VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 45 i 



their falling down when not required, and at the same time it prevents their 

 coming nearer to the tin plate than the proper limit. It is evident that when 

 the instrument stands as shown in fig. 1, the gilt surface of the paper xy, which 

 covers the inside of the lateral frames, stands contiguous and parallel to the tin 

 plate. 



When the instrument is to be used, it must be placed on a table, a window, 

 or other convenient support, a bottle electrometer is placed near it, and is con- 

 nected, by means of a wire, with one of the tin tubes ad, bc; and by another 

 conducting communication the tin plate must be connected with the electrified 

 substance, the electricity of which is required to be collected on the plate abcd : 

 thus, for instance, if it be required to collect the electricity of the rain, or of 

 the air, the instrument being placed near a window, a long wire must be put 

 with one extremity into the aperture a or b of one of the tin tubes, and with 

 the other extremity projecting out of the window. If it be required to collect 

 the electricity produced by evaporation, a small tin pan, having a wire or foot of 

 about 6 inches in length, must be put on one of the tin tubes, so that the wire* 

 going into the tube the pan may stand about 2 or 3 inches above the instrument. 

 A lighted coal is then put into the pan, and a few drops of water poured on it 

 will produce the desired effect. 



Mr. C. adds, that having actually used this new instrument in several experi- 

 ments, he had found it to answer perfectly well; one of its principal recommenda- 

 tions being the certainty of its operation. 



XFII. On the Conversion of a Mixture of Dephlogisticated and Phlogisticated 

 Air into Nitrous Acid, by the Electric Spark. By Henry Cavendish, Esq., 

 F. R. S., and A. S. p. 26l . 



In volume 75 of the Phil. Trans., p. 372, I related an experiment, which 

 showed, that by passing repeated electric sparks through a mixture of atmo- 

 spheric and dephlogisticated air, confined in a bent glass tube by columns of 

 soap-lees and quicksilver, the air was converted into nitrous acid, which united 

 to the soap-lees and formed nitre. But as this experiment has since been tried 

 by some persons of distinguished ability in such pursuits without success, I 

 thought it right to take some measures to authenticate the truth of it. For this 

 purpose, I requested Mr. Gilpin, clerk of the r. s., to repeat the experiment, 

 and desired some of the gentlemen most conversant with these subjects to be pre- 

 sent at putting the materials together, and at the examination of the produce. 

 This laborious experiment Mr. Gilpin performed in the same manner, and with 

 the same apparatus, which was used in my own experiments. The method used 

 for introducing air into the bent tube, was that described in the last paragraph 

 of p. 373 in that paper. The soap-lees, like those of my own experiments, 



3m 2 



