Again, tan MZ: tan MN:: rad.: cos ZMN, and considering M m n as a plane triangle, by the principles of plane trigonometry we have Mm: Mn:: rad.: cos ZMN; therefore, by equality, tan MZ: tan MN:: Mm: Mn. In the same manner, we have tan SZ: tan SN:: Ss: so. Now, Mn, and so, are the corrections; and it is evident from the figure, that when the angle M is acute, Mn must be subtracted from the apparent distance, but added when that angle is obtuse; also when the angle S, at the sun or star is acute, so must be added, but subtracted when it is obtuse. After the observation is made, a figure may be drawn by the hand, sufficiently exact for ascertaining the nature of the angles at M and S; and this is the only ambiguity which it is necessary to attend to in solving the problem by this method.

EXAMPLE. On the 7th of June, sea account, at 6 37′ P. M. in Longitude 120° W. by account, the observed distance of the moon's farthest limb from Aldebaran, was 39° 7′ 4′′, the observed altitude of the star 43° 18′, and the observed altitude of the moon's lower limb 52° 52′;-Required the true distance?

The apparent altitude of the star is found to be 43° 14', that of the moon 53° 04', and the apparent distance of the moon and star 38° 52', also the moon's horizontal parallax 54 35". Now, in the triangle MZS, we have MZ, the moon's co-altitude =36° 56, ZS 46° 46′, and MS 38° 52′, to find the necessary corrections. It is well known that M m is equal to the difference between the moon's parallax in altitude and the refraction, and that S s is the difference between the sun's parallax and refraction, where the sun and moon are observed; but S& is equal only to the refraction, when a star and the moon are observed, or 1925".

In this example, M m is found to be 32′ 05′′, or 1925", and Ss1', or 60".

ART. XIII.—On the Respiration of Plants. By W. H. GILBY, M. D., M. G. S. In a Letter to Professor JAMESON.

HAVING been much interested in that inquiry which my friend Mr Ellis has so ably and ingeniously conducted in his Treatise upon the chemical changes produced in the Atmosphere by the Respiration of Plants, and having myself executed many experiments upon the subject at the time of my graduation, I thought a short account of the nature of those experiments, and of the conclusions to which they gave rise, might not be unworthy your notice. I am the more anxious to give such a statement, because it frequently happens, that the pages of our journals are still filled with many frivolous observations, sometimes gravely announcing as a new discovery, that plants do really vitiate the air, and at other times repeating the old notions of Priestley, all tending to shew a complete oversight of Mr Ellis's truly philosophical work. In a work even written in a very acute and sensible manner by the Reverend Mr Keith, upon the Physiology of Vegetables, the reader is left very much in the dark with regard to the ultimate question of the real respiratory function of plants. He embodies, as far as I recollect, the opinions of the most sensible writers upon the matter, which are oftentimes in contradiction, but neglect to satisfy us, by drawing any reconciling conclusion. Several writers, in particular Sennebier, Ingenhousz, and Saussure, have shewn that the changes produced on the air are quite different, according as the plant is placed in the sunshine or in the

shade. In the shade, the air is vitiated by the disappearance of the oxygen, and the formation of carbonic acid; on the contrary, under insolation, the carbonic acid, if any be present, is rapidly decomposed, and the air is again improved by the restitution of the oxygen.

The rapidity with which this process takes place is truly astonishing, as the following experiment, among many I have made with this view, will demonstrate. I filled a glass jar, holding 21 cubic inches, with a mixture of common air and carbonic acid, in the proportion of 70 parts of the former to 30 of the latter. I then introduced into the jar a bundle of fresh grass, which displaced, by previous measurement, exactly 2 cubic inches of air, and submitted the whole to the full light of the sun during four hours, the inverted jar being surrounded by mercury in a saucer. At the close of this time, I found, that only 2 parts out of the 100 were subtracted by lime-water, while not less than 41 parts were consumed by phosphorus. In estimating, therefore, the proportion of carbonic acid and oxygen before and after the experiment, it appears, that 26.3 of oxygen were added in this short space of time, over and above the 14.7 originally existing; or if we compute the quantity of each gas before and after the experiment, this will be the proportion:

Very few, I imagine, would hesitate in admitting, that this conversion of carbonic acid into oxygen, is entirely the result of the chemical action of the sun's rays. I thought, however, it would be interesting to establish this point by experiment.

Mr Ellis had suggested to me the idea of confining plants in an artificial atmosphere of carbonic acid and common air, under jars of different coloured glass, and thus of insulating them. Finding it impossible, however, to procure jars of the required colours, I adopted the following expedient in performing the experiment. I procured three phials, holding exactly 3 cubic inches, into each of which I thrust a bundle of grass, occupying

fth of a cubic inch. I then filled them with atmospheric air, mixed with a certain proportion of carbonic acid. Having stoppered them accurately, I dipped one of them in a glass jar filled with a transparent infusion of litmus; a second in a similar vessel of a clear infusion of roses; and the third was placed in a jar of pure water. I then, by quickly turning them, contrived that each jar should stand inverted upon a plate; and thus prepared, I insolated them during four or five hours; and if there was not more than 20 or 25 per cent. of carbonic acid, I always found, that the same change had taken place in each phial, namely, that the whole of the carbonic acid, excepting 2 or 3 parts, had been converted into oxygen. But if the proportion of carbonic acid was greater, suppose 40 or 50 per cent. I invariably found, that there was more oxygen in the litmus and pure water phials, than in that immersed in the red tincture. I instance the following indifferently, out of very many experiments that I have performed, upon this point.

I exposed the three jars, with their respective phials, to a full sunshine on the 5th of July. At the beginning of the experiment, there were in each phial, of Carbonic Acid, 43.00; Oxygen 11.97; Nitrogen 45.03 100. At the expiration of four hours, the analysis gave me *,

=

It thus appears, that the red rays are sufficiently powerful to decompose the carbonic acid, if the proportion of it be small; but if a larger proportion of it be present, then it is evident that more oxygen is elaborated by the violet rays than by those of the red colour.

In repeating the experiment, but varied, by substituting cabbage leaves, I found that very little change had taken place in the mixed air, which seems to prove, that their leaves, on account of the delicacy of their coats, are much more permeable by air.

It is here fit that I should advert to a circumstance which is evident from the tabular statement I have given. It will be seen, both in that and in the one before it, that the proportion of nitrogen to the other constituents, was greater at the close than at the commencement of the experiment. This, however, is a complete illusion. There is in fact no increase of nitrogen. I am quite clear, that the apparent addition of it arises from some portion of carbonic acid, which the sun has not had time or power to decompose, being retained in the leaves, and in this way lessening the absolute quantity of air in the phial; so that the more carbonic acid is retained in the leaves, the greater will be the ratio of nitrogen. To make this perfectly clear:-In the experiment above related, if all the carbonic acid, with the exception of 19.5 (the quantity remaining in the red phial) had been decomposed, we ought certainly to have found 35.47 of oxygen; but it appears that only 29.5 were indicated by the eudiometer, and therefore, we cannot hesitate in admitting, that 6 per cent. were retained in the leaves. The result of one or two trials would not make me so confident as to this conclusion; but having invariably found, from a very frequent repetition of my experiments, that the carbonic acid that disappears is not accounted for in the oxygen formed, I think the inference which I have drawn is perfectly fair, and indeed unavoidable. This apparent accession of nitrogen is noticed by Saussure, in his admirable work entitled Recherches, &c. as a real production of that gas; but I conceive that the circumstance is much more naturally accounted for by the explanation I have just given. If this explanation be correct, it will serve to shew, that this decomposition of carbonic acid takes place within the substance of the leaf, and not exterior to it, as Mr Ellis imagines.

I shall now proceed with the chief object of this paper, to inquire how far the respiration of plants is conducive to the purification of the atmosphere. As far as is known, from the experiments and observations of the ablest inquirers, it appears, that plants in darkness, in the shade, and perhaps in the common light of the day, generate carbonic acid, and that only during sunshine is there any evolution of oxygen. The question, therefore, is, Has this partial production of oxygen any effect in di