Their aperient action is dependent upon the minimum of irritation of the bowel, and is exercised by their abstraction from the blood of water, which passes into the bowel to act as a diluent of the salt.
It is also very useful as a supplement to mercury, which needs a saline aperient to complete its action.
Some sufferers from constipation find that they get greater relief from salts dissolved in water, or from natural aperient water taken on rising in the morning, and others again find that the best way of opening the bowels is to inject one or two drachms of glycerine into the rectum, or use it as a suppository.
Manna possesses mildly laxative properties, and on account of its sweet taste is employed as a mild aperient for children.
In large doses oil of turpentine causes purging and may induce much haemorrhage from the bowel; it should be combined with some trustworthy aperient, such as castor oil, when given as an anthelmintic. It is readily absorbed unchanged and has a marked contractile action upon the blood vessels.
The south spring is aperient, but contains some iron; while the north or chalybeate spring is more tonic in its properties.
In medicine it is employed as an aperient, and is one of the safest and most innocuous known.