.V/A' \\-lLI.IA.M SIEMENS, F.R.S. IOI 



fresh water, had not quite attained the maximum after three hun- 

 (liv<l (lays' immersion, the relative absorptions of the several 

 materials under examination may safely be assumed to be in the 



following proportions : 



In Fresh Water. In Salt Water. 



Per Cent. Per Cent. 



Raw india rubber ... 25 8 



Unvulcanized block do. . 28 3 '8 



India rubber and mica . . 19 3'9 



Vulcanized india rubber . . 10* 14 2*9 



Gutta percha . . . . -1'5 1*0 



India rubber in its raw and in its unvulcanized state is thus 

 proved to absorb water in greater quantities than the other 

 materials ; while, next to gutta percha, vulcanized india rubber 

 shows, both in fresh and salt water, the greatest insensibility to 

 absorption. 



3rd. In pure water the rate of absorption with increase of tem- 

 perature is least influenced in regard to gutta percha, the rate 

 being little more than double for an increase of 39 Fahr. to 120 

 Fahr. For india rubber the rate is about eight times, and for 

 Wray's mixture about sixteen times greater at 120 Fahr. than at 

 89* Fahr., the latter being the temperature of water of the maxi- 

 mum density, as found at the bottom of deep oceans. In sea water, 

 india rubber absorbs about double the quantity at 120 Fahr. that 

 it does at 39 Fahr. ; for gutta percha and Wray's mixture the rate 

 of absorption is not materially altered by temperature. 



4th. On removing the india rubber from the baths, previously to 

 weighing, the surfaces were invariably found to be slimy, an obser- 

 vation which first led to the belief, that india rubber was soluble 

 to a small extent in water. This fact is corroborated, and settled 

 beyond doubt, by the curves of this material depressing towards 

 the ends, as well as by the weights of the specimens, taken after 

 the tests were concluded, showing a decrease from 0'4 per cent, to 

 1*2 per cent. It is also apparent, that the unvulcanized iudia 

 rubber is likewise subject to solution in water, but in a less degree. 

 The results of experiments, instituted with ;i view of ascertaining 

 the dependence of absorption on the thickness of the material, 

 only served to express a law, which' might have been foreseen, 

 th'at thicker plates absorb comparatively less than thinner ones. 

 The difficulty of procuring materials physically equal leaves 



