This may be proved by testing the digestive power on albuminous foods of the fluid removed after a test meal, or by Schwarz's method of examining by the Rontgen rays after the patient has swallowed a bolus of bismuth wrapped in an envelope of connective tissue obtained from the appendix vermiformis of a sheep.

In healthy stomachs six to seven hours after the bolus is swallowed the connective tissue envelope should have dissolved, and the bismuth will be distributed, but if digestion is impaired the bolus may remain unbroken for from ten to twenty hours later, and will be seen as a deep black spot when the X rays are used.