1851.] OF THE ROYAL INSTITUTION. 43 
are harder than Gutta Percha itself, while others never solidify, but 
remain in the condition of bird-lime or treacle. 
The process termed the vulcanizing of caoutchouc was discovered 
by Mr. Thomas Hancock in 1843.— A sheet of caoutchouc immersed 
in melted sulphur absorbs a portion of it, and at the same time it 
‘ndergoes some important changes in many of its characteristic 
properties. It is no longer affected by climatic temperature ; it is 
neither hardened by cold, nor softened by any heat which would not 
destroy it. It ceases to be soluble in the solvents of common caout- 
chouc, while its elasticity becomes greatly augmented and permanent. 
The same effect may be produced by kneading sulphur into 
caoutchouc by means of powerful rollers; or the common solvents, 
naphtha and spirit of turpentine, may be charged with a sufficient 
amount of sulphur in solution to become a compound solvent ‘of rub- 
ber. In these cases articles may be made in any required forms 
before heating for the change of condition. It is necessary, however, 
for this purpose, that the form should be carefully maintained 
during the exposure to the heat necessary to effect the vulcanization 
which leaves it in a normal state. A vulcanized solid sphere 
of 24 inches in diameter, when forced between two rollers 1 inch 
apart was found to maintain its form uninjured. In fact, it is the 
exclusive property of vulcanized caoutchouc to be able to retain any 
form impressed upon it, and to return to that form on the removal 
of any disturbing force which has been brought to act upon it. 
Caoutchouc slightly expands and contracts in different tem- 
peratures ; it is also capable of being condensed under pressure. 
A cube of 21 inches, impactly secured, was subjected to a force 
of 200 tons. The result was a compression amounting to ~,;— 
great heat appeared to have been evolved, and the excessive elasticity 
of the substance caused a fly-wheel weighing five tons to recoil with 
an alarming violence. 
The evolution of heat from caoutchouc under condensation is a 
property possessed by it in common with air and the metals. It 
differs, however, from the latter in being able to exhibit cold by re- 
action. Mr. Brockedon stated that he had raised the temperature of an 
ounce of water 2° in about 15 minutes by collecting the heat evolved 
by the extension of caoutchouc thread: he refers this effect to the 
change in specific gravity. He contends that this heat thus pro- 
duced is not due to friction; because the same amount of friction 
is occasioned in the contraction as in the extension of the sub- 
stance, and the result of this contraction is to reduce the caout- 
chouc thus acted upon to its original temperature, 
Among the latest applications of the elastic force of caoutchouc 
-—the chief purport of Mr. Brockedon’s lecture — attention was 
directed : 
1. ToMr. E. Smith’s Patent application of tubes of vulcanized 
caoutchouc as torsion springs to roller blinds,— adjusted to the heavi- 
est external blinds of houses, or the most delicate carriage blinds ; and 
