604
INDUSTRIAL A N D ENGINEERING CHEMISTRY
logra hic axes in monoclinic and triclinic should be designated as (la& a”, “angle B”, “angle 7”. Crystal system, and if possible symmetry claas, should be reported. A mere s t a t e m e n t of crystal system, without t h e s u p porting geometrical or optical evidence, is not acceptable. The geometrical description should be separated from t h e description of optical properties. Drawings a r e almost essential for geometrical descriptions, and m a y further be used t o represent the orientation of optical pro rties. Clinographic projection (perspective) is appropriate geometrical urposes, b u t for records of optical properties and microscopicaf a p arances orthographic projections (plans a n d elevations) a r e regrable. Optical roperties should be reported aa f 4 l y aa t g e specimen, the a v a i l s h e apparatus, and t h e author’s ability permit. Even brief a n d simple descriptions are better t h a n none, provided they are accurate aa far as t h e y go. Optical data a r e articular1 important in those cases where crystal faces a r e not welrdeveloped: It is essential t h a t reported optical properties be clearly related t o crystallographic features or to t h e geometrical appearance of t h e material aa seen under t h e microscope. In t h e order of importance and ease of observation, the following optical properties are desirable: 1. Isotropic or anisotropic character; may differ from different
views. 2. Locationofvibration directions (axesof elasticity) with reference to prominent edges, faces, or planes of symmetry. Parallel (symmetrical) extinction versus oblique extinction. The extinction angle should be clearly designated with respect to the face, edge, or crystal a x i s of reference, preferably by a drawing. Phrases such as “extinction angle 2oo to the long edge, meaaured in the acute (or obtuse) angle ,9” may well be used. Different extinctions, a8 obtained on different views of a crystal, should.be clearly indicated. 3. kefractive indices exhibited by crystals in readily available orientations such as lying on prominent faces. The view examined should be stated, and the refractive indices corresponding respectively to the vibration directions for that view should be clearly designated, preferably on a drawing. Refractive indices should be reported for the different principal views of the crystals, if possible. If refractive indices are not determined numerically, nevertheless mme statement should be made regarding their relative magnitude and a e r e n c e , for different principal views. The direction of vibration of the slower component (axis of less elasticity, or higher refractive index) should be observed by means of a compensator (first order red plate or quartz wedge). If crossed arrows are used to indicate on a drawing the axes of elasticity, the shorter arrow should correspond to the higher refractive index less “optical elasticity”. The strength of double refraction (difference in refractive indices) for each view may be roughly estimated from the “order” of the polarization colore, thickness being considered. Mere statement of the polarization colors exhibited is of little significance, unless the thickness is specified or is consistent for the method of crystalliration. The numerical values given for refractive indices should indicate the probable accuracy of the method as actually carried out; this may m e r in different ranges and for diffwent materials. Solubility in immersion liquids may well be reported. If monochromatic illumination is used, as will be necessary if marked dispersion exists, i t is advisable that determinations be made with d u m light (in addition to any other wave lengths employed) to aid in comparison with the approximate refractive indices obtainable with white light. Refractive indices should be designated by Greek letters or other symbols only if i t is known that they are the principal values, corresponding to the axes of the index ellipsoid. For crystals known to be uniaxial (in the tetragonal or hexagonal systems) e and w should be employed t o designate the principal indices; e’ may be used t o designate an intermediate value. For crystals known to be biaxial (orthorhombic, monoclinic, triclinic), a,@, and y should be employed to designate the principalindices; “primes” may be used to designate intermediate values, as a’,@”, etc. When the refractive indices as measured nre not readily related to the principal values, they may be designated aa m,n.t, na, and their relationship to the observed crystal habit should be clearly indicated by a drawing or otherwiee. N,, Nn.N P should not be used instead of the Greek letters, t o designate principal refractive indices. Principal vibration directions (axes of elasticity) should be designated by the Greek letters as used for the respective refractive indices. 0 and E , corresponding to w and e, and X , Y.Z,corresponding to a, 6, y , may also be used. The views which best exhibit principal values of refractive index should be indicated, for ease in checking values by other observers. 4. Color and pleochroism. These should be observed on thick arystals; unless very marked, the former may sometimes be more apparent with dark-field illumination.
Vol. 17, No. 9
Pleochroism should be sought in all colored anisotropic crystals, and should be described with reference t o the vibration directions for each observed. A mere statement t h a t “a red-yellow pleochroism is observed” is not adequate; the respective orientation for each color should be given. Surface color and reflection pleochroism may be observable on highly colored materials. 5 . Interference figures, indicating uniaxial or biaxial and positive or negative character. These should be obtained if a t all possible, even when refractive indices are not observed, or as a check on the latter. The quality of the interference figure obtained, and the orientation that best exhibits i t , should be stated. The axial plane, acute bisectrix, and optic axial angle should be reported, often best by means of one or more drawings. The value given for the optic axial angle should be consistent with the accuracy of the determination; 8V is preferable, provided 4, is known approximately; dE and ZH may also be given. I n any case, the method of determination (calculation from n’s, estimation, or measurement by a universal stage) should be stated. Dispersion of refractive indices, of birefringence, or of optic axes or axial plane should be reported if possible, particularly when marked enough to be readily observable with ordinary apparatus or specimens. Tabulation of optical properties is desirable where several substances are studied and compared, but usually the necessary condensstion and use of symbols will involve a aacrifice of clarity and ease of interpretation by semiskilled observers. Additional separate descriptive paragraphs on each substance are therefore highly desirable. Phare changes involving crystal8 (polymorphism, desolvation, etc.) should be reported whenever noticed. Observations and conclusions drawn from them should be clearly differentiated. If enahtiotropic (reversible) polymorphism exists, the approximate transition temperature should be determined, together with the method of observing it. The properties of the high- and low-temperature phases, and the ease and manner of the transformation from one to the othgr should be given. If monotropic (irreversible) polymorphism exists, the properties of the stable and metastable phases should be given, and also the method by which the metastable phase may be consistently obtained. Unless the system is fully established, the use of Greek letters (a for the lowest temperature phase, etc.) is not advisable; Roman numerals are preferable designations.
NBS Spectrographic Steel Standards Nickel-Molybdenum Steel (SAE 4620) is now being ieaued in two No. 419, inch rod, and standard No. 819,l/z-inch size-tandard rod-with the following percentage composition: Mn 0.72,Cu 0.080, Si 0.27,Ni 1.71,Cr 0.24,Mo 0.22,and Sn 0.009. Price per sample is 8.00.
Open Hearth Ingot Iron is now being issued in two sixes-standard No. 420. ’/:rinch rod, and standard No. 820, ‘/pinch rod-with the following percentage composition: Mn 0.020, Cu 0.050, Ni 0.020, Cr 0.006, Co 0.007, Mo 0.002,Al 0.002,and Sn 0.005. Price per sample is $3.00.
Certification of Values for Low Concentrations of Aluminum and Tin in Smples Previously Issued Caution. For determination of small amounts of aluminum! use of
alumina-baae abrasives in cleaning and ahaping rod8 munt be avoided and outer cylindrical surface of ‘/,rincb standards must be removed to a depth of n.002 inch. -. _ _ . -. -. fltandard Aluminum Standard Tin
% 403.803 406,806 407,807 414, . 416,816 417,817 420,820
.
0.005 0.023 0,055 0,020 1.06 0.013 0.002
% 412.812“ 414, 417, ai7 419,819 420,820
0.026 0.014 0,020
0.009 0.005
li 88n le 812 now out of stock but certified tin value will 8pply t o s a m p l e previousL issued.
A general certificate of analysis of steel standards revised t o include the new standards, other than boron steels, is furnished with shipments of standarda. Of the standards originally issued, Nos. 810,811,and 812 are out of etock and cannot be supplied.
