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in all cases, the quantity of air produced bears a certain general proportion to the capacity of the vessel in which the process is made.” Again, “ I have found a slower and a less produce of air from rain water than from pump water; owing, I suppose, to the rain water containing less air to operate upon, and generally also in a purer state, than that which is contained in pump water.” We now know that the latter contains more carbonic acid than the former.

(369.) These experiments were continued by Priestley with cabbage leaves, lettuce, the spurge, cucumber, potatoes, white lilies, and many other kinds of plants: in all of them proving the decomposition of fixed air (carbonic acid) by the living vegetable matter in the water under the influence of Light. We find philosophers, both at home and abroad, repeating Dr. Priestley's experiments, and gradually arriving at a correct interpretation of the observed pheno

Cavendish, in his experiments on air, wanders round the truth, but is continually drawn away from it by the hypothesis of Phlogiston. Sennebier found that plants yielded more dephlogisticated air (oxygen) in distilled water impregnated with fixed air than in plain distilled water. On this, Cavendish says, “For as fixed air is a principal constituent part of vegetable substances, it is reasonable to suppose that the work of vegetation will go on better in water containing this substance than in other water.”

M. Monge, in his memoir “Sur le Résultat de l'Inflammation du Gaz Inflammable et de l'Air Déphlogistique dans des Vaisseaux Clos,” also examines this question. About this time the complete explanation afforded by Lavoisier's annihilation of the Phlogistic Hypothesis, led to correct explanations of the facts; and we advance more steady in our inquiries.

(370.) My investigations on this subject were first published in the Philosophical Magazine for April, 1840, and they have, since that time, been continued with little interruption. The progress of the inquiry was recorded in

COLOURED MEDIA EMPLOYED.

211

the Reports of the Association for several years. It is necessary for a correct understanding of the results obtained, that all the conditions under which the experiments have been made should be distinctly stated.

(371.) Six boxes were so prepared, that air was freely admitted to the plants within them, without permitting the passage

of

any of the solar rays, except those which passed through the coloured media with which they were covered.

These media permitted the permeation of the rays of Light in the following order :

1. A Ruby Glass, coloured with Oxide of Gold. This glass permits the permeation of the ordinary red, and the extreme red rays only.

. 2. A BROWN-RED GLASS. — The extreme red ray appeared shortened; the ordinary red ray and the orange ray passed freely, above which the spectrum was sharply cut off.

3. ORANGE GLASS.— The spectrum was shortened by the cutting off of the violet, indigo, and a considerable portion of the blue rays. The green ray was nearly absorbed in the yellow, which was considerably elongated. The whole of the least refrangible portion of the spectrum permeated this glass freely.

4. YELLOW Glass, somewhat Opalescent. — This glass shortened the spectrum by cutting off the extreme red ray, and the whole of the most refrangible rays beyond the

. 5. COBALT BLUE GLASS.— The spectrum obtained under this glass was perfect from the extreme limits of the most refrangible rays down to the yellow, which was wanting. The green ray was diminished, forming merely a welldefined line between the blue and the yellow rays. The orange and red rays were partially interrupted.

6. DEEP-GREEN GLASS. The spectrum is cut off below the orange and above the blue rays. Although the space on which the most luminous portion of the spectrum falls, appeared as large as when it was not subjected to the ab

blue ray

sorptive influence of the glass, there was a great deficiency of Light, and on close examination with a powerful lens, a dark line was seen to occupy the space usually marked by the green ray:

(372.) A case was also prepared, containing five flat vessels filled with different-coloured fluids.

A. RED. Solution of Carmine in Supersulphate of Ammonia. - This gives a spectrum nearly in all respects similar to that given by the ruby glass (1.); all the rays above a line drawn through the centre of the space occupied by the orange being cut off.

B. YELLOW. A saturated Solution of Bichromate of Potash. — This beautifully transparent solution admits the permeation of the red and yellow rays, which are extended over the space occupied by the orange ray in the unabsorbed spectrum. The green rays are scarcely evident,

From the absorptive powers of the sulphurets of lime and potash in solution, I was very desirous of using them, but they were found to be so liable to decomposition when exposed to the sun's rays as to be quite useless for my purposes : sulphuretted hydrogen being liberated in such quantities as to burst the bottles with great violence.

C. GREEN. Muriate of Iron and Copper. - This medium is remarkably transparent; the blue, green, yellow, and orange rays pass freely, all the others being absorbed.

D. BLUE Cupro-sulphate of Ammonia. - This fluid obliterates all the rays below the green ray, those above it permeating it freely.

E. WHITE. — This is merely water rendered acid by nitric acid, for the purpose of securing its continued transparency. It should be noted that spaces in the boxes have been left open to the full influence of the Light, that a fair comparison might be made between those plants growing under ordinary circumstances, and the others under the dissevered rays.

(373.) It will be seen from the above, that the following combinations of rays have been obtained to operate with.

INSULATED RADIATIONS.

213

1. and A. The calorific rays well-insulated.

2. A smaller portion of these rays mixed with a small amount of those having peculiar illuminating powers.

3. The central portion of the solar spectrum well-defined, and all the rays of least refrangibility, thus combining the luminous and calorific rays.

4. The luminous rays mixed with a small portion of those having a calorific influence.

5. The most refrangible rays with a considerable portion of the least so; thus combining the two extremes of chemical action, and affording a good example of the influence of the calorific blended with the chemical spectrum.

6. Some portion of those rays having much illuminating power, with those in which the chemical influence is the weakest under ordinary circumstances.

B. The luminous rays in a tolerably unmixed state.

C. The luminous rays combined with the least actively chemical ones, as in 6.; but in this case the luminous rays exert their whole influence.

D. The most refrangible or chemical rays well-insulated.

E. White Light.

(374.). From these arrangements it will be evident, that, although we do not secure the complete isolation of the rays, as we should do with a prism, we obtain the great preponderance of one influence over others, which suffices to insure, to a certain extent, the decided action of that

I am well aware that we only arrive at approximations to the truth by the system adopted, but I am unacquainted with any method by which these experiments could be continued for any time, otherwise than with absorptive media.

(375.) When we look on a spectrum which has been subjected to the influence of some absorptive medium, we must not conclude, from the coloured rays which we see, that we have cut off all other influences than those

one.

which are supposed to belong to those particular colours. Although a blue glass or fluid may appear to absorb all the rays except the most refrangible ones, which have usually been considered as the least calorific of the solar rays; yet it is certain that some principle has permeated the glass or fluid, which has a very decided thermic influence, and so with regard to media of other colours.

The relative temperatures indicated by good thermometers placed behind the glasses and fluid cells, which I have used, will place this in a clear light. The following results present a fair average series, and distinctly mark the relative degrees in which these media are permeable by the heating rays:

GLASSES.
Colour.
Luminous rays not absorbed.

Temperature. 1. Ruby. Ordinary red, and the extreme red

. 87° 2. Red. Ordinary red, and orange, portion of extreme red 83° 3. Orange. Little blue, green, yellow, orange, red, and extreme red

- 104° 4. Yellow. Red, orange, green, and blue

88° 5. Blue. Violet, indigo, blue, little green, and some red 84° 6. Green. Orange, yellow, green, and blue

74° FLUIDS. A. Red. Ordinary and extreme red

78° B. Yellow. Ordinary red, and yellow

80° C. Green. Blue, green, yellow, orange

69° D. Blue. Green, blue, indigo, violet, and trace of red 73° E. White.

89°

All the rays

In these examinations the highest temperature was not obtained behind the red media, but behind those which have a yellow or orange tint.

Such were the arrangements adopted: these were sometimes slightly varied, but not to an important extent. The results obtained will be best understood by my report to the British Association in 1847, which I reprint with but little alteration.

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