There are few phenomena of common occurrence which have proved more perplexing to philosophers than those which attend the deposition of dew. Every one is familiar with these phenomena, and in very early times observant men had noticed them; yet it is but quite recently that the true theory of dew has been put forward and established. This theory affords a striking evidence of the value of careful and systematic observation applied even to the simplest phenomena of nature.

It was observed, in very early times, that dew is only formed on clear nights, when, therefore, the stars are shining. It was natural, perhaps, though hardly philosophical, to conclude that dew is directly shed down upon the earth from the stars; accordingly, we find the reference of dew to stellar influences among the earliest theories propounded in explanation of the phenomenon.

A theory somewhat less fanciful, but still depending on supposed stellar influences, was shortly put forward. It was observed that dew is only formed when the atmosphere is at a low temperature; or, more correctly, when the air is at a much lower temperature than has prevailed during the daytime. Combining this peculiarity with the former ancient philosophers reasoned in the following manner: Cold generates dew, and dew appears only when the skies are clear—that is, when the stars are shining; hence it follows that the stars generate cold, and thus lead indirectly358 to the formation of dew. Hence arose the singular theory, that as the sun pours down heat upon the earth, so the stars (and also the moon and planets) pour down cold.

Nothing is more common—we may note in passing—than this method of philosophizing, especially in all that concerns weather-changes; and perhaps it would be impossible to find a more signal instance of the mistakes into which men are likely to fall when they adopt this false method of reasoning; for, so far is it from being true that the stars shed cold upon the earth, that the exact reverse is the case. It has been established by astronomers and physicists that an important portion of the earth’s heat-supply is derived from the stars.

Following on these fanciful speculations came Aristotle’s theory of dew—celebrated as one of the most remarkable instances of the approximation which may sometimes be made to the truth by clever reasoning on insufficient observations. For we must not fall into the mistake of supposing, as many have done, that Aristotle framed hypotheses without making observations; indeed, there has seldom lived a philosopher who has made more observations than he did. His mistake was that he extended his observations too widely, not making enough on each subject. He imagined that, by a string of syllogisms, he could make a few supply the place of many observations.

Aristotle added two important facts to our knowledge respecting dew—namely, first, that dew is only formed in serene weather; and secondly, that it is not formed on the summits of mountains. Modern observations show the more correct statement of the case to be that dew is seldom formed either in windy weather or on the tops of mountains. Now, Aristotle reasoned in a subtle and able manner on these two observations. He saw that dew must be the result of processes which are interfered with when the air is agitated, and which do not extend high above the earth’s surface; he conjectured, therefore, that dew is simply caused by the discharge of vapour from the air.359 “Vapour is a mixture,” he said, “of water and heat, and as long as water can get a supply of heat, vapour rises. But vapour cannot rise high, or the heat would get detached from it; and vapour cannot exist in windy weather, but becomes dissipated. Hence, in high places, and in windy weather, dew cannot be formed for want of vapour.” He derided the notion that the stars and moon cause the precipitation of dew. “On the contrary, the sun,” he said, “is the cause; since its heat raises the vapour, from which the dew is formed when that heat is no longer present to keep up the vapour.”

Amidst much that is false, there is here a good deal that is sound. The notion that heat is some substance which floats up the vapour, and may become detached from it in high or windy places, is of course incorrect. So also is the supposition that the dew is produced by the fall of condensed vapour as the heat passes away. Nor is it correct to say that the absence of the sun causes the condensation of vapour, since, as we shall presently see, the cold which causes the deposition of dew results from more than the mere absence of the sun. But, in pointing out that the discharge of vapour from the air, owing to loss of heat, is the true cause of the deposition of dew, Aristotle expressed an important truth. It was when he attempted to account for the discharge that he failed. It will be observed, also, that his explanation does not account for the observed fact that dew is only formed in clear weather.

Aristotle’s views did not find acceptance among the Greeks or Romans; they preferred to look on the moon, stars, and planets as the agents which cause the deposition of dew. “This notion,” says a modern author, “was too beautiful for a Greek to give up, and the Romans could not do better than follow the example of their masters.”

In the middle ages, despite the credit attached to Aristotle’s name, those who cultivated the physical sciences were unwilling to accept his views; for the alchemists (who alone may be said to have been students of nature) founded360 their hopes of success in the search for the philosopher’s stone, the elixir vit?, and the other objects of their pursuit, on occult influences supposed to be exercised by the celestial bodies. It was unlikely, therefore, that they would willingly reject the ancient theory which ascribed dew to lunar and stellar radiations.

But at length Baptista Porta adduced evidence which justified him in denying positively that the moon or stars exercise any influence on the formation of dew. He discovered that dew is sometimes deposited on the inside of glass panes; and again, that a bell-glass placed over a plant in cold weather is more copiously covered with dew within than without; nay, he observed that even some opaque substances show dew on their under surface when none appears on the upper. Yet, singularly enough, Baptista Porta rejected that part of Aristotle’s theory which was alone correct. He thought his observations justified him in looking on dew as condensed—not from vapour, as Aristotle thought—but from the air itself.

But now a new theory of dew began to be supported. We have seen that not only the believers in stellar influence, but Aristotle also, looked on dew as falling from above. Porta’s experiments were opposed to this view. It seemed rather as if dew rose from the earth. Observation also showed that the amount of dew obtained at different heights from the ground diminishes with the height. Hence, the new theorists looked upon dew as an exhalation from the ground and from plants—a fine steam, as it were, rising upwards, and settling principally on the under surfaces of objects.

But this view, like the others, was destined to be overthrown. Muschenbroek, when engaged in a series of observations intended to establish the new view, made a discovery which has a very important bearing on the theory of dew: he found that, instead of being deposited with tolerable uniformity upon different substances,—as falling rain is, for instance, and as the rising rain imagined by the new361 theorists ought to be,—dew forms very much more freely on some substances than on others.

Here was a difficulty which long perplexed physicists. It appeared that dew neither fell from the sky nor arose from the earth. The object itself on which the dew was formed seemed to play an important part in determining the amount of deposition.

At length it was suggested that Aristotle’s long-neglected explanation might, with a slight change, account for the observed phenomena. The formation of dew was now looked upon as a discharge of vapour from the air, this discharge not taking place necessarily upwards or downwards, but always from the air next to the object. But it was easy to test this view. It was understood that the coldness of the object, as compared with the air, was a necessary element in the phenomenon. It followed, that if a cold object is suddenly brought into warm air, there ought to be a deposition of moisture upon the object. This was found to be the case. Any one can readily repeat the experiment. If a decanter of ice-cold water is brought into a warm room, in which the air is not dry—a crowded room, for example—the deposition of moisture is immediately detected by the clouding of the glass. But there is, in fact, a much simpler experiment. When we breathe, the moisture in the breath generally continues in the form of vapour. But if we breathe upon a window-pane, the vapour is immediately condensed, because the glass is considerably colder than the exhaled air.

But although this is the correct view, and though physicists had made a noteworthy advance in getting rid of erroneous notions, yet a theory of dew still remained to be formed; for it was not yet shown how the cold, which causes the deposition of dew, is itself occasioned. The remarkable effects of a clear sky and serene weather in encouraging the formation of dew, were also still unaccounted for. On the explanation of these and similar points, the chief interest of the subject depends. Science owes the elucidation of these difficulties to Dr. Wells, a London physician, who studied362 the subject of dew in the commencement of the present century. His observations were made in a garden three miles from Blackfriars Bridge.

Wells exposed little bundles of wool, weighing, when dry, ten grains each, and determined by their increase in weight the amount of mois............