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Charles Francois de Cisternay Du Fay discovered two types of static electricity, and that like charges repel each other whilst unlike charges attract. Du Fay announced that electricity consisted of two fluids: “vitreous” (from the Latin for “glass"), or positive, electricity; and “resinous,” or negative, electricity. This was the ‘two-fluid theory’ of electricity, which was to be opposed by Benjamin Franklin’s ‘one-fluid theory’ later in the century. Du Fay was also a pioneer in crystal optics.
Charles Francois de Cisternay Du Fay was born in Paris, September 14, 1698. Du Fay started his career in the French army, rising to the rank of captain. He had been soldier and diplomat betimes, but he left to become a chemist in the Academie Francaise. Du Fay was a highly educated savant, whose versatility and ability as a scientist is shown by the fact that he was the only man who had ever contributed to the annals of the academy investigations in every one of the six subjects admitted by that institution as worthy of recognition.
Du Fay upheld his reputation in this new field of science, making many discoveries and correcting many mistakes of former observers. In this work also he proved himself a great diplomat by remaining on terms of intimate friendship with Dr. Gray - a thing that few people were able to do. (Stephen Gray, a British chemist, is credited with discovering that electricity can flow, 1729).
Almost his first step was to overthrow the belief that certain bodies are “electrics” and others “non-electrics” That time scientists already knew, that some substances when rubbed show certain peculiarities in attracting pieces of paper and foil which others do not. Du Fay proved that all bodies possess this quality in a certain degree.
“I have found that all bodies (metallic, soft, or fluid ones excepted),” he says, “may be made electric by first heating them more or less and then rubbing them on any sort of cloth. So that all kinds of stones, as well precious as common, all kinds of wood, and, in general, everything that I have made trial of, became electric by beating and rubbing, except such bodies as grow soft by beat, as the gums, which dissolve in water, glue, and such like substances. This also to be remarked that the hardest stones or marbles require more chafing or heating than others, and that the same rule obtains with regard to the woods; so that box, lignum vitae, and such others must be chafed almost to the degree of browning, whereas fir, lime-tree, and cork require but a moderate heat.
Having read in one of Mr. Gray’s letters that water may be made electrical by holding the excited glass tube near it (a dish of water being fixed to a stand and that set on a plate of glass, or on the brim of a drinking-glass, previously chafed, or otherwise warmed), I have found, upon trial, that the same thing happened to all bodies without exception, whether solid or fluid, and that for that purpose it was sufficient to set them on a glass stand slightly warmed, or only dried, and then by bringing the tube near them they immediately became electrical.
I made this experiment with ice, with a lighted wood-coal, and with everything that came into my mind; and I constantly remarked that such bodies of themselves as were least electrical had the greatest degree of electricity communicated to them at the approval of the glass tube.”
His next important discovery was that colors had nothing to do with the conduction of electricity. “Mr. Gray says, towards the end of one of his letters,” he writes, “that bodies attract more or less according to their colors. This led me to make several very singular experiments. I took nine silk ribbons of equal size, one white, one black, and the other seven of the seven primitive colors, and having hung them all in order in the same line, and then bringing the tube near them, the black one was first attracted, the white one next, and others in order successively to the red one, which was attracted least, and the last of them all.
I afterwards cut out nine square pieces of gauze of the same colors with the ribbons, and having put them one after another on a hoop of wood, with leaf-gold under them, the leaf-gold was attracted through all the colored pieces of gauze, but not through the white or black. This inclined me first to think that colors contribute much to electricity, but three experiments convinced me to the contrary. The first, that by warming the pieces of gauze neither the black nor white pieces obstructed the action of the electrical tube more than those of the other colors.
In like manner, the ribbons being warmed, the black and white are not more strongly attracted than the rest. The second is, the gauzes and ribbons being wetted, the ribbons are all attracted equally, and all the pieces of gauze equally intercept the action of electric bodies. The third is, that the colors of a prism being thrown on a white gauze, there appear no differences of attraction. Whence it proceeds that this difference proceeds, not from the color, as a color, but from the substances that are employed in the dyeing. For when I colored ribbons by rubbing them with charcoal, carmine, and such other substances, the differences no longer proved the same.”
His great achievement was to discover the two kinds of electricity, positive and negative (according to the present terminology). From the mid-18th through the early 19th centuries, scientists believed that electricity was composed of fluid. In 1733 Du Fay announced that electricity consisted of two fluids: “vitreous” (from the Latin for “glass"), or positive, electricity; and “resinous,” or negative, electricity. When Du Fay electrified a glass rod, it attracted nearby bits of cork. Yet, if the rod touched the pieces of cork, the cork fragments were repelled and also repelled one another.
Du Fay accounted for this phenomenon by explaining that, in general, matter was neutral because it contained equal quantities of both fluids; if, however, friction separated the fluids in a substance and left it imbalanced, the substance would attract or repel other matter. This was the ‘two-fluid theory’ of electricity, which was to be opposed by Benjamin Franklin’s ‘one-fluid theory’ later in the century.
As early as the mid-18th century, Du Fay noted that electricity may be conducted in the gaseous matter - that is to say, plasma - adjacent to a red-hot body (hot metal). In connection with his experiments with his thread suspended on glass poles, Du Fay noted that a certain amount of the current is lost, being given off to the surrounding air. He recommended, therefore, that the cords experimented with be wrapped with some nonconductor that it should be “insulated” ("isolee"), as he said, first making use of this term.
In 1732 the king appointed Du Fay superintendent (director) of the royal gardens in Paris (Jardin du Roi) and in this capacity he was an active botanist. In the last years of his life Du Fay did his work on the optical properties of crystals. He had taken the first steps toward a correlation of crystal form and optical properties. Unfortunately his early death prevented the full publication of his results. He died on July 16, 1739, after a brief illness.
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