SECT. XXI.—SWEETNESS, ITS NATURE
NOR is it only in the touch that smooth bodies cause positive pleasure by relaxation. In the smell and taste, we find all things agreeable to them, and which are commonly called sweet, to be of a smooth nature, and that they all evidently tend to relax their respective sensories. Let us first consider the taste. Since it is most easy to inquire into the property of liquids, and since all things seem to want a fluid vehicle to make them tasted at all, I intend rather to consider the liquid than the solid parts of our food. The vehicles of all tastes are water and oil. And what determines the taste is some salt, which affects variously according to its nature, or its manner of being combined with other things. Water and oil, simply considered, are capable of giving some pleasure to the taste. Water, when simple, is insipid, inodorous, colourless, and smooth; it is found, when not cold, to be a great resolver of spasms, and lubricator of the fibres; this power it probably owes to its smoothness. For as fluidity depends, according to the most general opinion, on the roundness, smoothness, and weak cohesion, of the component parts of any body; and as water acts merely as a simple fluid; it follows that the cause of its fluidity is likewise the cause of its relaxing quality; namely, the smoothness and slippery texture of its parts. The other fluid vehicle of taste is oil. This too, when simple, is insipid, inodorous, colourless, and smooth to the touch and taste. It is smoother than water, and in many cases yet more relaxing. Oil is in some degree pleasant to the eye, the touch, and the taste, insipid as it is. Water is not so grateful; which I do not know on what principle to account for, other than that water is not so soft and smooth. Suppose that to this oil or water were added a certain quantity of a specific salt, which had a power of putting the nervous papillœ of the tongue into a gentle vibratory motion; as suppose, sugar dissolved in it. The smoothness of the oil, and the vibratory power of the salt, cause the sense we call sweetness. In all sweet bodies, sugar, or a substance very little different from sugar, is constantly found. Every species of salt, examined by the microscope, has its own distinct, regular, invariable form. That of nitre is a pointed oblong; that of sea-salt an exact cube; that of sugar a perfect globe. If you have tried how smooth globular bodies, as the marbles with which boys amuse themselves, have affected the touch when they are rolled backward and forward and over one another, you will easily conceive how sweetness, which consists in a salt of such nature, affects the taste; for a single globe, (though somewhat pleasant to the feeling,) yet by the regularity of its form, and the somewhat too sudden deviation of its parts from a right line, is nothing near so pleasant to the touch as several globes, where the hand gently rises to one and falls to another; and this pleasure is greatly increased if the globes are in motion, and sliding over one another; for this soft variety prevents that weariness, which the uniform disposition of the several globes would otherwise produce. Thus in sweet liquors, the parts of the fluid vehicle, though most probably round, are yet so minute, as to conceal the figure of their component parts from the nicest inquisition of the microscope; and consequently, being so excessively minute, they have a sort of flat simplicity to the taste, resembling the effects of plain smooth bodies to the touch; for if a body be composed of round parts excessively small, and packed pretty closely together, the surface will be both to the sight and touch as if it were nearly plain and smooth. It is clear from their unveiling their figure to the microscope, that the particles of sugar are considerably larger than those of water or oil, and consequently, that their effects from their roundness will be more distinct and palpable to the nervous papillœ of that nice organ the tongue: they will induce that sense called sweetness, which in a weak manner we discover in oil, and in a yet weaker, in water; for, insipid as they are, water and oil are in some degree sweet; and it may be observed, that the insipid things of all kinds approach more nearly to the nature of sweetness than to that of any other taste.