We have not found this method in Boerhaave (1727), nor in Lemery (1675), nor does Faraday, in his "Chemical Manipulations," make any allusion to it. As a process of purification of solutions, it seems to have been lost sight of in modern laboratory practice.
We have made trial of the method rendered noteworthy by more than two thousand years' practice, and find that, while the process is quite slow, it has certain advantages. Chief among these is the fact that, when the capillary siphon is adjusted, it requires no further attention; there is no pouring into a constantly emptying funnel, and there is no possibility of overflow. When properly arranged, the last drop of liquid passes from the upper vessel to the lower, and, except in certain cases of extreme divisibility, the filtration is perfect. The process seems particularly adapted to the purification of weak solutions, as of mineral waters, where the insoluble portion is not to be conserved. There is obviously much choice in the material of which the capillary siphon is made. We have tried cotton, wool, lamp-wick, and asbestus, and we find stout, silky wads of the latter most serviceable; it is also useful for filtering very acid and alkaline solutions.
The rapidity of filtration does not seem to be hastened by lengthening the longer arm of the siphon, and is chiefly dependent on the number of threads in the filter, and on their fineness. In one experiment, thirty-two strands of cotton yarn filtered twice as fast as sixteen strands, and only half as fast as sixty-four strands. Oil filters much slower than saline solutions, and the latter much slower than pure water.
We have used the expression "capillary siphoning" in describing anethisis, and perhaps it needs justification. The ascension of the liquid is due to capillarity, and the descent through the longer arm of the siphon is in obedience to gravity. Siphoning is dependent on atmospheric pressure, and can not be strictly applied to the present case; yet we find by experiment that, if the lower arm of the woolen threads be raised to the level of the liquid to be filtered, the action ceases, and the form of a siphon is at all events essential to the process.
The quotations from ancient authors show that they must have been quite familiar with capillary attraction, yet the first observance of this phenomenon is attributed by some authorities to Franciscus Aggiunti, physician to the Grand Duke of Tuscany, about the beginning of the seventeenth century,[1] (Aggiunti died 1635.)
In conclusion, the object of this paper is not to propose a return to this ancient method of filtration, the modern rapid processes being more in accordance with present needs; but we have thought it not altogether useless to call attention to a neglected process which can in certain cases be employed with advantage. Where the object of filtration is to collect the insoluble portion, it is obviously of no value; whether the process could be advantageously used on a large scale remains to be tested.
- ↑ Gehler's "Physikalisches Wörterbuch," article "Capillarität."
