But

sweet, another a bitter, and a third a sour taste. science has not yet been able to give sufficient information upon this question by any experiments or observations.

Still less are we able to say why one substance should taste sweet, and another bitter. At least the chemical composition of substances can give no explanation of the fact, for many substances of an entirely different composition, have the same taste. Besides sugar, which is composed of carbon, hydrogen and oxygen, acetate of lead has a sweet taste also. Again, many substances have a bitter taste, such as quinine, sulphate of magnesia, and others, which differ entirely in their composition, and have nothing in common in their chemical characteristics.

A better agreement between taste and chemical properties is shown by the acids and alkalis. There are certain compounds in chemistry, which are distinguished by a sour taste, and are therefore termed acids. They have also the property of altering certain colouring matters, e.g. turning blue litmus red. In opposition to these acids stand bases, and the soluble bases, or alkalis, which are caustic like the acids, and turn reddened litmus blue again. In the combination of acids and bases to form salts, both lose their characteristic peculiarities, and even their peculiar taste, and then have a different one, either a saline taste, as in chloride of sodium, or a bitter taste, as in sulphate of magnesia, or even a sweet one, as in acetate of lead.

The acid, alkaline, and saline tastes belong, generally speaking, to three different series of compounds, which differ from each other by definite chemical characteristics

so that we have some reason for comparing their taste with their chemical properties. But we must add that neither the chemical nor gustative properties of these compounds are separated by sharply defined limits.

It is very remarkable that the acid and alkaline tastes may be artificially induced by an electric current. Let an electric current be passed through the tongue, the positive pole being placed on the tip of the tongue, and the negative pole on the nape, so that the current passes from the tip of the root of the tongue; a sour taste will then be experienced on the tip of the tongue. If, however, the negative pole is placed on the tongue, the taste will be different, and is generally described as alkaline. Now we know that the electric current decomposes salts, and that the acids appear at the positive, and the alkalis at the negative pole, and also that salts are found in the saliva of the mouth, which might lead us to suppose that the salts are decomposed at the poles into acid and alkali, both of which we recognise by the sense of taste. But it is not so; for even when the pole is not brought into contact with the tongue, if liquids or the lips intervene, we still experience the electric taste. Now it is possible that the tissue of the tongue itself is decomposed, which causes the taste, but so little is known of these processes, that no satisfactory explanation has RADCLIFFI at present been offered.

Even weak currents are sufficient to produce the taste, which may most easily be induced by placing polished pieces of zinc and copper upon the tongue, so that the edge of one will touch the tip, the other the under surface of the tongue. If the outer edges are now brought into contact with each other, we experience a

distinct taste, which is acid when the zinc is placed beneath the tongue, and alkaline when the copper is placed on the tip. The moisture of the tongue here forms a weak galvanic element with the two metals.

The sensitiveness of our gustative organs for certain substances is very considerable, but not to be compared with that of smell. We can recognise by taste a solution of one part of sulphuric acid in 1,000 of water. A drop placed upon the tongue would contain about

400

of a gramme ( of a grain) of sulphuric acid, an infinitesimally small quantity, the detection of which by chemical analysis would be difficult.

In the ordinary course of life we meet with other kinds of tastes in addition to those which we have already mentioned, such as corrupt, rancid, oily, aromatic, and similar tastes. We must remember however, that in such cases, our judgment is seldom founded upon the sensation of taste alone, but is assisted both by the sensation of smell and the sensation of touch. The former is caused by the vapours from the substance which is being eaten, rising through the fauces into the cavities of the nose; and the latter is produced by the form and cohesion of the particular kind of food. The taste or flavour of wine is principally decided by the smell of several kinds of ethers. The potato has no taste of its own, and yet we imagine it to taste differently in different forms. It is clear that in all such cases we experience the combined sensations of smell, taste, and touch.