Part 19 (1/2)
[Sidenote: LONG WAVES MOST ABSORBED.]
It has already been stated that a layer of water less than the twentieth of an inch in thickness suffices to stop and destroy all waves of radiant heat emanating from an obscure source. The longer waves of the obscure heat cannot get through water, and I find that all transparent compounds which contain _hydrogen_ are peculiarly hostile to the longer undulations. It is, I think, the presence of this element in the humours of the eye which prevents the extra red rays of the solar spectrum from reaching the retina. It is interesting to observe that while bisulphide of carbon, chloride of phosphorus, and other liquids which contain no hydrogen, permit a large portion of the rays emanating from an iron or copper ball, at a heat below redness, to pa.s.s through them with facility, the same thickness of substances equally transparent, but which contain hydrogen, such as ether, alcohol, water, or the vitreous humour of the eye of an ox, completely intercepts these obscure rays. The same is true of solid bodies; a very slight thickness of those which contain hydrogen offers an impa.s.sable barrier to all rays emanating from a non-luminous source.[A] But the heat thus intercepted is by no means lost; its _radiant form_ merely is destroyed. Its waves are s.h.i.+vered upon the particles of the body, but they impart warmth to it, while the heat which retains its radiant form contributes in no way to the warmth of the body through which it pa.s.ses.
[Sidenote: FINAL COLOUR OF ICE AND WATER BLUE.]
Water then absorbs all the extra red rays of the sun, and if the layer be thick enough it invades the red rays themselves. Thus the greater the distance the solar beams travel through pure water the more are they deprived of those components which lie at the red end of the spectrum.
The consequence is, that the light finally transmitted by the water, and which gives to it its colour, is _blue_.
[Sidenote: EXPERIMENT.]
I find the following mode of examining the colour of water both satisfactory and convenient:--A tin tube, fifteen feet long and three inches in diameter, has its two ends stopped securely by pieces of colourless plate gla.s.s. It is placed in a horizontal position, and pure water is poured into it through a small lateral pipe, until the liquid reaches half way up the gla.s.ses at the ends; the tube then holds a semi-cylinder of water and a semi-cylinder of air. A white plate, or a sheet of white paper, well illuminated, is then placed at a little distance from one end of the tube, and is looked at through the tube.
Two semicircular s.p.a.ces are then seen, one by the light which has pa.s.sed through the air, the other by the light which has pa.s.sed through the water; and their proximity furnishes a means of comparison, which is absolutely necessary in experiments of this kind. It is always found that, while the former semicircle remains white, the latter one is vividly coloured.[B]
When the beam from an electric lamp is sent through this tube, and a convex lens is placed at a suitable distance from its most distant end, a magnified image of the coloured and uncoloured semicircles may be projected upon a screen. Tested thus, I have sometimes found, after rain, the ordinary pipe-water of the Royal Inst.i.tution quite opaque; while, under other circ.u.mstances, I have found the water of a clear green. The pump-water of the Inst.i.tution thus examined exhibits a rich sherry colour, while distilled water is blue-green.
The blueness of the Grotto of Capri is due to the fact that the light which enters it has previously traversed a great depth of clear water.
According to Bunsen's account, the _laugs_, or cisterns of hot water, in Iceland must be extremely beautiful. The water contains silica in solution, which, as the walls of the cistern arose, was deposited upon them in fantastic incrustations. These, though white, when looked at through the water appear of a lovely blue, which deepens in tint as the vision plunges deeper into the liquid.
[Sidenote: ICE OPAQUE TO RADIANT HEAT.]
Ice is a crystal formed from this blue liquid, the colour of which it retains. Ice is the most opaque of transparent solids to radiant heat, as water is the most opaque of liquids. According to Melloni, a plate of ice one twenty-fifth of an inch thick, which permits the rays of light to pa.s.s without sensible absorption, cuts off 94 per cent. of the rays of heat issuing from a powerful oil lamp, 99-1/2 per cent. of the rays issuing from incandescent platinum, and the whole of the rays issuing from an obscure source. The above numbers indicate how large a portion of the rays emitted by our artificial sources of light is obscure.
When the rays of light pa.s.s through a sufficient thickness of ice the longer waves are, as in the case of water, more and more absorbed, and the final colour of the substance is therefore blue. But when the ice is filled with minute air-bubbles, though we should loosely call it _white_, it may exhibit, even in small pieces, a delicate blue tint.
This, I think, is due to the frequent interior reflection which takes place at the surfaces of the air-cells; so that the light which reaches the eye from the interior may, in consequence of its having been reflected hither and thither, really have pa.s.sed through a considerable thickness of ice. The same remark, as we have already seen, applies to the delicate colour of newly fallen snow.
FOOTNOTES:
[A] What is here stated regarding hydrogen is true of all the liquids and solids which have hitherto been examined,--but whether any exceptions occur, future experience must determine. It is only when in combination that it exhibits this impermeability to the obscure rays.
[B] In my own experiments I have never yet been able to obtain a pure blue, the nearest approach to it being a blue-green.
COLOURS OF THE SKY.
(7.)
[Sidenote: NEWTON'S HYPOTHESIS.]
In treating of the Colours of Thin Plates we found that a certain thickness was necessary to produce blue, while a greater thickness was necessary for red. With that wonderful power of generalization which belonged to him, Newton thus applies this apparently remote fact to the blue of the sky:--”The blue of the first order, though very faint and little, may possibly be the colour of some substances, and particularly the azure colour of the skies seems to be of this order. For all vapours, when they begin to condense and coalesce into small parcels, become first of that bigness whereby such an azure is reflected, before they can const.i.tute clouds of other colours. And so, this being the first colour which vapours begin to reflect, it ought to be the colour of the finest and most transparent skies, in which vapours are not arrived at that grossness requisite to reflect other colours, as we find it is by experience.”
M. Clausius has written a most interesting paper, which he endeavours to show that the minute particles of water which are supposed by Newton to reflect the light, cannot be little globes entirely composed of water, but bladders or hollow spheres; the vapour must be in what is generally termed the _vesicular_ state. He was followed by M. Brucke, whose experiments prove that the suspended particles may be so small that the reasoning of M. Clausius may not apply to them.
But why need we a.s.sume the existence of such particles at all?--why not a.s.sume that the colour of the air is blue, and renders the light of the sun blue, after the fas.h.i.+on of a blue gla.s.s or a solution of the sulphate of copper? I have already referred to the great variation which the colour of the firmament undergoes in the Alps, and have remarked that this seems to indicate that the blue depends upon some variable const.i.tuent of the atmosphere. Further, we find that the blue light of the sky is _reflected_ light; and there must be something in the atmosphere capable of producing this reflection; but this thing, whatever it is, produces another effect which the blue gla.s.s or liquid is unable to produce. These _transmit_ blue light, whereas, when the solar beams have traversed a great length of air, as in the morning or the evening, they are yellow, or orange, or even blood-red, according to the state of the atmosphere:--the transmitted light and the reflected light of the atmosphere are then totally different in colour.
[Sidenote: GOETHE'S HYPOTHESIS.]