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

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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

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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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COLLOIDAL CHEMISTRY SOME DENTAL APPLICATIONS NINETEEN SIXTEEN precipitate. If now we pour the same amount of milk into another beaker, add some gelatin, and then add vinegar, we obtain no precipitate. These conditions exist in the stomach. The casein of the milk is very easily precipitated by the Hydrochloric Acid, causing digestive disturbance. This sub- stance is an irreversible colloid. Soap is an example of a reversible colloid. It crystallizes in an alcoholic solution and readily forms a colloidal solution when transferred to water. The adding of gelatin to milk prevents precipitation. Cow's milk has a comparatively large percentage of casein 131521 and a small percentage of albumin CIWWJ. Human milk has a greater percentage of albumin than casein, while the ass's milk has the least amount of casein. Hence the latter is the best milk to give when the former two have been found unsat- isfactory, as it has a large amount of protective colloid in the form of albumin and very little irreversible colloid in the form of casein. For the same reason iced cream containing gelatin is both more palatable and more digestible. Colloidal chemistry is of the highest importance in medical science as well as in biology. To quote Prof. Stewart W. Young of Stanford University: When one considers the relatively infrequent occurrence in biological systems of either crystalline substances or of substances that may readily be made to crystallize from water fthe universal biologic dis- persing mediumj, it immedately becomes evident that the chemistry and physics of such systems must be in the main colloidal. All bio-chemistry is thus in the main colloidal chemistry. To bring the above statement home more strik- ingly, it is but necessary to add that the formation of crystals between joints causes gout, while violent poisons such as cor- rosive sublimate, are transformed into irreversible colloids and form crystals in the cells. Hence we give albumin in mercurial poisoning to protect the poisonous solution and pre- vent its crystallization, a condition which permits of easy passage through the cell-membrane. Colloids do not diffuse or dialyse. Living cells show the Brownian movementg histology is properly a study of colloidal chemistry. Muscular movement likewise may be explained on the colloidal basis as was done by Prof. Janeway in the Acorn of April, 1915. All our foods and the entire composition of our body are mostly col- loidal. Some commercial applications are interesting from the dentist's point of view. India-rubber, for instance, in both the coagulation and vulcanization process undergoes distinc- 22