oeticeen Metals and Dissolved Halogens. 239 



diameter and H liters capacity supported in a thermostat con- 

 taining about 50 liters of water. Most of the experiments 

 were conducted at 25° ± 0*1° ; one, however, at 35°. In the 

 reaction vessel was a two-bladed glass stirrer of the form 

 illustrated, the blades standing at an angle of about 45° to the 

 vertical stem. The latter was attached to a brass shaft run- 

 ning in a bearing rigidly supported above the beaker and 

 terminating in a horizontal pulley. A speed indicator provided 

 with a bell which sounded every 100 revolutions was attached 

 above the pulley and kept permanently in position. 



The different metals were taken for experiment in the form 

 of circular disks -J-0 mm in diameter and usually 0*6 mm in thick- 

 ness. For the experiments with mercury the disk was of pure 

 gold heavily amalgamated. The holder for the disks was 

 made of a thin glass rod provided at its lower end with three 

 branches bent at right angles to the stem and tipped with an 

 easily fusible glass, which grasped the disk at three points on 

 its circumference. By softening one or more of these tips in 

 the flame the disk was inserted so as to be firmly held with the 

 minimum of contact between glass and metal. The glass stem 

 was mounted in a cork which fitted into a brass socket in the 

 form of a vertically slotted ring held by a stationary support 

 above the beaker, so that the disk with its glass holder could 

 be quickly removed from the apparatus or returned to exactly 

 the same position. When in place the disk was held as shown 

 in the figure, with its plane vertical, opposite the blades of the 

 stirrer and 5-7 mm distant from the wall of the beaker, so that 

 the liquid circulated freely behind it. Care was taken that 

 the relative position of disk, stirrer, and beaker should be the 

 same in all the experiments. The stirrer was driven by an 

 electric motor of -§- horse power, and the different speeds 

 obtained partly by interposing pulleys of various sizes and 

 partly by adjustment of a rheostat in series with the motor. 

 Variations in the speed of the stirrer were small and for the 

 most part negligible in comparison with the other errors of 

 experiment. 



The rate of fall in concentration of the free halogen was 

 determined by removing samples of the liquid with a pipette 

 at definite intervals and titrating. From each two successive 



V C 



titrations the value of the expression In — was calcu- 



1-2 ~ ll G 2 



latecl and found, as required by the theory, to be practically 

 constant throughout each series ; c x and c, being the concentra- 

 tions of the halogen at times t 1 and £,, and v the volume of 

 the solution, which remains constant during the interval in 

 question. 



