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Southern Counties Adult Education Society. (K 89) 8vo. 1862.
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Athenæum for February, 1863. 4to.
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Mechanics' Magazine for February, 1863. 8vo.
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Friday, March 6, 1863.

SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. Vice-President,

in the Chair.

WILLIAM ALLEN MILLER, M.D. Treasurer R.S. On the Photographic Transparency of Bodies, and on the Photo

graphic Spectra of the Elementary Bodies.

AFTER a few preliminary remarks upon the triple nature of the force, calorific, luminous, and chemical, associated together in the radiation which emanates from luminous sources, the speaker stated his intention of limiting himself in great measure to some recent investigations upon the chemical rays. *

It is well known that bodies which are transparent to light are not equally so to radiant heat. Glass, for example, which to the eye is perfectly transparent and limpid, arrests a large portion of the rays of heat emitted by bodies which are not sufficiently hot to become luminous. Pure rock-salt, on the other hand, transmits rays of both light and heat from all sources freely. In like manner, in the case of rays which produce chemical action, corresponding effects have been observed ; glass absorbing many of the chemical rays, whilst quartz transmits such rays freely.

The chemical rays emitted by luminous objects vary greatly both in quantity and in quality, some sources of light emitting rays of much higher refrangibility than others. Thus, the flame of ordinary coalgas burned in admixture with air, so as to produce the blue light of a smokeless gas flame, gives out scarcely any rays capable of affecting an ordinary photographic plate ; whilst the same amount of gas, burned

* The expressions here employed are simply used as descriptive of the effects ordinarily produced by the different portions of the spectrum, not as necessarily implying that the rays which produce the effects of heat, light, and chemical action respectively are essentially different, except in the number and frequency of the vibrations by which they are produced, the most refrangible rays being produced by the shortest and most frequent vibrations.

in the ordinary manner for illumination, emits a very decided though limited amount of rays capable of producing chemical action. The rays emanating from the intensely hot jet of the oxyhydrogen flame, are nearly without action upon a sensitive surface of collodion ; whilst if thrown upon a ball of lime, the light then emitted contains as large a proportion of chemical rays as the solar light, and of very nearly the same refrangibility. But the most remarkable source of the chemical rays is afforded by the light of the electric spark or of the voltaic arc, the chemical spectrum of which is three or four times as long as the chemical spectrum obtainable from the sun itself.

1. Photographic Transparency of various Media.- Amongst the methods of testing the extent of chemical action of any given radiant source, the most convenient is that which is dependent upon the extent of photographic effect exerted upon a surface of collodion coated with iodide of silver, on which the spectrum is allowed to fall.

In no case does it appear that any non-luminous source can emit chemical rays of sufficient intensity to traverse ordinary refracting media ; and amongst the rays given off by various luminous objects, it is found that the chemical effects upon the collodion plate are not perceptible in those portions upon which the first three-fourths of the visible spectrum has fallen, but they commence powerfully in the last fourth; and in the case of the electric spark are prolonged to an extent equal to between four and five times the length of the visible portion.

A diagram exhibiting the relative lengths of the visible solar spectrum, and the photographic spectrum obtained from the electric spark between silver points, showed this fact in a striking manner.

It was known to those who have studied the spectrum, that many @lourless substances besides glass exert an absorptive action upon some of these chemical rays; but the subject had not hitherto received that careful experimental examination which its importance seems to warrant. Exact knowledge upon these points became requisite in the course of an investigation upon the photographic spectra of the metals in which the speaker was engaged.

In the prosecution of these inquiries it was a desideratum to procure some substance which should possess a higher dispersive power than quartz, and which, whilst avoiding the double refraction of quartz, should yet allow the free passage of the chemical rays. The speaker was hence led to try a variety of substances which, owing to their transparency to light, might reasonably be hoped to possess chemical transparency also.

The inquiry soon extended itself beyond the limits originally proposed, and ultimately embraced a large number of bodies in the solid, liquid, and gaseous conditions.

Before, however, proceeding to detail the results obtained, the speaker alluded to the discovery of Professor Stokes, that many bodies exist which, when placed in the invisible extra-violet, or more refrangible portion of the spectrum, exhibit the remarkable power of absorbing

the rays of this portion, and radiating them forth again in a visible form. Bodies which possess this power Mr. Stokes termed fluorescent. Now the chemical rays are exactly those which occasion fluorescence most powerfully. The light thus rendered visible is, however, very feeble, compared with that of the ordinary luminous portions; yet it may be rendered distinctly visible to those who are sufficiently near by allowing this chemically active portion of the spectrum to fall upon a screen consisting of some fluorescent substance, such as one of the salts of uranium, or a solution of æsculin, which latter was the material employed on the present occasion.

Returning then to the course of his own experiments, the speaker stated that although he had found in rock-salt, fluor-spar, water, and some few other substances, compounds which were almost as diactinic, or chemically transparent as quartz, he had not succeeded in finding anything which could be advantageously substituted for quartz in the preparation of the prisms and lenses required in the investigations in which he was engaged.

Among the most remarkable results upon the photographic transparency of bodies which had been observed by the speaker in his researches, were the following:

1. Colourless solids which are equally transparent to the visible rays, vary greatly in permeability to the chemical rays.

2. Bodies which are photographically transparent in the solid form, preserve their transparency in the liquid and in the gaseous states.

3. Colourless transparent solids which absorb the photographic rays, preserve their absorptive action with greater or less intensity both in the liquid and in the gaseous states.

4. Pure water is photographically transparent, so that matte compounds which cannot be obtained in the solid form sufficiently transparent for experiments, may be subjected to trial in solution in water.

The mode in which the experiments were conducted was the following:

The source of light employed was the electric spark obtained between two metallic wires, generally of fine silver, connected with the terminals of the secondary wires of an induction coil, into the primary circuit of which was introduced a condenser, and into the secondary circuit a small Leyden jar. The light of the sparks was then allowed to fall upon a vertical slit, either before or after traversing a slice or stratum of the material of which the electric transparency was to be tested; the transmitted light was then passed through a quartz prism, placed at the angle of minimum deviation. Immediately behind this was a lens of rock crystal, and behind this at a suitable distance the spectrum was received upon the sensitive surface of collodion Liquids were contained in a small glass cell with quartz faces, and gases and vapours in long tubes closed at their extremities with thin


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plates of polished quartz. The following tables exhibiting the relative diactinic power of various solids, liquids, and gases and vapours were then commented on.


Rock Crystal
74 Water
74 Oxygen

74 Ice .. 74 Alcohol 63 Nitrogen

74 Fluor Spar 74 Chloroforin 26 Hydrogen

74 Topaz 65 Benzol.

21 Carbonic Acid

. 74 Rock Salt 63 Wood Spirit

20 Olefiant Gas Iceland Spar 63 Ether

16 Marsh Gas Sulphate of Magnesia 62 Acetic Acid

16 Coal Gas.

37 Borax ...

62 Oil of Turpentine 8 Benzol Vapour 35 Diamond .

62 Bisulphide of Carbon. 6 Hydrochloric Acid · 55 Bromide of Potassium 48

Hydrobromic Acid . 23 Thin Glass . 20

Hydriodic Acid . . . 15 lodide of Potassium 18

Sulphurous Acid 14 Mica 18

Sulphuretted HydroNitrate of Potash. 16


14 The effects indicated in these tables were rendered visible to the audience, by illuminating the photographic negative of the electric sparks from silver points by the light of the electric lamp, and bringing the image to a distinct focus upon the screen by means of a condensing lens.

Each photograph was obtained under circumstances varying only in the nature of

the transparent medium through which the rays of the spark from the silver points were made to pass, before they were allowed to fall upon the collodion plate. When absorption occurs, it is almost always exhibited upon the most refrangible rays; but in the case of the coloured gases and vapours, chlorine, bromine, and iodine, the absorption differs from the general rule, and is by no means proportioned to the depth of colour. A column of chlorine with its yellowish green colour cuts off the rays of the less refrangible extremity through fully two-thirds of the spectrum ; the red vapour of bromine cuts off about one-sixth of the length of the spectrum, the absorbent action being limited to the less refrangible extremity, whilst the deep violetcoloured vapours of iodine allow the less refrangible rays to pass freely for the first fourth of the spectrum; then a considerable absorption occurs, and afterwards a feeble renewal of the photographic action is exhibited towards the more refrangible end.

Diactinic bases, when united with diactinic acids, usually furnished diactinic salts ; but such a result was not uniformly observed ; the silicates were none of them as transparent as silica itself in the form of rock crystal. Again, hydrogen is eminently diactinic, and iodine vapour, notwithstanding its deep violet colour, is also largely diactinic; but hydriodic acid gas is greatly inferior to either of them.

The same substance, however, whatever may be its physical form, whether solid, liquid, or gaseous, preserves its character; no chemically opaque solid, though transparent to light, becoming transparent pho

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