516 
ME. W. CEOOKES ON ATTEACTION AND 
In small tubes, and taking certain special precautions, I could prepare a vacuum with 
my Sprengel pump which would hardly allow an induction-current to traverse it, or would 
only show a faint, cloud-like discharge ; but it was impossible to effect this in the large 
tubes required for these experiments, and, in fact, all the Sprengel vacuum balance- 
Fig. 5. 
tubes which I had hitherto prepared became brightly luminous 
when the current from an induction-coil was passed through 
them. I accordingly fitted up the apparatus shown in figure 5, 
in which a chemical vacuum could be prepared by a method 
first devised by Dr. Andrews (Philosophical Magazine, February 
1852). 
51. a b is the tube containing a straw beam with pith-ball 
terminals ; at b two platinum wires are passed through, to con- 
nect with an induction-coil ; at a the tube is contracted to allow 
the apparatus to be sealed off ; c is a portion of the tube con- 
taining a copper boat filled with freshly cast sticks of caustic 
potash ; d is a tube bent as shown, and nearly full of strong 
sulphuric acid, which has been previously boiled for some 
minutes and then allowed to cool in a vacuum ; e is a mercury 
joint, connecting the apparatus to the Sprengel pump. At the 
upper part of the tube d is a stopper fitting into a funnel-joint 
and capable of being replaced (as shown in figure) by a tube 
through which I could pass carbonic acid when desirable. The 
carbonic acid was prepared by the action of hydrochloric acid on marble ; when not 
being passed into the exhausted tube, the gas was kept bubbling through mercury, 
U 
