Pleasant Ways in Science

by Richard A. Proctor

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) founded their hopes of success in the search for the philosopher’s stone, the elixir vitæ, 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 new 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 studied 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 moisture which had been deposited upon them. At first, he confined himself to comparing the amount of moisture collected on different nights. He found that although it was an invariable rule that cloudy nights were unfavourable to the deposition of dew, yet that on some of the very clearest and most serene nights, less dew was collected than on other occasions. Hence it became evident that mere clearness was not the only circumstance which favoured the deposition of dew. In making these experiments, he was struck by results which appeared to be anomalous. He soon found that these anomalies were caused by any obstructions which hid the heavens from his wool-packs: such obstructions hindered the deposition of dew. He tried a crucial experiment. Having placed a board on four props, he laid a piece of wool on the board, and another under it. During a clear night, he found that the difference in the amount of dew deposited on the two pieces of wool was remarkable: the upper one gained fourteen grains in weight, the lower one gained only four grains. He made a little roof over one piece of wool, with a sheet of pasteboard; and the increase of weight was reduced to two grains, while a piece of wool outside the roof gained no less than sixteen grains in weight.

Leaving these singular results unexplained for a while, Dr. Wells next proceeded to test the temperature near his wool-packs. He found that where dew is most copiously produced, there the temperature is lowest. Now, since it is quite clear that the deposition of dew was not the cause of the increased cold—for the condensation of vapour is a process producing heat—it became quite clear that the formation of dew is dependent on and proportional to the loss of heat.

And now Wells was approaching the solution of the problem he had set himself; for it followed from his observations, that such obstructions as the propped board and the pasteboard roof kept in the heat. It followed also, from the observed effects of clear skies, that clouds keep in the heat. Now, what sort of heat is that which is prevented from escaping by the interference of screens, whether material or vaporous? There are three processes by which heat is transmitted from one body to another,—these are, conduction, convection, and radiation. The first is the process by which objects in contact communicate their heat to each other, or by which the heat in one part of a body is gradually transmitted to another part. The second is the process by which heat is carried from one place to another by the absolute transmission of heated matter. The third is that process by which heat is spread out in all directions, in the same manner as light. A little consideration will show that the last process is that with which we are alone concerned; and this important result flows from Dr. Wells’ experiments, that the rate of the deposition of dew depends on the rate at which bodies part with their heat by radiation. If the process of radiation is checked, dew is less copiously deposited, and vice versâ.