In the course of laboratory experiments tackiness has often been induced by the use of traces of copper salts. The rate at which tackiness is induced appears to be dependent upon the amount of copper salt used, but once it begins, the rubber molecule is very rapidly broken down, and resins are formed. As the formation of resins is accompanied by the inclusion of oxygen in the chemical constitution, it would be expected that dry rubber becoming tacky should increase in weight. This is found to be the case, and to give an idea of how this weight increases with the progress of tackiness, the results below may be studied. It will be seen that the maximum quantity of copper sulphate used amounted to 0·025 per cent, (approx.) upon the weight of latex taken. Now it is highly probable that only a fraction of this quantity was retained in the rubber on coagulation, the remainder being in solution in the serum. Furthermore, as the rubber was well washed and worked down to thin crepe, _the total quantity of copper salt remaining in the dry crepe must have been exceedingly small_. Yet the effect is most marked and should impress upon all managers the necessity for guarding against any possible contamination caused by brass or copper. ----------+-----------------+-------------------------------------------- _Sample._ | | _Weight of Rubber._ +------+ +------+--------+--------+--------+---------- | | | After |Further |Further | Percent- |_Amount of Copper Salt._| When |Interval|Interval|Interval|age in In- | | Dry. |of Four |of Seven|of Three|crease in | | | Weeks. | Weeks. | Weeks. | Weight. ---+------------------------+------+--------+--------+--------+---------- | | Grms.| Grms. | Grms. | Grms. | 1 |0·02 grms. copper | | | | | |sulphate per 100 c.c. | 430 | 441 | 482 | 488 | 13·5 |latex | | | | | 2 | Ditto | 428 | 439 | 481 | 486 | 13·55 3 |0·01 grms. copper } | | | | | |sulphate, per 100 c.c.} | | | | | |latex } | 962 | 987 | 1035 | 1036 | 7·7 |0·01 grms. copper } | | | | | |acetate, per 100 c.c. } | | | | | |latex } | | | | | 4 |0·025 grms. copper | | | | | |sulphate, per 100 c.c. | 502 | 513 | 558 | 560 | 11·5 |latex | | | | | ---+------------------------+------+--------+--------+--------+----------