1864. | Blectricity. 349 
of ice, and the interest excited by his remarkable book “ Heat as a 
Mode of Motion,” have caused physicists in other countries to direct 
their attention to this subject. Professor Reusch, of Tubingen, in a 
letter to Dr. Tyndall,* describes some observations which he has made 
on this body. A long narrow plate of clean ice was suspended by its 
two ends in loops of silk, whilst a third loop, hung from its centre, had 
a small weight attached to it. After the lapse of 20 or 80 minutes a 
bending was plainly seen, the ice comporting itself like a plastic body ; 
once indeed he was able to bend a thin lamella of ice between the fingers 
of both hands. In preparing these plates it was noticed that in sawing 
through ice, the saw after a time ceases to act, the space, between its 
teeth becoming filled with freshly-formed ice, so that it passed along 
almost without friction The saw, in fact, melts through the ice, the 
heat necessary for that being the equivalent of the work applied to the 
saw. In dividing plates of ice it is necessary to handle them like 
glass. If the convex blade of a knife be passed over a piece of ice 
with a certain pressure a sharp crack will result, and the plate may be 
broken in the direction of this crack, provided the temperature of 
the ice and of the air be below 0° C. Obviously the knife acts in this 
instance like a diamond, which depresses minute particles of glass, and 
through the wedge action of which a progressive linear cracking is pro- 
duced, which renders fracture possible. A mere scratch suffices neither 
for glass nor for ice. 
VIII. ELECTRICITY. 
A new insulating material has been recently imported by Sir W. 
Holmes from Demarara, which bids fair to be a formidable rival to 
gutta-percha. It is the dried juice of the bullet tree (sapota muller’) and 
is called balata ; it appears likely to be more valuable than India-rubber 
or gutta-percha by themselves, as it possesses much of the elasticity of 
the one and the ductility of the other, without the intractability of 
India-rubber, or the brittleness and friability of gutta-percha, whilst 
it requires a much higher temperature to melt or soften it. Since the 
Exhibition of 1862, Sir W. Holmes, who was the Commissioner repre- 
senting the colony of British Guiana, has been engaged in investiga- 
tions how to produce the material cheaply, and how to dry or coagulate 
it rapidly ; he has now succeeded so far as to warrant the importation 
of steam machinery to be applied for its extraction, and there appears 
to be every probability that balata will become an important article of 
commerce, supplying the great want of the day, a good insulating 
medium for telegraphic purposes. Messrs. Silver & Co. have already 
imported many tons of it, and Professor Wheatstone is now investiga- 
ting its electrical and insulating properties. Another substitute for 
- gutta-percha, the juice of the alstonia scholaris, a tree belonging to 
the natural order apocynca, has been forwarded from Ceylon by Mr, 
Ondaatjie ; it is stated to possess the same properties, and to be as 
* Philosophical Magazine,’ vol. xxvii, p. 192. 
