214 CAOUTCHOUC AND GUTTA-PERCHA. 
priate for so many uses, is its relation to heat. Above 50° it becomes more 
flexible and somewhat elastic, but still maintains its hardness and remarkable 
power of resistance; on severe tension its contracts but litile. At 65° to 70° 
it becomes soft and very plastic, and loses much of its toughness. In this con- 
dition several pieces of it can easily be kneaded into one whole. Mere immer- 
sion into bot water of these temperatures suflices to give the mass every shape 
desired, which it also preserves after cooling off, when it reassumey its former 
hardness at: every temperature below +45°. It is, further, easily inflammable, 
and burns with a bright flame, and amid strong sparkling, dripping off a dark, 
glutinuous residue. In regard to solvents, gutta-percha is like caoutchouc ; its 
decomposition in increased heat and its composition are similar to those of that 
substance. Like it it consists of carbides of hydrogen. The little oxygen 
which has been found in analyzing it was probably received from the air during 
_the purifying operation. ‘The article of trade has frequently been subjected to 
chemical experiments, but we cannot say that a clear result has been elicited. 
We omit them, therefore, mentioning only one circumstance which has often 
caused confusion in practice. 
The surface of a carefully cleaned plate is found to be, in parts, covered with 
a bluish bloom, which, however often wiped away, reproduces itself as long as 
the plate remains flexible. After years the whole surface appears to be faintly 
grayish blue, and under the microscope can be perceived an exceedingly thin 
layer of very fine, white, little dots. A higher temperature, to which the gutta- 
percha had been exposed, greatly promotes this alteration, and therefore the 
darker sorts suffer by it most. he physical property resulting from this change 
of the surface is noteworthy. Unchanged gutta-percha is a good insulator of 
electricity, and occupies so low a rank in the scale of electric affection by rubbing 
as to remain strongly negative when rubbed by almost any substance. Only 
gun-cotton and electric-paper make it positively electric. he changed surface 
does not destroy the power of insulation, but the gutta-percha rises thereby high in 
the scale of electrical excitement, and when rubbed with almost any substance 
becomes positively electric in a high degree. The exceptions are only mica, 
diamonds, and furs. Hence we learn the remarkable fact that we possess in 
gutta-percha a plate, one plane of which—the blue—when lightly rubbed with 
the hand, with linen, glass, rock crystal, the barb of a feather, or flannel, 
develops intense positive, and the other—the brown—intense negative elee- 
tricity when going through the same processes. 
Gersdorf finds the cause of this change of surface, which resembles a similar 
appearance on the surface of ripe plums, in the attraction of water from the 
atmosphere. Gutta-percha, deprived by cautious melting of all water, of which 
it, as a rule, imbibes some 5-6 per cent. in the process,of formation, and there- 
fore assuming a dark brown color, is soon covered, especially on its cut planes, 
with this vapory coating; while this does not take place, or does only a great 
deal later, with the light-brown substance, which has not been rendered anhy- 
drous to so extreme a degree, except when, and only where, it is traversed by 
dark veins. Still, it is a question whether the change of color is a consequence 
of a mere deprivation of water, or rather attributable to the influence of heat, 
that is, to a change of the substance itself. 
Therefore, it is likely that the change of the gutta-percha is caused by a se- 
cretion of a component part of the mass by the influence of air and heat. 
The excessive positive irritability of the altered surface, which we find in 
no other vegetable substance, is an indication that the process is connected 
with the two kinds of resin which Payen has obtained from gutta-percha. A 
closer inquiry into the question, especially in the indicated direction, would be 
useful, for only thus could we discovar the causes of the unfortunate transfor- 
mation of gutta-percha into a very brittle mass, a change noticeable chiefly in 
small articles manufactured out of that substance. 
