656 Original Articles. [ Oct., 
Comparing the numbers in this table with those in the first series, 
which were made under precisely similar conditions in all respects, 
except temperature, which then did not exceed an average of 40° or 
45° Fahr., it becomes evident that temperature has a considerable effect 
on the amount of water absorbed. Thus, gutta-percha at 45° absorbed 
0:044 grains; at 75°, 0°27 grains, or six times as much. In lke 
manner, india-rubber absorbed 0°17 erains at a lower temperature, 
and 0°45 at the higher, or two-and-a-half times as much. Wray’s 
compound, 0-072 at the lower temperature, and 0°58 at the higher, or 
seven times as much. 
Reasoning upon the foregoing experiments, a question arises as to 
the ratio or quantity of water absorbed in different times, and the con- 
dition of the specimens after a much more lengthened immersion. The 
present experiments, although showing the relative permeability of 
different insulators, do not afford data to determine the ultimate con- 
dition of the material intended to surround and insulate the conducting 
wires of the electric cable. To ascertain these facts, a much more 
enlarged series of experiments is required, extending over a much 
greater length of time. If, for example, gutta-percha absorbs -015 
grains of water in 100 hours, under a pressure of 20,000 lbs. on the 
square inch, we want to determine the corresponding quantity absorbed 
in 1,000 hours; and further, at what period will the continuous ab- 
sorption cease? These are questions of vital importance as regards 
the porosity of the specimens; and, when ascertained, we should still 
require to know to what extent the insulation of the electric current 
would be impaired in the cable saturated with moisture. 
Should our best insulators, such as Chatterton’s compound or 
gutta-percha, as given in the experiments, arrive at a point at which 
they will absorb no more water under a given pressure, it then becomes 
necessary that we should ascertain whether the water imbibed is suf- 
ficient to carry off the whole or a part of the voltaic current, and 
whether the passage of the current through the imsulator would acce- 
lerate, in turn, the oxidation and consequent destruction of the con- 
ductor. To solve these questions, we require, in my opinion, a long 
series of carefully-conducted experiments, which would tend to give a 
reliability to these important undertakings which at present they have 
not attained. 
The earlier experiments on the insulating power of various cores 
when placed under pressure were made with voltaic electricity ; but, 
owing to the shortness of the specimens, it was found impossible to 
destroy their insulation by the absorption of water so as to permit a 
current from a small battery to pass through the covering. 
Failing in this, recourse was had to frictional electricity, which, 
from its high intensity, passed with greater or less facility through 
the insulating coverings of the wire. Still the difficulty of deciding 
upon the period at which, after remaining under pressure, the imsu- 
lation began to grow less perfect, remained to a large extent unremoved. 
This difficulty was very much increased by the necessarily short period 
in which the experiments had to be completed. It was impossible in 
many cases to leave the cores long enough under pressure to ascertain 
