suffer in a greater or less degree when exposed to strong light; the retina from its transparency allowing it to pass through without injury."

The black or dark matter of the "retina" is technically called the "nigrum pigmentum," and is probably not absolutely necessary for vision, but only provided as a defence against strong light, since it is found to be much darker in the Ethiopian than in the European, and is of a lighter colour in fair people than in dark, and therefore lightest in those countries farthest removed from the effects of the sun.

The scorching power of the solar rays being destroyed when received by black surfaces is an incontrovertible fact, and the only explanation that the chemist can offer regarding it is that the mixture of heat and light in the solar rays is absorbed by black surfaces, and converted into sensible heat.

the sun

The sudden and fatal infliction called stroke" that frequently seizes reapers and labourers, in the intense heat of harvest time, is referred to the scorching effect of the solar rays, upon the scalp of the head, producing inflammation of the brain, and probably the first record of this is in the following Scripture passage.

"And when the child was grown, it fell on a day that he went out to his father to the reapers. And he said unto his father, My head, my head! And he said to a lad, Carry him to his mother. And when he had taken him, and brought him to his mother, he sat on her knees till noon, and then died."

In extremely hot climates the traveller is often "sun-stricken," and dies; and in allusion to the watchful care of Providence over His chosen people, it is said, "They shall not hunger nor thirst; neither shall the heat, nor sun smite them for He that hath mercy on them, shall lead them, even by the springs of water shall he guide them."

The difference between the heating of dark and light-coloured soils, when equally exposed to solar heat, and their difference in cooling when equally placed in the shade, as mentioned at page 104, may be now stated in technical terms, viz., that dark soils absorb heat more powerfully, and radiate heat more energetically than light-coloured soils, which reflect a great portion of heat; we have now to examine how these facts, and others concerning the mere alteration of surfaces in affecting both absorption and radiation of heat, are applicable to the explanation of a beautiful phenomenon of Summer and Autumn.

The night has been serene, the moon and stars have shed their brilliant light, no clouds have obscured the heavens, no rain has fallen, and yet when we step forth at daybreak, we find the grass and the flowers of the field loaded with myriads of drops of water, sparkling like gems in the golden rays of the rising

sun.

We recognize this beautiful phenomenon as Dew ;but from whence has it silently journeyed and arrived during the hours of night?-can the chemist reply?

He can; and the reply will furnish another example of the power and goodness of God, for "His favour is as dew upon the grass;" another proof of the everwatchful care of Him with Whom "the darkness and the light are both alike," Whose Hand is equally extended for the protection of the animated creation during its noontide activity and its midnight repose.

Throughout the fervent glow of a Summer or Autumnal day, the solid opaque earth absorbs heat; this abides upon its mere surface, and is not conducted beneath; but at sunset, if the sky be cloudless and calm, the earth immediately radiates part of the heat upward, and soon becomes many degrees colder than the air directly incumbent upon its surface; accordingly the watery vapour that is present in the yet warm air, is chilled or condensed into drops of water, and these" distil as the dew" upon the earth, for the refreshment of its productions.

This phenomenon cannot fail of appearing remarkable, even to the most careless observer, and it becomes yet more so, when accurately investigated by the chemist. Examine a garden immediately after sunrise at this season; probably the grass-plat is saturated with dew; the gravel walk is nearly dry; the leaves of the hollyhock are dripping with water; the leaves of the laurel are free from moisture; but all these objects were similarly exposed to the night air, and if dew were a fine rain, as some persons imagine it to be, all should be equally covered with its drops; why is this difference observed?

Because the surfaces of the various objects differ in their radiating power; the grass-plat and the leaves of the hollyhock are excellent radiators; they throw off heat with great energy, and so becoming cold, they induce a more copious deposition of water from the air than the gravel-walk and the laurel leaves, which, being bad radiators, retain heat, and remain so warm that the watery vapour in the air wafts over their surfaces without being chilled or condensed, and therefore they are free from dew.

Rough or woolly leaves, like the painted or sanded surface of the tin-plate, radiate heat most rapidly, whilst smooth or varnished leaves, like the polished or bright surface of the tin-plate, do not radiate with such energy, and as a consequence of this, the former leaves, ensure a more plentiful deposition of dew than the latter.

From the limits of the garden, let us carry forth these observations and facts into the boundless fields of Nature, and discover the miraculous workings of Providence.

Barren rocks and soils, by reason of their peculiar hard and compact structure, have neither the power of absorbing nor of radiating heat with great energy, they do not speedily acquire a low temperature during the clear nights of Summer and Autumn, and as a consequence, dew is scarcely, certainly not abundantly, deposited upon them; it is not required for their support, and they have no vegetable life, or but little of the lowest grade to maintain.

Like every treasure from the bountiful Hand of God, the precious "dew of Heaven" is not lavished and scattered upon objects that are unfitted for its reception, and therefore can derive no benefit from its genial influence, but only abundantly upon such as are properly prepared, and accordingly these obtain wholesome nurture, and bring forth good fruit, whilst the former remain as barren stones.

Naturally fertile soils, and such as are artificially rendered productive, are generally of a loose porous structure; this physical peculiarity enables them to absorb heat abundantly during the day, and to radiate it so powerfully at night, or, in other words, to become so many degrees colder than the ambient air, that its watery vapour condenses upon them and their productions as dew in abundance.

It is indeed a subject well calculated to arrest our attention, and excite our admiration and gratitude, when we thus find every soil, plant, shrub, and tree, has, by its own physical peculiarity of structure, induced the deposition of dew in proportion to its wants; and when we reflect that not a single dewdrop is created in vain, but each is measured by The Hand of Infinite Wisdom, and appointed to fulfil some useful end.

Some tracts of land, when clothed with vegetation, radiate heat with such vast energy that their low temperature not only causes the plentiful deposition of dew, but subsequently changes it into frost; but this extreme of cold, if continued, would be equally fatal