252. Formation of Dew. These remarks lead naturally to the subject of Dew. For the true theory of this phenomenon we are indebted to Dr. Wells, a London physician, who published an account of his experiments in 1818. By these he ascertained the following laws :

I. Dew is most copiously deposited under a clear sky; 2. And with a calm state of the atmosphere.

3. It is most copiously deposited on those substances which have a clear view of the sky;

4. And which are good radiators (thus a gilded thermometer is less liable to be covered with dew than one ungilt);

5. And which are placed close to the earth.

6. The deposition of dew is always accompanied with a lowering of temperature; and at those places where dew falls most copiously the temperature sinks lowest.

Dr. Wells soon saw that the deposition of dew was owing to radiation. A solid substance which is a powerful radiator and is exposed to the clear sky gives off a great deal of its heat by uncompensated radiation into space, and is thus reduced below the temperature of the surrounding air. The particles of air in contact with the substance share in this reduction until they reach a temperature at which they can no longer retain their aqueous vapour, but must deposit it on the solid body.

We thus see why dew is most copious under a clear sky; and if we reflect that the solid body is colder than the surrounding air, and can only obtain dew by cooling the particles of air in contact with it, we see the necessity for calmness of atmosphere. We see also why the substance must have a clear view of the sky.

Since the deposition of dew is the effect of cooling, we see why it is always accompanied with a fall of temperature; and the hypothesis that the lowering of temperature is caused by radiation is also quite in accordance with those experiments

in which Dr. Wells found that dew was most copiously deposited on good radiators.

It is not at first sight so obvious why a substance suspended a little above the earth should have less dew than one at the earth's surface; but Dr. Wells' explanation is no doubt the correct one; he remarks that, in the case of a substance above the earth, when the air in contact with it becomes cooled it also becomes heavier and sinks down, its place being supplied by warmer air. The cooling in this case, just as in that in which there is a horizontal movement of the air, is not sufficiently intense to produce dew.

253. Artificial formation of ice. Dr. Wells by means of this theory was able also to explain the formation of ice in Calcutta, where shallow pans containing water are during night exposed to the clear sky, and are often in the morning covered with a layer of ice.

Professor Tyndall has justly remarked that the formation of ice under these circumstances, while it is due to radiation, demands nevertheless an absence of aqueous vapour from the air; and he quotes in favour of this view the remark of Sir Robert Baker, who, speaking of the formation of ice in Bengal, says that the nights most favourable for its production are those which are clearest and most serene, and in which very little dew appears after midnight.

CHAPTER VI.

Radiant Heat.-Phosphorescence and Fluorescence. 254. Phosphorescence. In the preceding chapters it has been attempted to shew that the radiation of a heated body is, both in quantity and quality, dependent upon the

temperature of the body, and on it alone; and this law may be regarded as a very accurate expression of a great number of facts. Accordingly, if a thermometer were raised to 100° Fahr., either by exposure to the direct rays of the sun, or by immersion in hot water, we should expect to find its radiation precisely the same in both cases, and we should be right in our expectation.

Nevertheless, there are certain substances in which the nature of the radiation depends not altogether on their present temperature, but to some extent upon the kind of radiant heat to which they have recently been exposed.

Such bodies are said to be phosphorescent. It has been shewn by E. Becquerel that the property of becoming phosphorescent belongs to a great number of substances. Thus, if we take a tube containing powdered sulphide of calcium, or sulphide of strontium, and expose it to sun light or the electric light, if viewed afterwards in the dark it will remain luminous for several hours. Evidently this luminosity is not due to the temperature of the powder; and since this appearance takes place whether the powder be in vacuo or in air, it cannot be due to chemical action, and is probably rather due to a modification in the molecular state of the body caused by the action of light.

The same phenomenon may be observed in many diamonds, in fluor-spar, arragonite, chalk, heavy spar, and a number of other minerals; also among organic substances, in dry paper, silk, cane-sugar, &c.

255. In many instances this exhibition of light lasts for a considerable time after the exposure of the substance to the exciting source; but in some cases it disappears in a few seconds, or even in a very small fraction of a second. In order to investigate the duration of this effect Becquerel has invented an instrument called a phosphoroscope. In this instrument two disks are placed alongside of one an→

other, having the same axle, and the substance to be tried for phosphorescence is placed in a fixed support between these disks; there are apertures in these disks, but the aperture in the one does not correspond to that in the other, so that it is impossible to see through the disks. Suppose now that on the other side of that disk which is further from the eye there is a source of light, such as the sun or the electric light, and that the disks by means of a suitable train of wheels are made to revolve with great rapidity; then at the time when the aperture of the disk further away exposes the substance in the support to the rays of light it will be hidden from the eye of the observer by the nearer disk, but a moment afterwards the opening of this disk will reveal it, and if phosphorescent it will then appear luminous. It is clear that by this arrangement the length of time elapsing between the exposure of the substance to the source of light and afterwards to the eye of the observer will depend upon the rapidity of rotation, and may therefore be made as small as possible. The observations must be made in a dark chamber. Becquerel found by this means that compounds of uranium are luminous only if viewed .003 or .004 of a second after exposure.

256. Fluorescence. Sir David Brewster was the first to remark that when the sun's light is condensed by a lens and admitted into certain solids or fluids, there appears to be an internal dispersion of the rays. Some time afterwards, in 1845, Sir J. Herschel began to study a very curious phenomenon connected with sulphate of quinine-which is in reality a colourless liquid—namely, that under certain aspects a solution of this substance exhibits a beautiful blue colour. This may be readily verified by viewing by daylight a solution of this substance placed in an ordinary test tube. Sir J. Herschel shewed that the incident light, after passing through a small thickness of the fluid, although not

sensibly enfeebled or coloured, had lost its power of producing this effect. In a solution of quinine therefore there is a copious dispersion of light, which takes place near the surface, while there is also a feeble dispersion for a long distance within the fluid; and Sir D. Brewster was led to the belief that the dispersion produced by sulphate of quinine was only a particular case of internal dispersion.

257. Professor Stokes has since explained this phenomenon with great success. The circumstance to which he directed his attention was the fact that a very thin stratum of the fluid is sufficient to deprive the light of the power of again producing the same effect. The rays producing dispersion, he was led to see, are very quickly absorbed; but he remarked that the dispersed rays themselves are able to travel many inches of the fluid with great freedom. The rays producing dispersion are therefore of a different nature from the dispersed rays.

Professor Stokes was thus led to recognize a change in the refrangibility of the rays when they become dispersed. If it further be supposed that the rays causing dispersion are the invisible rays beyond the violet, this will account for the circumstance that the visible appearance of the light is not changed when it is deprived of its power of producing this phenomenon; and it will also account for the fact that the blue appearance can hardly be seen by candle light, which is deficient in chemical rays beyond the violet.

258. In order to prove this change of refrangibility Professor Stokes instituted a very extensive series of experiments, from which he deduced the following results:

1. In the phenomenon of true internal dispersion the refrangibility of light is changed.

2. The refrangibility of the incident light is greater than that of the dispersed light to which it gives rise.