In order to be certain of the true state of things, glass must not be employed; the supports must be made of polished tin, which hardly radiates at all, and which sufficiently isolates the thermometers from the soil; moreover, all the parts of the thermometers must be covered with metal. Then, the metal being polished, the thermometers will give the true temperature of the air; and when the casing is varnished, blackened, covered with leaves or any other substance, we obtain by a simple comparison with the polished thermometer, the degree of cold produced by the radiation of this substance.

By means of such instruments as these, I have found that leaves of plants, glass, varnish and lampblack, always cool on calm fine nights, from one to two degrees below the surrounding air. On looking at the smallness of these differences, one might be led to suppose that the fall of temperature observed by Wilson and Wells, and which amounted to seven or eight degrees, was much exaggerated. But when we remember that in their experiments the thermometers for showing the temperature of the air were raised four or five feet above the earth, while those covered with the radiating substance were close to the soil, we can easily see why their results and my own differ so much. For Pictet has long ago shown that the temperature of the air decreases rapidly, on calm fine nights, as we approach the earth. This fact alone would render the temperature of the radiating substance, placed close to the surface of the earth, lower than that of the air in which the higher thermometers were placed; so that, in this arrangement of the instruments, the difference between the two thermometers by no means indicates the amount of cooling of the body below the surrounding air.

In another of Wells' experiments there was a thermometer covered with wool placed at the same level as a free thermometer, and the difference of temperature observed was 5°,3. Here the wool certainly cooled two or three times as much as the lampblack in my experiments; and I know that the radiating power of wool is not greater than that of lampblack.

To explain the cause of this extraordinary cold observed by Wells, we must first clear up any doubt that may be attached to it. It was for this purpose that I covered a thermometer with wool, and exposed it to the air with two others of the same size, one of which was coated with lampblack and the other with polished metal; in a few minutes the thermometer with the wool fell twice as low as that coated with lampblack. A fourth thermometer covered with an equal quantity of wool, pressed close to the instrument by means of a metallic wire, gave a result intermediate between the other two. Lastly, I covered a fifth thermometer with two pieces of flannel, and it fell still less than the last. These experiments were repeated, cotton being used instead of wool, and the results were perfectly similar. I then began to suspect that the superiority of the cotton and wool over the lampblack, was owing to a certain modification in the radiating power of these bodies, caused by the presence of the air filling their interstices.

But how can air increase the cold resulting from radiation? The answer is simple. We have known for many years, that the nocturnal cooling of a body does not vary with the temperature of the atmosphere. Thus Capts. Parry and Scoresby state that on calm fine nights in the polar regions, the snow was cooled about 9° below the air four or five feet above it when the temperature of the latter was 0° - or 25° - or 30°. M. Pouillet has found that Swan's down is cooled 7° below the air at 0o or - 25°. And I for my part, have found that the blackened or varnished thermometers cool a certain fixed number of degrees, whatever the temperature of the night may be. Now it is clear that the tufts of cotton or of wool spread out on the upper part of the bulb of a thermometer, after having cooled by radiation, will communicate the cold so acquired to the surrounding air, which becomes by this means heavier, will descend in the interior to fall on the ground; but a certain time is required for the passage of this air through the interstices of the wool or cotton.

The threads then, of these last, will be in contact with air that is colder than it was at the beginning of the experiment; and as the fall in this temperature below the surrounding medium is invariable, they must necessarily become colder still. This increase of cold will cause a new fall of temperature in the medium; the latter gives rise to another cooling in the radiating body; and so on until the weight acquired by the condensed air is sufficent to overcome the obstacles opposing its exit.

The same phenomena take place naturally in many circumstances. Indeed, plants with hairy leaves are colder than those with smooth ones. The temperature of grass and that of other low plants which clothe the fields, falls, in consequence of this reaction of the air, much below that of elevated bodies, because of their vicinity to the soil which supports the surrounding medium, and compels it to remain in contact with the radiating surfaces. The truth is, that the layer of air by which the grass is surrounded, is not steady; it changes its position, on the contrary, in precisely the same way as water in a vessel over the fire; the particles of air condensed by the cold on the tops of the blades of grass, descend towards the earth, become heated by contact with the latter, and rise again towards the tops of the leaves, and so on; but it is clear that, in spite of this state of motion, the air on the whole cools, and in order that the grass may be of the same constant temperature below that of the surrounding medium, it must cool more still; and thus a gradual cooling and an increasing moisture in the layer of air are caused.

I cannot enter here into all the necessary details to show how the frigorific reaction of the air explains ail the facts' preceding and accompanying the appearance of dew, and many other phenomena connected with this interesting question, which have not as yet been satisfactorily accounted for. They will ail be found, however, in my memoir, which I shall soon, I hope, have the honor to present to the academy.