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we were to calculate the quantity of carbon in all the carboniferous deposits at two thousand billion pounds (a quantity which must be much under the truth); then during its formation no less than 64,000,000,000,000,000 cubic feet of carbonic acid must have been extracted from the atmosphere, and a like quantity of oxygen gas returned to it.* This is equal to į of the quantity of carbonic acid present in the whole extent of the atmosphere. And when we consider that this is but a portion of the carbonic acid removed, we may reasonably conclude that the atmosphere contained, at the commencement of the great carboniferous epoch, more than double the quantity of carbonic acid which it does now.

We have no grounds for affirming that there is a less vegetation now than in early times. On the contrary, it is highly probable that the vegetation now is much greater than that of former periods ; but it is no less certain that the vegetation of former times was vastly more luxuriant at given places. For the continents being of smaller dimensions, more carbonic acid could be spared to support a luxuriant vegetation in a confined area. It is owing to this great luxuriance of vegetation, within a limited district, that vegetable remains were accumulated in such quantity as to defy even a remote analogy at the present day.

Without reference to geological epochs, I may here state in what manner coal and lignites are produced. The two principal kinds of coal are, the wood, or brown coal of Germany, and the stone, or mineral coal so abundantly found in our own country.

The wood coal, from its composition, has evidently been formed by a regular decay of plants with limited access of air. Hence the hydrogen is still present, whilst the oxygen has disappeared along with carbonic acid. Mineral coal, on the other hand, is distinguished from wood coal by containing a very small portion of hydrogen. Wood coal has been formed by the evolution of carbonic acid from the sub

• 1000 lbs of charcoal in burning produce above 32,000 cubic feet of carbonic acid. 1000=32,000, or l=32: 2,000,000,000,000,000 lbs. will produce 64,000,000,000,000,000 cubic feet. 2,000,000,000,000,000 lbs.

:=892,857,142,857 tons.

2240 lbs. But the assumed number, 2,000,000,000,000,000, is empirical, and we have, therefore, to shew it is not above the truth, howerer far it may be below it. Now we bare already seen that Manchester by fuel, for domestic purposes alone, sends into the air, every year, 23,614,285,714 cubic !eet of carbonic acid. And, taking its manufactories into the calculation, we may safely suppose, that the total amount will not be less than 46,000,000,000. Now

64,000,000,000,000,000=1,391,304

46,000,000,000 That is, Manchester would consume the total amount we have supposed to exist in 1,391,304 years. Or supposing that there were in the world about 65,000 places consuming the same amount of fuel as Manchester, the total amount of coal in the great carboniferous deposits would be consumed in about twenty-one years. But this is obviously absurd, for we know that there is a much greater snpply than this. Hence our original empirical number, instead of being above, must be much under the truth.

stance of the plants composing it; whilst mineral coal has been formed from the expulsion of part of its elements in the form of combustible oils. These oils may often be procured from the coal by distillation. Heat appears to have been the cause of the expulsion of these oils. A remarkable example occurs in a quarry within a few miles of St. Andrews, between that town and Cupar. A basaltic rock which has penetrated through the carboniferous strata, forms a hill in the locality alluded to: this rock is thoroughly impregnated with coal naptha. At whatever part a fragment may be broken off, the fresh surfaces are quite humid with an imprisoned fluid, which almost instantaneously evaporates ; this fluid has the smell and all the properties of coal naptha. The great difference, therefore, in the formation of wood and mineral coal is, that in the production of the former, carbonic acid is evolved ; in that of the latter a hydrocarbon. Hence it is that no combustible gases exist in the mines of wood coals, whilst they abound in those of mineral.

But be this as it may, our conclusion is still the same : that during the formation of the carboniferous deposits, much carbonic acid was abstracted from, and much oxygen furnished to the atmosphere. From the very low numbers which we assumed as indicating the weight of coal in the carboniferous strata, we have seen that its conversion into carbonic acid would nearly double the quantity of that gas now in the atmosphere. But the marine plants, which probably abounded in the same proportion as the terrestrial, have left few evidences of their former existence. They are so perishable in their nature, and surrounded by an element which aids their decay, that their preservation was highly improbable. But during their decay, the carbonic acid from which they were formed, must have been given to the surrounding water; and probably entering into chemical combination with some of its materials, was not again restored to the atmosphere. From whence came all the carbonic acid in the limestones not formed by the accumulation of shells ? Some of it, certainly, may have been derived from the source just mentioned. Nor are we to allow ourselves to be misled by the belief that the quantity of carbonic acid evolved from such a source, would be too small to exercise an appreciable effect. The decomposing organic matter has perceptibly affected the whole mass of the ocean in its vast extent; for all the recent analyses of sea water prove the presence of sulphuretted hydrogen-a gas only generated by the action of decomposing organic matter on salts of sulphuric acid. There are salts of lime in sea water, particularly the sulphate of lime, now this salt is very easily decomposed by a carbonate. Supposing that during the decay of the marine plants, which every analogy leads us to suppose must have existed in quantity proportional to the terrestrial, an alkaline carbonate was produced: this acting upon the sulphate of lime would occasion a precipitation of carbonate of lime, and give rise to those soluble alkaline sulphates, which we find in such quantity in sea water. By this suggested explanation, I by no

means infer that this was a general mode by which the stratified limestone was produced : the undivided limestone could not possibly have been produced in this way. But it is possible the thin layers of limestone which occasionally alternate with the shale, sandstone or coal, in the coal formation, may be due to such a cause; or might we not conceive that the bituminous limestone shale might also owe its production to this? The immense mass of undivided mountain limestone could by no possibility have been thus produced, but may have well been by the deposition from solution through the instrumentality of the causes I have formerly described. Once allow, with many geologists, that the ocean was in a heated state during a great part of the primary period, and we are furnished with another mighty means of abstracting the carbonic acid from the atmosphere. The heated waters of the ocean could not dissolve carbonic acid, but as they cooled, this gas would be absorbed from the superincumbent air; and the water which evaporated and again descended as rain, would bring down in solution large quantities both of carbonic acid and ammonia.

Taking such things as these into consideration, together with their possibility or probability, it is obvious that we have lost all data for calculating the amount of carbonic acid in the air, at the commencement of the secondary period. Allowing them even a shade of probability, we could not deny that the former atmosphere may have contained more than twenty times the amount of carbonic acid that it does at present; and admitting that it did so, we can account for the extraordinary luxuriance of the primeval vegetation and for the absence of land animals whilst that vegetation lasted.

During the period at which the carboniferous strata were deposited, neither reptiles, birds, nor mammalia appear to have existed : nor was it possible that they could have existed, were these views of the state of the atmosphere correct.

It is not my intention to detain you, nor is it my province to wander with you step by step over the various geological epochs. In our brief sojourn in the carboniferous strata, we have seen that several, possibly many causes were in operation to remove carbonic acid from the air, and consequently to fit it for the support of animal life. But let us not suppose that these causes ceased with the termination of the carboniferous era. They still operated, though in a less striking degree during all the divisions of the secondary period; but, during the deposition of the new red sandstone, they appear to have been in a great measure dormant. It is the duty of the geologist to explain what physical causes then existed, which were so unfavourable to animal and vegetable life. But the causes which acted during the carboniferous period were again revived with the oolitic system ; and, accordingly, from the coal occurring in it, we draw evidences of a removal of carbonic acid from the atmosphere, and a supply of oxygen to it. But here also we are struck with the new forms of animal life which have now sprung into existence : the

saurians which began to appear in the new red sandstone, have now multiplied, and play on the shores of the colitic land ; insects inhabit the luxuriant forests which cover that land ; and that most extraordinary of all created beings, the pterodactylus, or flying lizard, executes the functions for which it was designed. The sea has acquired new inhabitants : not only monstrous reptiles, but new forms of fishes-zoophyta, mollusca, and articulosa ; but still we find neither birds nor mammalia. The plants of this series afford evidence sufficient of a tropical climate, but the saurian animals furnish proof yet more conclusive. You may remember the cause of animal heat, as I stated it to you in a former lecture.

It is a combustion of certain unazotised ingredients of food by means of the oxygen of the air ; and as a product of the combustion, carbonic acid and water are formed. The heat occasioned by the conversion of the carbon and hydrogen into carbonic acid and water in the interior of the body, must be as great as if the elements were burned in the open air

We mentioned that it was quite possible that marine animals may have existed when terrestrial animals could not. Of course the grand object of respiration is the same in both classes of animals, viz. transformation of the food, and of particular constituents of the blood, by means of the oxygen of the air. Hence aquatic respiration differs from aerial only in that the water becomes the medium of conveying the air to the respiratory organs. In the lower classes of marine animals, the respiration is entirely cutaneous, the air not being supplied through distinct channels, but by a transudation through membranes permeable to it. As we go higher in the scale, we find a bronchial respiration, or respiration by means of lungs. In these the water holding oxygen in solution meets with a net work of veins, and aerates the blood circulating within them. The cause of animal heat being a combustion of carbon and hydrogen by means of oxygen, it is obvious that in the cold-blooded animals, the quantity of oxygen required will be much less than in the warm-blooded. Accordingly we find this to be the case. A tench lives for some time in water containing only são its bulk of oxygen, whilst river water generally contains from į to 1 per cent of this gas. Unfortunately I am not aware of any experiments which show how much carbonic acid may be in water without being detrimental to the life of marine animals ; but certain it is, that the luxuriant vegetations which analogy leads us to believe must have existed in the sea at this period, would extract carbonic acid from, and furnish oxygen to the surrounding water.

The amphibians require very little oxygen for the support of their vital functions. Frogs will live for four or six hours in an atmosphere of pure hydrogen or nitrogen, which does not contain a particle of oxygen. And although the amphibians require less oxygen than the terrestrial saurians, still we find that the economy of the latter requires much less oxygen than that of higher animals. Their lungs are,

therefore, comparatively imperfect, and the two systems of circulation incomplete; for their arteries circulate a mixture of venous and arterial blood. I do not know if there are any experiments on record, of the powers of any particular saurian to live in an atmosphere deficient in oxygen and surcharged with carbonic acid, such as we suppose existed during " the age of reptiles." I hoped to have been able to show you some experiments of their power to do so; but from their general torpidity at present I have not been able to meet with any lizards to secure for this purpose. In a few weeks, however, they will be coming out of their holes, and I shall introduce them into an atmosphere of the olden times, such as their progenitors revelled in when they were masters of the world, and multiplied themselves to such an amazing extent.

All reptiles are distinguished by the small amount of food which they consume, and by their tenacity of life under very trying circumstances. The food which they do take is not of a nature to be transformed into carbonic acid ; nor are their organs of respiration suited for this transformation, even it were. Hence it is, that they do not require much oxygen for the support of their respiratory functions, and that they expire so little carbonic acid. Hence it is also, that they depend upon the warmth of the air to keep up the temperature of their bodies, having no means of generating heat within. Considering then these facts, and the supposed nature of the atmosphere during this period, we discover a sufficient cause why this should have been (as Mr. Mantell aptly denominates it)

an age of reptiles ;"—they did not require much oxygen, which the air could not have afforded. Their respiratory functions were not retarded by an excess of carbonic acid, which would have proved fatal to animals of a higher organization; nor did they come in collision with the plans of the intelligent Creator to remove from the atmosphere by means of organic life the excess of carbonic acid.

From these facts we see that reptiles could have had no difficulty in living in an atmosphere containing less oxygen than at present; but we find that in the oolitic period, a particular kind of quadruped existed. This seems to have been a marsupial animal allied to the didelphys. It was evidently an insectivorous animal, from the conformation of its jaw bones found in the Stonesfields oolitic beds, where the elytra of land beetles are found accompanying them. Now can we find nothing in the respiratory system of the marsupials which would lead us to believe that they might have been in an atmosphere such as we have supposed to exist ? The marsupials of course breathe, like other mammals, like man himself, by the lungs alone; and if any peculiarity of the respiratory system existed in the primeval didelphys, we could scarcely expect to find anything but mere traces of it in its modern congeners, changed as they must have been to suit the varied conditions of the atmosphere ; but let us try to discover whether such traces may not have been preserved.

Now in saurians, chelonians, and fishes, two canals are observed

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