Place a tin or pewter bason filled with clean water upon a silver mug: with both your hands, which must previously be wet with a solution of salt in water, grasp the silver vessel, and put your tongue into the water, taking care not to touch the tin or pewter vessel with any part of your body; you will now perceive an acid taste; which will be more sensible, if you withdraw your hands from the silver vessel while your tongue remains in the water, and then replace them.
To prove that Earth-Worms have a nervous system.
Place an earth-worm upon a plate of zinc, resting on a larger plate of silver.—The animal, as soon as it approaches the silver, seems to be repulsed by a painful sensation, and at last becomes fatigued by its repeated and fruitless exertions to make its escape, which nothing apparently prevents.
This evidently proves that the animal is provided with a nervous system, as experiments have proved that Galvanic irritation is excited only in the nerves.
CHAP. IX.
Some common Effects which are supposed to be occasioned by Galvanism.
We have already remarked, that a sub-acid taste is perceptible when two different metals are applied to the tongue and fauces: it has also been found that Galvanism affects the taste, when two different fluids and a single metal are in contact with the tongue. Upon this principle a variety of known facts have been accounted for.—For example—It has long been observed that beer, cyder, &c. when drunk from a tin or silver vessel, were more palatable than when received from a vessel of glass, or any other substance not metallic. The supposed explanation of this, is as follows.—When the outer extremity of the vessel is applied to the under lip, rendered moist by the saliva, and the tongue is extended so as to be in contact with the liquid contained in the vessel, a Galvanic arc is formed, which produces the brisk and lively taste.
It has been supposed, by persons fond of this theory, that snuff, when taken from a metallic box, excites a more agreeable sensation than when taken from a box of tortoise shell, or leather.
The fact that a silver spoon becomes discoloured by being used for eating eggs, is familiar to every one. This, also, is attributed to the Galvanic action. By experiment, sulphur has been discovered in both the albumen and yolk of an egg.—The Galvanic combination is between the sulphur of the egg, the silver spoon, and the saliva; for no tarnish is produced on the spoon when it is immersed in either the albumen or yolk; and that part of the spoon which enters the mouth is most discoloured. In every Galvanic experiment, water is decomposed into its constituent parts, hydrogen and oxygen gases. These things being premised, the fact is easily accounted for.—The hydrogen, which before the operation is nascent in the water, (which holds the sulphur in solution) now readily unites with the sulphur, and forms sulphurated hydrogen gas, which produces the tarnish on the silver.[[23]]
We shall mention a few other common appearances, and leave the solution to the ingenuity of the reader.
When copper sheathing is fastened on a ship with iron nails, the nails, and particularly the copper, are found to be corroded about the places of contact.