and the author stipulates that the water added must be a large quantity, because if we were to add a small quantity, it would become impure upon coming into contact with the impure water.
For example, suppose we have a vessel containing half a qullah of impure water, while the vessel itself has a capacity of more than two qullahs. If we wish to purify it, we bring two qullahs and pour them over the half-qullah. We will then have added a large quantity of water, and it becomes tahur if its change has disappeared. But if we add only one qullah and the change disappears, it does not become tahur; rather, it remains impure, because it came into contact with the impurity while it was a small quantity, and thus became impure through it and cannot purify it.
The addition of water must also be continuous. If we add half a qullah and then bring another half, the first portion will already have become impure, and so on. Thus, the water added must be a large quantity of tahur water. As for the water to which it is added, it is not stipulated that it be either a large or a small quantity. Thus, if we have a vessel containing two qullahs of impure water, though it has a capacity of four qullahs, and we add two qullahs to it so that its change disappears, it becomes pure, even though the impure water amounted to two qullahs.
Or if the change in a large quantity of impure water disappears on its own, ...........
His statement, “Or if the change in a large quantity of impure water disappears on its own,” means that a large quantity is that which reaches two qullahs. This is the second method of purifying impure water: its change disappears on its own, provided that it is a large quantity.
For example, water in a vessel amounting to two qullahs may be impure, but after remaining for two or three days its odour disappears and no trace of the impurity remains, without our having added anything to it. It then becomes tahur, because a large quantity of water is strong enough to purify something else; so its purifying itself is all the more so.
In summary, if it amounts to two qullahs, it is purified in two ways:
- By addition, as mentioned above.