﻿Prof. R. Bunsen on the Washing of Precipitates. 7 



air-pump constructed on the principle of SprengeFs mercury- 

 pump, and which appears to me preferable to all other forms of 

 air-pump for chemical purposes, since it effects a rarefaction to 

 within 6 or 12 millimetres pressure of mercury. 



Fig 4 shows the arrangement of this pump. On opening the 

 pinchcock a, water flows from the tube / into the enlarged glass 

 vessel b, and thence down the leaden pipe c. This pipe has a 

 diameter of about 8 millims., and extends downwards to a depth 

 of 30 or 40 feet, and ends in a sewer or other arrangement ser- 

 ving to convey the water away. The lower end of the tube d 

 possesses a narrow opening; it is hermetically sealed into the 

 wider tube b, and reaches nearly to the bottom of the latter. A 

 manometer is attached to the upper continuation of this tube d 

 by means of a side tube at d l ; at d^ is attached a strong thick 

 caoutchouc tube possessing an internal diameter of 5 millims. 

 and an external diameter of 12 millims. ; this leads to the flask 

 which is to be rendered vacuous, and is connected with it by 

 means of the short narrowed tube k. Between the air-pump and 

 the flask is placed the small thick glass vessel/, in which, when 

 one washes with hot water, the steam which may be carried over 

 is condensed. All the caoutchouc joinings are made with very 

 thick tubing, the internal diameter of which amounts to about 

 5 millims., the external diameter to about 17 millims. The 

 entire arrangement is screwed down upon a board fastened to the 

 wall, in such a manner that each separate piece of the apparatus 

 is held by a single fastening only, in order to prevent the tubes 

 being strained and broken by the possible warping of the board. 

 On releasing the pinchcock a, water flows from the conduit / 

 down the tube c to a depth of more than 30 feet, carrying with 

 it the air which it sucks through the small opening of the tube d 

 in the form of a continuous stream of bubbles. No advantage 

 is gained by increasing the rapidity of the flow, since the friction 

 exerted by the water upon the sides of the leaden pipe acts di- 

 rectly as a counter pressure, and a comparatively small increase 

 in the rapidity of the flow is accompanied by a great increase in 

 the amount of this friction. Accordingly at^ris a second pinchcock, 

 by which the stream can be once for all so regulated that, on 

 coirrdetely opening the cock a } the friction, on account of the 

 diminished rate of flow, is rendered sufficiently small to allow of 

 the maximum degree of rarefaction. Such an apparatus, when 

 properly regulated once for all by means of the cock g> ex- 

 hausts in a comparatively short time the largest vessels to within 

 a pressure of mercury equal to the tension of aqueous vapour at 

 the temperature possessed by the stream*. The tension exerted 



* The time required to obtain the above degree of exhaustion in a flask 

 of from 1 to 3 litres capacity ranges from six to ten minutes ; the quantity 

 of water necessary amounts to about 40 or 50 litres. 



