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9a. Upon bringing the hand toward the brush the pitch increased. ssss1. With still smaller balls and points, in which case the brush could hardly be distinguishable, the sound was not heard. He alluded to the rotating mirror of Wheatstone as becoming not only useful but necessary at this stage. He considered the brush as the form of discharge between the contact and the air or else some other non or semi-conductor, but generally between the conductor and the walls of the room or other objects which are nearest the electrodes, the air acting as the dielectric. One experiment, he performed with hydrochloric acid led him to believe that that particular gas permitted of a dark or invisible discharge. Sometimes the air was electrically charged ssss1 to a less distance than the length of the brush or light.

10. Brush in Different Gases. Striae Cathode Brushes. In the air, at the ordinary pressure he found the color to be “purple;” when rarefied still more purple, and then approaching to rose; in oxygen, at the ordinary pressure, a dull white; when rarefied, “purple;” and with nitrogen, the color was particularly easily obtained at the anode, and when nitrogen was rarefied the effect was magnificent. The quantity of light was greater than with any other gas that he tried. Hydrogen, as to its effect, fell between nitrogen and oxygen. The color was greenish grey at the ordinary pressure and also at great rarity. The striae were very fine in form and distinctness, pale in color and velvety in appearance, but not as beautiful as those in hydrogen. With coal gas, the brushes were not easily produced. They were short and strong and generally green, and more like an ordinary spark. The light was poor and rather grey. Also in carbonic acid gas the brush was crudely formed at the ordinary pressure as to the size, light and color. The tendency of the discharge in this case was always towards the formation of the spark as distinguished from the brush. When rarefied, the light was weak, but the brush was better in form and greenish to purple, varying with the pressure and other circumstances. As to hydrochloric acid, it was difficult to obtain a brush at the ordinary pressure. He tried all kinds of rods, balls and points, and while carrying on all these experiments he kept two other electrodes out in the air for comparison, and while he could not obtain any satisfactory brush in the hydrochloric acid gas, there were simultaneously beautiful brushes in the air. In the rarefied gas, he obtained striae of a blue color.