OF THE PARAFFINS AND THEIR MONOHALOGEN DERIVATIVES. 
9 
for exhausting the apparatus, a Sprengel for extracting a sample of the gas for 
analysis, or a water pump for regulating the pressure of the contents. 
At K is a side tube connected through a pressure gauge with the gas holder and 
drying apparatus. 
With the connections made as thus described, there was found to be very little 
leakage in the apparatus. When it was exhausted as completely as possible the rise 
of pressure was only a very small fraction of a millimetre per hour. 
Different methods of filling were adopted according to the material used. In the 
earlier experiments a water pump only was used. By means of this, the apparatus 
was exhausted to from 15 to 20 millims., and the gas admitted slowly through the 
purifying train, the process being repeated several times ; but the method was too 
extravagant for the more costly materials, and took too much time, so, in the 
later experiments, a Topler pump was used. This has a large reservoir, and with an 
hour’s pumping the pressure was reduced so low that it was often difficult to see 
whether the gauge or the barometer by the side of it stood the higher. 
When the gas was admitted through purifying and drying apparatus, this 
apparatus was usually exhausted with the rest, but as in most cases Geissler bulbs 
were used, the vacuum got gradually worse in the successive bulbs from the pressure 
of the contained liquid, so they were first filled with the gas by exhausting them two 
or three times and allowing it to stream in. 
When a volatile liquid was used, it was contained in a small bottle with a tight 
cork, through which passed a glass tube, and wired on the end of this was a piece of 
thick-walled india-rubber tube. Before attaching it to the Kundt apparatus, the 
liquid was made to volatilize freely by warming it, or by connecting the bottle to a 
water pump, and when the air was driven out the india-rubber tube was closed with a 
clamp, and joined to the dust tube, so that when the exhaustion had been completed, 
by opening the clamp, the vapour could be admitted free from air. 
For the hydrocarbons and methyl and ethyl chlorides, lycopodium powder was used 
in the dust tube, as it gave decidedly better figures than silica, but with the dense 
gases it was found to become sticky, so that it could not be made to move, and for 
these silica was used. 
A preliminary experiment was always needed to fix the position of the piston, 
for though with the denser gases figures of some sort could be got with it in any 
position, yet they were generally unsymmetrical unless it was carefully placed. The 
position which gave the strongest agitation gave the most symmetrical figures, but 
they were seldom perfect in this respect, and for this reason the proper distribution 
of the dust in the tube is a matter of importance. There is usually a tendency 
for the powder to encroach on the node from one side more than from the other, 
and this is more marked the greater the quantity of dust used, so that if the 
quantity per centimetre varies from end to end of the tube, the result is an 
apparent shifting of the nodes to an extent that is not the same at different points, 
MDCCCXCIV —A. C 
