and it would seem that the chief difference between this process and that of decay consists partly in the degree of this resistance, and partly in the destruction being masked in the one case by the repairs of nutrition. There is no difference in the essential nature and products of the acts, but simply in the conditions in which they take place in the economy.

Viewed in this relation, the force connected with the process of respiration must be considered, in great part at least, as physical and mechanical in its character,—for it is difficult to attach the idea of vitality to a change, so closely allied to combustion and putrefactive disorganization: and the more so, as the effects upon the capillaries may be explained by this means. The dilated condition of the vessels, when the function of respiration is vigorous, may be regarded indeed as the natural consequence of the free extrication of heat, which must take place under these circumstances, and the shrunk and contracted condition when the process flags, as an equally an equally necessary consequence of the diminished supply of the same agent.*

* The function of respiration, allied as it is to the processes of decay and combustion, is essentially destructive to the organism; and if certain circumstances did not exist, by which it was kept within bounds, the body would soon kindle up into fever or actual flame. The distinctive gaseous product, moreover, is highly poisonous in its character, and hence a second reason why the function would be inimical to the wellbeing of the creature unless properly restrained and checked. It may be interesting,

The phenomena of capillary movement in perfect organisms appear to point to a common law in the operation of force, and this equally, whatever may be the nature of the force. Except in a very rudimentary

therefore, to pause a moment and investigate the antagonizing influences which prevent and correct these several evils.

The history of respiration appears to imply that this function is most vigorous in the absence of light. In plants, for example, the formation of carbonic acid is nearly suspended during the day, or at any rate is much less marked than at night. In animals these results are obscured by many circumstances, but here also, all other things being equal, the same facts are observable. In that form of force, therefore, which is accompanied with the sensation of light, we find, as it would seem, a condition unfavourable to the formation of carbonic acid, and light, on this account, may be supposed to be unpropitious to the act of respiration. Light, moreover, is not merely unfavourable to the formation of carbonic acid, but it is entirely inimical to this process, for the history of vegetation affords sufficient and ample proof that this gas is actually decomposed under the influence of this agent. And thus it would appear, that the conditions unfavourable to the act of respiration are those which restore the equilibrium of the atmosphere by destroying the poisonous gas which is the product of the act.

The actual mode in which light prevents the formation, or effects the decomposition of carbonic acid, is but very imperfectly understood. The belief is, that the gas in question, though poisonous to animals, is a necessary element of food to plants: it is supposed that the latter are enabled to assimilate this gaseous food under the influence of light, and in this way to preserve the air in the requisite degree of purity. It is an apparent fallacy, however, to imagine that plants are the agents solely concerned in the process of atmospheric depuration. The difference between animal and vegetable is one of degree and not of kind, and though all parts of the creation

condition of the vessels, or else at times when the heart flags there is no evidence of the existence of any power in these vessels which could aid in the circulation by rhythmical contractions and dilatations; but

are bound up as a common whole in mutual subserviency each to the other, each part is nevertheless a complete entity in itself, and not merely a complementary appendage to another.

It may well be doubted, indeed, that plants are the sole agents endowed with the power of destroying carbonic acid. If it were so we might imagine (what is not the case) that the quantities of this gas present in the atmosphere would be greater in proportion as the forests disappear before the progress of civilization. In India and China also it may well be doubted whether the vegetable covering of the soil is sufficient to render the air fit for healthy breath. And in the colder realms of the north, where the vegetable carpet is so scantily spread that mighty hordes of animals are compelled to migrate year by year into more southern regions, the insufficiency of the vegetation to the proper purification of the atmosphere is very apparent: and so also in the ocean, for the scanty fields of seaweeds can scarcely be proportionate to the wants of the countless creatures which breathe in the waters.

And if the vegetable world be insufficient to the task of purifying the air and waters, where are we to turn in order to find the co-operating agencies? It may not be to the realm of animate nature, for here, though under certain circumstances there may be evidence of a partial decomposition of carbonic acid; yet, at all times, a larger amount of this gas will be generated than can by any possibility be decomposed. It is to the inorganic world, therefore, that we are obliged to turn, and that here are to be found the complementary agencies for which we seek, appears for a threefold reason.

In the first place: the stability of the atmosphere is an argument that the cause of this stability rests in an agency which

there is evidence of a general dilatation under the operation of force, by which these vessels are preserved as open channels for the current which flows from the heart. We may imagine circumstances which would

remains constant and unchanging. This fact seems to point beyond the kingdom of life; for, if the forms of vegetation were solely concerned in maintaining this equilibrium, it might be supposed that the quantity of carbonic acid present in the air would be greater in winter than in summer.

In the second place: it may be supposed that inorganic bodies, under certain circumstances, are endowed with a catalytic power, by virtue of which they are enabled, without any manifest change in themselves, to effect changes in other bodies, and hence it may be argued, without any improbable assumption, that the phenomenon of light is expressive of a condition in certain inorganic as well as in certain organic substances, by which the formation of carbonic acid is prevented and its decomposition effected.

In the third place: the history of sulphuretted hydrogen, (which gas is a joint product with carbonic acid in the process of decay, and not uncommonly also in respiration,) furnishes a strong argument that inorganic bodies are actually concerned in this depuratory office. Both these gases present many points of relationship, and they are closely allied in origin, in properties, in the poisonous effects upon the system, and also in the mode in which they are affected by light, for this agent is unfavourable to their origin and fatal to their existence; and therefore it is allowable to seek some explanation of the action of light in the one case by what is more clearly seen in the other. What, then, it may be asked, is the class of bodies employed in the destruction of the sulphuretted hydrogen which is continually passing off into the atmosphere? As is the case with carbonic acid, it cannot be supposed that this gas is absorbed as food by vegetable structures, neither can it be supposed to be in any way sub

induce an undue and partial expansion in certain vascular tubes, and then suppose as the consequence of this an increased afflux of blood, which again, by its own peculiar molecular changes, would keep up and

servient to the wants of the animal system, for to animal and vegetable it is a direct and deadly poison. We are obliged to admit, therefore, that inorganic bodies are concerned in the decomposition of this gas-for agents must be employed to conquer the affinities of its component elements; and as this is the case here, so also we may argue, in relation to carbonic acid, that inorganic bodies under the influence of light are endowed with a similar depuratory office.

As to the identical bodies which may be concerned in the removal of carbonic acid and other poisonous gases from the atmosphere, we may infer, from the history of spongy platinum, that these are the porous rocks and earths. We know that the peculiar properties of the metal are chiefly owing to the infinite extension of surface by which the substance to be acted upon is brought into intimate contact with the agent, and therefore a similar property may, without any improbability, be claimed for other mineral bodies in which this condition exists. And hence we may see new proofs of design and beauty in the world around us. The porous rocks, when lighted by the sunbeams, may be intended to filter and purify the air as well as the waters, and so made to serve a purpose which could not be answered by crystalline solids like marble or granite. The belt of sand which encircles the sea may also have a similar office, and the activity of its several particles be increased perhaps by the continual washings of the tides. And, finally, the boundless deserts of torrid climes may have some relation to the exuberant life of other parts of the same regions, and instead of being wild and dreary desolations, they may be important means by which the atmosphere is purified, and health secured to man, and beast, and plant.