of gold were formed on ordinary wood charcoal blocks before the common mouth b!owpipe ordinarily employed in blowpipe analysis. The "synthetic" charcoal blocks currently on the market provcd unsatisfactory because of a fusible ash or flux formed on the metallic beads when such blocks were employed. These synthetic blocks appear to have been made by pressing charcoal fragments together in some sort of a binder. The wood charcoal blocks formed a substantially infusible ash that was blown away instead of adhering to the bead. No diRculty was experienced in connection with perfect sphericity of the beads, as representative data in Table I1 make clear. The measurements given were taken in all possible orientations. They were facilitated by use of the rotating stage of the microscope for directions in a horizontal plane and the beads were turned over manually for different orientations in other directions.
The apparatus was also calibrated directly by means of a spherical pure gold bead, the diameter of which was calculated from its weight obtained on a sensitive microbalance and its known density. This diameter applied to the unknown scale of the eyepiece micrometer as a primary standard of lengtb serves to calibrate the instrument. Concordant values obtained by the two different methods serve as conclusive evidence of validity of gravimetric calibration of the scale in the eyepiece micrometer. DISCUSSION
Although satisfactory results have been obtained with pure gold beads and almost as good agreement has been experienced in connection with pure silver, the reverse is true with beads of gold-silver alloys submitted t o the same treatment. The results are so far from satisfactory that no instance can be cited where theoretical percentages could be calculated from weight-volume
Table I. Gold Beads
Av. Micrometer Calculated Divisions Diameter of Subtended by Bead Bead, Cm. Diameter* 0.0688 44.4 1 3.30 1 . 7 1 X 10-4 0.0674 43.1 2 3.08 1 . 6 0 X 10-4 0.0616 38.9 1 . 2 3 X 10-4 3 2.37 0.0668 42.7 1 . 5 6 X 10-4 4 3.00 4 Optical system of 7.5 x eyepiece, 10.0 X objective, and 160-mm. t u b e length. WeighUl9.32. b Summarized representative results from Table 11.
No. of
Weight of Bead, Mg.
Table 11.
Volume of Bead", Cc.
Sphericity Tests on Gold Beads of Table I
(Small divisions of micrometer subtended by different diameters of gold beads") Gold Bead 3, Gold Bead 4. Gold Bead 2, Gold Bead 1, Diameter Diameter Diameter Diameter 0,0616 Cm.. 0.0868 Cin., O.O67+.Cm., O.O688~Cm,, Divisions Divisions Divisions Divisions 42.4 39.6 44.3 43.5 42.5 38.1 44.3 43.1 42.5 39.7 44.6 43.1 42.8 39.1 44.4 43.0 42.6 39.0 44.4 42.8 42.8 38.5 44.4 43.1 43.0 39.5 44.3 43.1 42.9 38.6 44.4 43.0 43.0 38.8 44.5 43.2 42.9 38.3 44.3 43.0 44.4 4i:r Av 44 4 43:l 3i:9 0.0674/43.1 0.0616/38.9 0.0668/42.7 Dia
