Columbia University School of Dental and Oral Surgery - Dental Columbian Yearbook (New York, NY)

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NINETEEN SIXTEEN COLLOIDAL COLLOIDAL CHEMISTRY-SOME DENTAL APPLICATIONS. CHEMISTRY C :OLLOIDAL chemistry concerns itself with a SOME DENTAL study of the fine subdivisions of matter APPLICATIONS I!-o after a certain stage. A colloidal solution I'l 5 is one in which the particles are so finely subdivided that they remain in permanent 65-gg:-g9.g,,' suspension. This definition permits of the assumption that any substance can be suffi- ciently subdivided to represent a transformation into a colloid,i' which assumption has been verified. Sodium Chloride, a typical crystalline substance, can be transformed into a colloid by such a subdivision. Thus we see that there is no sharp demarcation between colloidal and ordinary chemistry but rather a transition. With the advent of the ultra-microscope great strides have been made in the study of this comparatively new branch of chemistry. Under the ordinary high power micro- scope, the smallest particle visible has a diameter of one tenth of a micron, which is taken as the distinguishing limit between precipitates and colloids. With the ultra-microscope, par- ticles become visible whose diameter is four tenths of a milli- micron. This means four ten millionths of a millimeter, an inconceivable dimension. The principle of the ultra-micro- scope is very ingenious and easy to comprehend. If a beam of light is thrown across a totally darkened room, one may ob- serve the dust particles of the air in rapid vibratory motion. This phenomenon can be accentuated by throwing a handful of mica particles within the beam. Instantly bright irride- scent particles are seen dancing in the light, producing a fas- cinating spectacle. This principle is the distinctive feature of the ultra-microscope. The ray of light is allowed to pass parallel to the stage across a dark field instead of being re- flected directly thru the lens by the mirror. The latter method has the disadvantage of dazzling the eye and thus obscures the vision. A common example of this is our failure to see the stars during the day on account of the superior brilliancy of the sun. The physical difference, therefore, be- tween the ultra-microscope and the ordinary microscope is that the former makes use of the principle of refracted light while the latter is based on the principle of reHected light. The most interesting feature of the ultra-microscopic particles is their tremendous rate of movement which increases directly with their decrease in size. The most common form of movement is the Brownian, characteristic of the cocci germs, which, indeed, represent colloidal particles. 20

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YEAR BOOK of writing a few pages and labelling them a history just be- cause others have stupidly done so. There is no such a thing as a history. For that means recording something that is gone and past-dead. Nothing is ever dead. In fact the incidents that occurred during the past three years of college will hence- forth be more alive than they were even while they were actually occurring. They will now live in the imagination and each one will have a personal interesting manner of seeing them. All of which is infinitely more interesting than getting them down in one black and white form. Moreover how could one possibly write the history-the past-of one hundred and thirty human beings who are first to go out and accomplish their deeds '? Perish the thought! Let others who will, write histories,-not I. This has only been the whim of a momentg an interesting and pleasant pastime: a winding thought that turned every now and then aside to get some pleasure out of a particular impression and then continued. . . . M. A. JAGENDORF. HERE AND THERE. A freshie once was asked to give By Youngs rule. I was told, The dose of some mild laxative For a child eleven months old. So this is what the freshie did And thought it quite a stunt: - He found the dose for a year old kid And marked it, Wait a month. M. M. A STRANGE BUT TRUE HISTORY OF OUR CLASS OOM sy OUR POOR EDITOR

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YEAR BOOK The color of a colloid may be of any kind and is deter- mined by the size of the particles. Thus gold, whose particles are of the finest, presents a yellow-to-an-orange color, whereas increasing the size of the particles successively, causes a con- comitant change in color from red to violet to blue, and finally to green. This serves to explain why gold foil presents a green appearance by transmitted light. Silver shows similar changes. When composed of the smallest particles, it is colorless, while as they increase in size, it becomes yellow, and in silver foil, blue. Rubies and other precious stones are colloids of various subdivisions. The fact that color varies with the size of the particles of a substance is accepted by the dye-stuff industry as the basis for the modern method of studying dyes. The size of the particles also determines the surface of the substance exposed. This is an important factor in physiolo- gical processes. Thus, a unit of one cubic centimeter presents a surface of six square centimeters. A volume of one cubic centimeter containing particles of one-tenth of a micron pre- sents a surface of sixty square meters. The same volume con- taining particles of one-tenth of a milli-micron presents a surface of sixty thousand square meters, which compares favorably with the total surface of the leaves of a tree. There are two methods for making colloids. One is by starting with a molecular solution and allowing the molecules to grow. This is the condensation method. The other is to start with a solid and subdivide it into sufiiciently small particles. The latter is called the dispersion method. When a substance like glue or gelatin is dissolved in water, it dis- perses into very minute particles. These remain in permanent suspension due to the formation of an envelope around them, and to their Brownian movement, which prevents their coalesc- ing with the consequent formation of particles sufficiently heavy to precipitate. A very important property of colloids is their so-called protective action in solutions. lf we add a small percentage of gelatin to a solution of gypsum or Calcium Sulphate, the crystallization of the Plaster of Paris is markedly delayed. This fact is employed in medicine, altho its explanation was only recently made possible by colloidal investigations. It was a custom with physicians to prescribe the addition of gelatin to milk when curdling took place. The gelatin pre- vented the curdling and aided digestion. The chemical ex- planation is found in the following experiment :-If some milk is poured into a beaker, and vinegar added, we obtain a 21 COLLOIDAL CHEMISTRY SOME DENTAL APPLICATIONS