Composition of Waters of Land-Drainage and of Rain. 155 



the nitrate and hydrochloric acid mutually decompose each other, 

 Avith the separation of nitric oxide and chlorine ;■ the latter acts 

 upon the iodide of silver, liberating iodine, the vapours of which, 

 when the nitrate present is considerable, will be seen to fill the 

 flask. When the operation is supposed to be complete, the 

 flask and its contents are allowed to become perfectly cool, or 

 may be dipped into cold water to hasten this period. The 

 clamp I is now shifted, and the neck of the bottle restored to its 

 place in the India-rubber tube, when the same alternate pumping 

 and admission of carbonic acid are gone through for the removal 

 of the nitric oxide as were before employed for abstracting the 

 air. When this is accomplished the flask is separated from the 

 rest of the apparatus (which is at the time filled with carbonic 

 acid), and the first part of the process is at an end. The second 

 part is an application of Professor Bunsen's method — namely, of 

 converting the iodine into hydriodic acid, by means of an excess 

 of a standard solution of sulphurous acid, and estimating the 

 amount of excess of this latter by a standard solution of iodine. 

 To save reference I will shortly mention the relative strength of 

 these solutions and the method of using them. 



The test solution of iodine is made by dissolving 25 grains of 

 carefully purified iodine in 1 pint (7000 grs.) of distilled water. 

 In using it a burette containing 700 grs. is employed, and this 

 being divided into 100 parts, each part (or septem) contains • 025 

 (or Vu^h) of a grain of iodine, and represents • 00356 grains of 

 nitric acid. As it Is easy by practice to read to one-half or even 

 one-third of a measure, the estimation may be made to * 001 of 

 nitric acid. 



The sulphurous acid solution is of no absolute determinate 

 strength, but is standarded every day or oftener when experi- 

 ments are in progress. It is made by mixing 1 part by measure 

 of a saturated solution of sulphurous acid with about 250 parts 

 of water. A pipette containing 100 septems (700 grains) is 

 used for measuring this standard sulphurous acid, and this 

 quantity will generally require from 30 to 35 measures of the 

 standard iodine solution for its neutralization. 



The sulphurous acid solution is so weak that a portion thrown 

 on to the palm of the hand will hardly be detected by the smell. 

 To ascertain its standard value, a pij)ette-full diluted with water 

 is placed in a pint flask, a few drops of solution of starch arc 

 added, and the iodine solution is dropped into the mixture 

 till the* blue colour becomes permanent; a single drop is 

 enough to produce the change when the point has been arrived 

 at. When the solution becomes weak, a little more of the 

 strong sulphurous acid is added. It is conveniently kept in a 

 large loosely corked bottle, furnished with a syphon tulx", upon 



