78 Tfmehri. 
tion largely practised, whereby the sulphur is worked 
into the white rubber, over which it is sprinkled after it 
has become plastic by mechanical pressure with heated 
rollers. Mineral colouring matter is also incorporated. 
Sometimes a great variety of matter is wrought in to re- 
duce the quality, such as fuller's earth, white lead, mag- 
nesia, sulphate of lead, clay, chalk, zinc, &c. Rubber 
containing this addition has lost its elasticity in a large 
degree. Red rubber, which is recognised as superior to 
grey, is vulcanised by sulphide of antimony to which 
about twenty per cent of sulphur has been added. 
Gutta-percha is a closely allied substance to india- 
rubber, their chemical composition being almost identical ; 
but in practical application they are found to differ widely. 
Gutta-percha is characterised by a considerably greater 
plasticity under heat, but possesses little power of tension 
and is much less porous ; so that for the great majority 
of purposes one material could not be substituted for the 
other. For certain purposes, however, the two are some- 
times combined. It was introduced to England nearly 
forty years ago, and the trade very speedily developed. 
It forms an excellent insulator in electrical operations, 
and is one of the best non-conductors of heat. It is 
largely used for covering wires and cables, and, when 
vulcanised, in water and in other ways. When protected 
from light and air, it seems almost unaffected by lapse of 
time. It can be vulcanised in the same way as india- 
rubber. Uncured, at a temperature of 50 deg. centigrade it 
becomes soft, and at 100 deg. it is sticky and paste- 
like. The fresh milk hardens by mere exposure to the 
air in shade, without being subjected to smoke. 
