154 Comj)osition of (''/aters of Land- Drainage and of Rain. 



ing upon the withdrawal of the air. The acid-tube and globular 

 flask being now in position, a vacuum is made in them by a few 

 strokes of a small air-pump ; the amount of the vacuum can be 

 observed by the barometer tube c, which at its lower end dips 

 into a small vessel of mercury. The bottles d and e are 

 intended to supply carbonic acid ; d consists of a two-necked 

 bottle, of a capacity of about 2 pints, and containing fragments of 

 marble ; it is joined to e by means of the tube g, which reaches 

 nearly to the bottom of each, e is also a two-necked bottle, 

 holding about 4 pints, and furnished at one of its openings with 

 a cork and a tube, containing bi-carbonate of potash, for the 

 purpose of preventing any nitric acid from reaching the apparatus 

 from external sources. On pressing for a second the little 

 brass spring clamp A, which is placed on an India-rubber joint, 

 the carbonic acid diffuses itself through the apparatus, and the 

 dilute hydrochloric acid contained in the bottle e is forced over 

 into d, where it gives rise to a further supply of carbonic acid, 

 which forces the acid back into the bottle e. In this way any 

 unnecessary waste of the materials is avoided, and the apparatus 

 does not require renewal for a long time,* The carbonic acid 

 mixed with the small quantity of air remaining in the flasks is 

 now removed by the pump, and the vacuum is as before again 

 destroyed by recourse to the spring h. By repeating 4 or 5 

 times these operations, which do not occupy as many minutes, 

 the last portions of air are effectually removed. f The vessels 

 being now nearly vacuous, the acid in b is, by a little manage- 

 ment, made to flow into the bottle containing the nitrate and 

 iodide of silver. The T-piece connecting these is made of glass. 

 The parts m m m, where one tube joins on at right angles to 

 another, are short T-pieces either of glass or metal. 



By withdrawing the bottle a about a quarter of an inch from the 

 tube connecting it with the rest of the apparatus, the operator is 

 now able to shut it off from the latter by means of the clamp z, 

 which is of course placed in readiness at the beginning of 

 the operation, Tlie bottle now containing the nitrate, the iodide 

 of silver, and hydrochloric acid, is placed in a water-bath, which 

 is conveniently supported on a retort stand, and remains in the 

 boiling water for about ten minutes. Under these circumstances 



* The same object may 'he attained by the use of a double cylinder arrange- 

 ment, such as is employed in tlie Dobereiner lamp. 



"t* It is quite possilde that the air might be removed, though perhaps hardly so 

 effectually by a stream of carbonic acid m ithout the use of a pump, but there are 

 many advantages in the employment of this latter which more than compensate 

 for the extra trouble which it involves in the fitting up of the apparatus, especially 

 that the vacuum enables us to boil the substances in the flask A without risk of 

 bursting, which otherwise would be almost certain to occur if, as it must be, the 

 flask is closed. 



