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. He compared the appearances also of the anode and cathode brushes in different gases at different pressures. He noticed that in air, the superiority of the anode brush was not very marked (§ 41 at end.) In nitrogen, this superiority was greater yet. A line of theory ran through Faraday’s mind in connection with all these experiments, whereby he held that there is “A direct relation of the electric forces with the molecules of the matter concerned in the action.” § 47. He made a practical application of the principles in the explanation of lightning, because nitrogen gas forms 4/5 of the atmosphere, and as the discharge takes place therein so easily. [Illustration: FROM MAGNETOGRAPHS BY PROF. MCKAY. p. 25. 1. Platinum wire. 2. Copper gauze. 3. Iron gauze. 4. Tinfoil. 5. Gold-foil. 6. Brass protractor. 7. Silver coin. 8. Platinum-foil. 9. Brass. 10. Lead-foil. 11. Aluminum. 12. Magnesium ribbon. 13. Copper objects. ] [Illustration: FROM SCIAGRAPH OF VARIOUS OBJECTS. p. 130. By Prof. Terry, U. S. Naval Academy. ]