Nov., 1922
THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Given a certain paint paste whose yield value and mobility have been measured, if we add varying quantities of vehicle and measure the yield value and mobility, we will obtain curves like those shown in Fig. 1. The mobility increases in a linear manner and reaches the mobility (the fluidity) of the vehicle at zero concentration of pigment. The mobility would have a zero value at a definite concentration, which is probably connected with the relation of the volume of the pore space in the pigment to the concentration of the vehicle. We have, then, a third property which seems capable of exact definition. The yield value on dilution decreases in a linear manner first, and therefore might be expected to reach a zero value a t some finite concentration which we define as the theoretical Concentration of zero yield value. This is a fourth property, which may be of considerable importance since it dcpends upon the size of the particle and the adhesion between the particles and the vehicle, but apparently not upon the fluidity of the vehicle. When the concentration exceeds the theoretical concentration of zero yield value, the material is plastic. However, even when the concentration is very much lower than this so that the particles of the disperse phase do not come within range of each other, the flow is effected and there seems to be always a yield value, although it is no longer a linear function of the concentration, as shown in the figure. We may say that these colloidal solutions are pseudoplastic. We conclude, therefore, that colloidal solutions all exhibit a yield value, whereas true solutions never do. True solutions give us one property of fluidity, which suffers an abrupt change a t the melting and boiling points. Colloidal solutions give us several properties, and these properties differ in a marked manner for the different types of colloids. A simple paint is a colloid of the nonpolar type. The effect of temperature upon it is t o increase the mobility in just the ratio that the fluidity of the vehicle is increased, without affecting the yield value. Quite different is the behavior of a colloid of the polar type, represented by glue and nitrocellulose in acetone. The yield value decreases in a linear manner if the temperature is raised and passes through a zero value, so that we have at this temperature the transition from a plastic material t o a viscous liquid, quite analogous to a melting p i n t . We have here a fifth property which appears to be clearly defined. With the plastometer it seems possible to distinguish between the different types of colloids, as well as to distinguish a colloidal solution from a true solution. The measurements of fluidity of colloidal solutions recorded in the literature must all be regarded as apparent fluidities only, and are therefore subject to a plastic correction. This error should be kept in mind wherever measurements of viscosity are used in the control of colloidal materials. Flueckiger Medal Award On September 15, 1922, Dr. Frederick B. Power, in charge of the Phytochemical Laboratory of the Bureau of Chemistry, was awarded the Flueckiger Gold Medal by the Swiss Pharmaceutical Society, for his invaluable researches on medicinal plants and essential oils. This inedal was established in 1892,in honor of Professor Flueckiger when he retired to private life in Switzerland, after many years of distinguished service as a teacher, writer, and investigator a t the University of Strasburg. Without regard to nationality, it is awarded, every five years, alternately by the Swiss and the German Pharmaceutical Societies, for “distinguished services in the promotion of scientific pharmacy.” It has hitherto been given to representatives of England, Germany, France, Switzerland, and Austria, this being the first time the honor has been bestowed upon an American. The medal is especially prized by Dr. Power because i t bears the name of a most highly esteemed teacher and friend.
1015
Glass Pressure Gage’ By S. Karrer, E. H. Johnston, and 0.R. Wdf FIXEDNITROGEN RESSARCH LABORATORY, DEPARTMENT 01 AQRICTJLTUBB, WASHINGTON, D. C.
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T IS often necessary to measure the pressure of a gas or vapor that will react chemically with the materials out of which ordinary pressure gages are constructed. An all-glass gage has been developed for this purpose, and several of them have been in continual use for almost two years. Although a number of glass gages are described in the literature, the present form has proved so satisfactory that i t seems worth mentioning.
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One form of this gage is shown in the accompanying figure.
It consists essentially of a thin glass diaphragm, B, blown on the end of the small tube F, which is sealed into the larger tube H. The diaphragm separates the region D, in which the pressure of the corrosive gas is to be determined, from the region C, containing air whose pressure is measured by a suitable mercury manometer. A change in the gas pressure in either of these regions will cause the diaphragm to deflect. The platinum wire anchored to the glans frame G and resting on the diaphragm at B follows its motion, and will touch a second platinum wire, also fastened to this frame, when the diaphragm moves upward far enough. This contact a t A between the platinum wires opens or closes an electrical circuit, including a galvanometer and small electromotive force. Thus, by changing the air pressure on the diaphragm, it may be brought to a definite position, as indicated by the opening or closing of this electrical circuit. The contact A 1
Published by permission of the Department of Agriculture.
