values for the increases in refractivity and density per degree centigrade decrease in the effective annealing temperature, and a slight revision of some of the values previously reported have resulted from new data. Some of the more reliable tentative values are included in the following table:

These values are valid only for the particular glasses for which they were determined, and are subject to further revision as the investigations proceed.

Light-Colored First Coats.-The main difficulty in applying a light-colored vitreous enamel directly to sheet iron or steel is that without the cobalt oxide, which incidentally produces a dark color, the adhesion is usually unsatisfactory. Since nothing better than cobalt for promoting the adhesion of enamel is known, it was decided to retain cobalt, but to develop a means for placing it at the plane of contact between the enamel and the metal, where its benefits could be had without impairing the color of the enamel. The following procedure has been developed:

The metal is first prepared for enameling in the usual manner. It is then dipped in a 4 per cent solution of cobalt nitrate at about 95° C., and allowed to remain until hot. Upon withdrawal, the hot metal quickly dries, after which it is placed in a furnace at about 300° C. for five minutes, when the cobalt nitrate apparently decomposes and the metal simultaneously acquires a coating of its own oxide and of cobalt oxide. As both of these oxides promote adhesiveness when at the surface, it appears that practically any enamel composition that is satisfactory in other respects will adhere well when the iron is prepared in this way. Such a solution of the problem seems especially fortunate, since it makes possible the use of a wide range of compositions, thus allowing individual requirements to be met instead of limiting the user to a single composition or to a narrow range of compositions.

HEAT AND TEMPERATURE MEASUREMENTS

Properties of Water and Steam.-In cooperation with the American Society of Mechanical Engineers, the heat capacity of water over the range 0 to 270° C. has been determined. Values of the heat content of saturated water at 10° intervals have been obtained, each value being the result of nine or more complete determinations.

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Experimental data on latent heat at 100°, 130°, 150°, and 200° have been obtained.

At the invitation of the British Electrical and Allied Industries Research Association two members of the bureau have attended an international steam-table conference.

Vapor Pressure of Carbon Dioxide.-Additional measurements of the vapor pressure of carbon dioxide at 25° and at 0° C. have been made. They confirm the results at 25° C., but give a slightly higher value at 0° C. (26,140 millimeters of mercury). This is in good agreement with the value published by the Massachusetts Institute of Technology.

Freezing Point of Nickel. The location of the freezing point of nickel makes it a very convenient fixed point in precise temperature measurements at very high temperatures. It has been found to be constant, and reproducible, and satisfactory in every way as a secondary fixed point for optical pyrometry.

Two separate lots of nickel were used, the average value for the freezing point of each lot being 1,454.9° C. This value has been rounded to 1,455° C. and is believed not to be in error by more than (',

International Temperature Scale. An intercomparison of the temperature scales used by the Physikalisch-Technische Reichsanstalt, the National Physical Laboratory, and the National Bureau of Standards has been inaugurated for the purpose of determining how closely these three laboratories agree in the practical realization of the international temperature scale.

The bureau's work on the thermoelectric portion of this scale has been completed, and work on the remaining portions is in progress. Comparison of Thermoelectric Temperature Scales.-After the adoption of the international temperature scale, that part of the sale based upon the platinum versus platinum-rhodium thermocouple (660° to 1.063° C) was compared with the three other thermoelectric solos which had been used by the Bureau of Standards since 1912. It was found that none of the older scales differ from the international sole by more than 0.3° C. at any point. This work also established the fact that the freezing point of copper is 20.0° C. above the gold point, or at 1.08$.0o C

Gas Thermometry. Apparatus for gas thermometry is being designed and constructed for use in determining the thermodynamic komponature of the see point and the temperatures of certain important Avod poizes in the low comporane range, such as the freezing point að Mánený, Norma, sedlimation point of carbon dioxide, normal doing point DE DLIGON, DIE

Standards for Low Temperatures. The cryogenic laboratory maintains and establishes temperature standards for low temperatures. A cryostat, an apparatus for maintaining constant low temperatures for long periods, is being constructed. This will enable the laboratory to calibrate thermometers at any temperature between -50° and 183° C. A very pressing need for investigations below -183° C. is the establishment of a practical scale of temperatures, and the construction of better thermometers for that range. A new liquid hydrogen plant is being built to supply liquid hydrogen for this work.

Standardization of Thermal Conductivity Apparatus.-Through the cooperation of the Celotex Co., a program for standardizing the thermal conductivity apparatus used in laboratories in various parts of the world has been prepared and carried out. First, an improved 8 by 8 inch hot-plate thermal conductivity apparatus, which has been found to be correct to within about 1 per cent, was designed and built. Then standard conductivity samples called "conductimeters" were developed. These were constructed of a permanent, low conductivity material, and were equipped with thermocouples by means of which an independent measurement of the heat flow could be obtained. These conductimeters were sent to several laboratories, where their conductivities were measured. In most cases the values found agreed with those of the bureau within 3 per cent.

Thermal Conductimeters.-The thermal conductivities of samples. of rubber containing various percentages of graphite, and prepared by the electrical division of this bureau, have been determined. These samples are to serve as standards with which the thermal conductivities of electrical insulating materials are to be compared.

Standard Calorimetric Samples.-A new supply of benzoic acid for use as standard calorimetric samples has been compared with the older lots, and found to have the same heating value (6,329 calories 20° per gram, weighed in air).

Effectiveness of Radiators.-Preliminary investigations of heating. radiators at the bureau have led to the following conclusions: The effectiveness decreases slightly with increasing radiator temperature. If all other factors are considered equal, hot water or low pressure steam heating tends to be more economical than the ordinary mode of steam heating. Pronounced differences have been observed between radiators of different sizes. Low radiators are more effective than high ones. For a given height, the radiator with the least number of tubes is the most effective. Effectiveness, however, seems to be affected by the height more than by the number of tubes.

IDENTIFICATION PROBLEMS

Typewriters and Documents.-Methods for the identification of typewriters and typewritten documents were studied during the year and instruments were constructed for use in such work. Several important documents have been identified for other departments. The subject is one in which precision measurements are applicable. When sufficient material is available, the identifications are of the most conclusive nature.

It is often possible to prepare the cases in such a way that personal opinions are practically eliminated, and the evidence can be made available from the documents in such clarity that those interested must form correct opinions.

Bullets, Guns, and Shells.-The bureau has equipped a laboratory for research in the identification of bullets, guns, and shells.

The bureau's interest in such work is primarily one of standardization of methods and instruments, so that all experts may use equipment suitable for the purpose.

The fine markings left on bullets and shells by firearms are excellent means for their identification. The subject is now being studied with microscopes and special measuring instruments, and such evidence is being obtained as proves unambiguously which gun fired a particular bullet.

Such research calls for skill both in precision measurements and in the photography of difficult objects.

LEATHER

Effect of Acid.-The investigation of the effect of acid on the properties of leather has been actively continued. Approximately 10,000 chemical and physical tests have been made, and 840 samples of standard leather have been tanned for this work in the bureau's experimental plant. Aging tests on representative commercial leathers treated with sulphuric acid have also been started. Aging tests take much time, but results already obtained indicate that quality standards can be improved.

The type of vegetable tanning material used has been found to affect greatly the leather's resistance to the deteriorating action of sulphuric acid. Additional samples of leather of the same type but tanned with various vegetable materials have been prepared for test.

A study of the effect of humidity has already shown that the deterioration produced by sulphuric acid is greater at 85 per cent relative humidity than at 65 per cent.

An investigation of aging properties on vegetable-tanned leather treated with oxalic acid has been begun.

Properties of Leather. The chemical properties of a large number of commercial leathers have been determined. The data obtained were used in preparing the revised specification now being considered by the industry.

During the year, a comparison of the vegetable and the chrome methods of tanning as affecting the strengths of different leathers was completed. In general, the vegetable-tanned leathers are stronger than the chrome tanned, particularly when the leather is made from steer hides or from sheepskins. The difference is not so pronounced for calf leathers.

The way the strength and the stretching of leather are affected by its splitting has been studied. It appears that failure in service has occurred from a lack of such information.

Publication. Properties of Leather-Committee Report, 1927–28; Jour. Am. Leather Chemists Assn., 22, No. 1, p. 2, January, 1929.

Sole Leather. An investigation to determine the comparative durability of vegetable-tanned and of chrome-tanned leather retanned with vegetable tanning materials has been finished. Actual service

tests on 500 pairs of soles, representing seven types of retanned or combination tanned leathers, have been completed, and a report of the work is being prepared for publication. Two new types of retanned leathers were added to the tests.

Chrome leathers lightly retanned with vegetable materials wear about 75 per cent longer per unit of thickness than do the ordinary vegetable leathers; those heavily retanned with vegetable materials wear about 25 per cent longer. Though less durable, the latter approaches more nearly to the firmness, water-resistant, and thickness properties characteristic of vegetable-tanned leather.

From such data the producer and the user can estimate the durability of such leathers.

Salt for Curing Skins.-The chemical compositions of 36 samples of salt representative of the types commonly used for curing skins and obtained from various places in this country and abroad have been determined. This completes the study requested by the industry in connection with their investigation of the causes of damages to skins. The results have been compiled and forwarded to the advisory committee for criticism and suggestions. These data include information on properties to which no reference was found. in the literature.

LENGTH

Fundamental Length Calibrations.--During the past year an extensive series of fundamental length calibrations has been made with the bureau's high precision (longitudinal) length comparator. This work has involved not only intercomparisons and determinations of the corrections of the subintervals of line standards of this bureau but also intercomparisons of the standards of the Topographical Surveys Laboratory of Ottawa, Canada. In the latter, measurements were also made at Ottawa, using the same standards and observers as were used in Washington. The Canadian standards have since been compared with the standards at the National Physical Laboratory of Great Britian and at the International Bureau of Weights and Measures at Sevres, France. Valuable data have thus been obtained regarding not only the corrections for the several length standards but also the personal errors of observers and the accuracy obtainable with different types of comparators.

Adjustments and alterations have been made on the bureau's circular dividing engine during the past year in an endeavor to fit it for dividing geodetic theodolite circles; and some auxiliary equipment for that purpose has been constructed. Two divided circles have been delivered to the Coast Survey for actual field tests. During the past year several circles have been submitted for test.

Use of Light Waves in the Ruling of Line Scales. At present line scales are ruled by dividing engines in which the spacing of the lines is controlled by the rotation of a screw. Unavoidable imperfections in the screw introduce errors in the intervals, making it necessary to calibrate the finished scales and to apply appropriate corrections whenever the scale is used.

The use of interference methods in making end gages having resulted in such an accuracy as eliminates every correction, it was attempted to use the same method for controlling the intervals between the lines of a line scale. Six-inch scales free from microscopic errors were