ME. C. GEEVILLE WILLIAMS ON ISOPEENE AND CAOETCHINE. 
243 
distilled between 116° and 193°. Only one-half came over below 204°. In the second 
rectification a large fraction was obtained between 40° and 50°, then scarcely any distilled 
until between 170° and 180°, which latter fraction was very large. At 171° the thermo- 
meter was often perfectly steady while an ounce distilled over, the entire contents of 
the retort not being much more than one and a half ounce. The lower fraction oscil- 
lated between 36° and 44°, the variability in its boiling-point beiag principally due to 
the presence of oxygen ; for, by careful and repeated rectifications over sodium, it may 
be made to distil almost entirely between 37° and 38°. This fiuid lies at the boundary 
line between gases and liquids : it is scarcely possible to wet the hand with it ; and if 
poiued from a moderate height in small drops, it is volatilized before reaching the floor. 
A very pretty way of observing the influence of small pressures upon its physical con- 
dition, consists in passing a few drops into a mercurial eudiometer standing vertically. 
The warmth of the hand applied to the top of the instrument is sufficient to convert it 
into vapour ; but if the position of the eudiometer be then altered so as to stand almost 
horizontally, the increase of pressm'e is sufficient to condense the gas into a liquid, 
and, on the other hand, it is instantly vaporized again on restoring the eudiometer to its 
original position. 
This hydrocarbon possesses a property which, even more than its extreme volatility, 
renders it troublesome to the analyst. Unlike very volatile fluids in general, it lines 
with a slight film of carbon the bulbs in which it is contained during combustion. 
It is true that the amount thus left is very small, but it is sufficient to render an exact 
determination of the carbon difficult. It being of course necessary to have all the 
apparatus, oxide of copper, &c. as cold as possible, it was also not easy to keep the 
hydrogen as low as could be desired. The substance used in the first three analyses 
boiled between 37° and 38°, and was prepared from caoutchouc. The fourth and fifth 
analyses were made on a product from gutta percha. No. IV. boiled between 42° and 
43°, and No. V. between 43° and 44°. 
I. 0T848grm. yielded 0’5985 carbonic acid and 0T998 water. 
II. 0T770grm. yielded 0'5696 carbonic acid and 0T963 water. 
III. 0‘2076grm. yielded 0-6689 carbonic acid and 0*2264 water. 
IV. 0-1730 grm. yielded 0*5570 carbonic acid and 0-1898 water. 
V. 0*1362 grm. yielded 0*4398 carbonic acid and 0*1504 water. 
Or, in per-centages : — 
Experiment. 
A 
Mean. 
Calculation. 
Carbon 
f 
I. 
. 88*3 
II. III. 
87*8 87*9 
IV. v.' 
87*8 88*1 
88*0 
(— 
QIO 
60 
88*2 
Hydrogen 
. 12*0 
12*0 12*1 
12*2 12*3 
12*1 
8 
11*8 
leading to the empirical relation 
68 
100*0 
2 K 2 
