silica. This carbonate of potash again meeting with the sulphate of lime in solution, would occasion the double decomposition of which we have spoken. And that these chemical changes did take place, the composition of the chalk shews us; for it every where contains both potash and silica. The organic remains converted into silica owe their origin to a like cause; that is, to a gradual deposition of silica, from the silicate of potash decomposed by means of the carbonic acid evolved during their decay. The infiltration and decomposition of this salt would then give the coherence to the chalky beds which they now possess. Indeed, to the infiltration of silicate of potash may be ascribed the conversion into stone of all the argillaceous and calcareous deposits; assisted, of course, by heat.

I am well aware that the supposition which we have advanced has to encounter a very serious objection in the paucity of vegetable remains, found in these beds; but this objection is by no means conclusive against the opinion. The plants found in them are all marine, and from their perishable nature could not be accumulated in quantity. Besides, the loose nature of the deposit would long admit of the access of water, which would ensure their decomposition. This circumstance must be allowed its due importance; for the impermeability to water in chalky beds is vastly inferior to that of arenaceous and argillaceous deposits; and the experiments of Dr. Lindley have proved that long immersion in water destroys most plants. Allow, then, even this suggestion as a partial cause (but we see no obstacle in assuming it as the universal one), and there is no difficulty in accounting for the withdrawal of carbonic acid from the atmosphere during the deposition of the cretaceous beds. In consequence of this withdrawal, the atmosphere becomes fitted to sustain higher forms of organic life, and these accordingly sprung into being during the tertiary period. During the tertiary period itself, this withdrawal of carbonic acid from, and supply of oxygen to, the atmosphere, was constantly proceeding, as the vast beds of lignite on the Rhine amply testify; and a gradual increase of forms of animal life is accordingly observed, from the basis of the tertiary rocks, to the summit of the series. It would be desirable that more conclusive evidences were furnished of the age of the brown coal: in absence of information to determine to what part of the series it should be placed, we must keep from drawing conclusions regarding it; the general inference, however, may be drawn, that these entombed lignites do certainly not belong to the newer part of the series.

Although we find very many species in the tertiary strata similar or identical with those now inhabiting the earth, still neither in these nor in the post tertiary or alluvial deposits, do we find any trace of man or of his works. But previous to his creation it became indispensable that a balance should be established between animal and vegetable life. It was the commencement of a grand era that ushered into the world an intellectual being. Nor can we conceive that a creature so

noble, stamped by the image of its Creator, could be destined, like those animals which had become extinct, to be swept in its turn from the earth by causes similar to those which had effected their extinction. The creation of an intellectual being was not an occurrence coming in the common order of events; but man must have been placed upon the earth by a fiat of the All-powerful Creator. The present order of the globe was ordained for him, and all things proceeded to fit it for his habitation. We will not deny that he who made man might likewise ordain that he should be swept from the earth; but we do deny that the exhibitions of the causes destined to produce this would be in uniformity with the usual designs of providence. It would be ill-accordant with a divine wisdom to suppose that it had not arrested the causes which led to the extinction of whole tribes of animals, when it called man into being. But it would as little harmonize with our conceptions of the Creator's works to imagine that any divine interposition or miracle altered the face of nature, immediately antecedent to the creation of that intellectual being. Laws were instituted, by which this earth is governed, and we must look for such changes in the natural current of their operation, not in their annihilation or alteration. Now if I have carried you along with me in the description of events which have proceeded on the earth from the first dawn of organic life, you will find no difficulty in discovering, òr admitting with me, that the grand causes for the extinction of animal life were now removed. We have seen all the primeval lands, which we have hastily traversed, covered with a luxuriant vegetation, but containing a disproportionably small number of forms of animal life. We have approached more nearly to our own epoch, and remarked the gradual increase of these forms; and, at the same time, we have seen that the atmosphere which covered those lands, gradually changed its character. We have remarked that the first forms of land animals were such as could live in an atmosphere destitute of its present proportion of oxygen, and such also as did not return any notable quantity of carbonic acid to the atmosphere. But we remarked also a great change as we approached our era:-the animals existing in strata antecedent to modern deposits, were all furnished with respiratory organs like our own; that is, with organs fitted for abstracting oxygen from the air, and returning carbonic acid: functions quite opposed to those of vegetables, which abstract carbonic acid and return oxygen. Hence, as soon as animals became sufficiently multiplied to supply the amount of carbonic acid to the air, equivalent to the quantity abstracted by growing plants, a balance would be struck, and the air could not experience any appreciable variation. For even supposing that by some adventitious circumstances, such as by volcanic agencies, an increased supply of carbonic acid was furnished to the atmosphere, the effect would simply be to induce an increased vegetation, and the excess would be withdrawn. On the other hand, if the amount of carbonic acid became

T2

diminished, vegetation would be retarded, and it would attain its normal standard. Such would be the natural effects of vegetation, when the original amount of carbonic acid in the air had become reduced to the point at which it ceased being detrimental to animals, but was still sufficient to administer to an ordinary, but not excessive luxuriance of vegetation.

But here a curious question arises-Does the progress of human society not occasion a greater demand for carbonic acid, and thus serve to destroy the equilibrium between plants and animals? It can scarcely be denied that civilization causes an increase in the aggregate of animal life. The preponderance is great between the animals depending upon man, and those destroyed by him. Nay, even the increase of human beings produced by an augmentation of the comforts of civilized life, would demand our serious attention to the enquiry. Once admit, and I do admit it for the sake of argument and probability, that civilization increases the aggregate of animal life, and it is evident we must find some means of compensating to the air for the carbon and nitrogen abstracted and retained by this increase. For were there no means of compensation, other parts of the earth, existing in a state of nature, would suffer for the supply of those parts subject to the dominion of man. The difficulty of finding an answer to this question has furnished a specious argument to those who refuse to admit that the food of plants is derived from animals; and yet the explanation is not so difficult. Which would require the most carbonic acid-America with its forests and extensive prairies, or America peopled by man, and covered by fields of waving corn? Civilised man enters America, he expels its tenants from their native soil, cuts down their forests, and plants in their stead agricultural produce. Strange as it may appear, he has caused neither an increase nor a diminution in the amount required; for it has been proved by Liebig that the same extent of land produces the same quantity of carbon, whether it be covered with forests or agricultural produce. He increases himself to an amazing extent, and thickly peoples that once thinly populated country. True, by his increase, he removes from the air quantities of carbonic acid, but he compensates for this by the habits of civilization. He has cultivated in the place of forests plants requiring a large supply of nitrogen. This nitrogen he is peculiarly fitted to supply them with, from the effete matter discharged by him. The air contains much more ammonia than is necessary for the purposes of wild plants; he abstracts out of its superabundance, and merely concentrates it at particular spots, where it meets with plants which peculiarly require it, but none of it is lost to vegetation. The peculiar habits of civilised life cause him to return to the air carbonic acid, amply compensating for that abstracted by his increase. In the form of coal he digs from the earth, and returns to the air the carbonic acid of former times. The great stream of air which, by the revolution

of the earth moves from the equator to the poles, wafts in its return this food to tropical climates, where nature yet revels in all her wildness. The food thus sent is immediately appropriated by a luxuriant vegetation, and oxygen of course emitted, which the same stream of air in its progress towards us, brings back to supply that consumed in the formation of the food. Thus man, by the habits of civilisation, fully compensates for that which he retains by his own increase, and that of the animals dependant upon him. Upon the compensation of ammonia, I have almost said enough. The plants of former ages were not such as to require much nitrogen; hence, although the decrease of carbonic acid from the air was great, that of ammonia was insignificant. The greatest exhausting cause in operation would be the rain which carried it to the sea, and caused it to remain there; that falling on the land would again evaporate. And though the increase of animal life may, as we have already said, remove for a time the superabundance, this will finally benefit vegetation. But all this time, I have been silent regarding volcanic agencies. I have been so, because I consider them merely as an auxiliary means of furnishing to the air the carbonic and oxygen retained by animals during life; but by no means as being the primary sources of food.

I would I could dwell longer upon these subjects: they are of too extensive a nature to be embraced in a single lecture; but for you I have been already far too long. My end will be gained if some of you have been convinced that the grand causes formerly in operation for the destruction of animal life have ceased, and that man is not liable to the destructive agencies of former times-if you are convinced that by wonderfully wise plans of Providence, the grand medium of animal and vegetable life, the atmosphere, has become fitted for the reception of man, and attained a state of repose and perfect equilibrium, by the exquisite adjustment that Death has become the source of Life. And if my illustrious namesake, when supporting the theories of Hutton, in his assertion that the world shewed no traces of a beginning nor none of an end-meant to include that the organic life on it is also destitute of such traces, we cannot assent with him. But that no evidences of a beginning nor of an end exist in the world itself, we fully admit; yet that that beginning was, and that an end will be, we have the word of one "that cannot lie."

Atmospheric Electricity.-By Mr. T. SPENCER.

SECTION IV.

Liverpool Albion, Feb. 14, 1842.

I shall occupy a portion of the present section by explaining the electrical phenomena attendant on hail storms, taking the undisputed fact, as already granted, namely, that hail and pieces of atmospheric

ice are formed from the water contained in cloud and furthermore, the position already assumed in the present papers, that cloud, as a necessary condition of its existence, possesses a definite portion of electricity. The explanations that follow, arise inductively, out of these points. The brief remarks I shall make on this head will have no relation to the formation of the hail itself, but only to the attendant electrical phenomena, together with the whiteness assumed by hail and atmospheric ice, both of which are, as yet, unattended with any satisfactory explanations.

All observers have been particular in remarking, that violent hailstorms are attended by lightning, and often by thunder; but whether these phenomena are the cause, or only an effect, produced by these occurrences, is I believe, undetermined.

Another important fact has also been noted, that the lightning given out is seldom so intense or dangerous in its effects as that given out during ordinary thunder storms, accompanied with rain, although there are rare exceptions to this rule; but the rule and exception are, I submit, easily explained according to the views already given in former portions of this paper.

Let us here, however, in some degree, recapitulate. Aqueous vapour possesses upwards of 1200° Fah. of latent calorie. It pos

sesses this amount as it floats in an invisible state at all times in our atmosphere. When condensed into globules of water, in which state it is true vapour, to form cloud, it parts with 1072° of caloric. Still, it will be observed, there is an important quantity yet held latent, amounting to 140° Fah., which latter portion is termed the caloric of fluidity, or, in other words, that portion of caloric that is necessary to the water, in order that it may maintain its fluidity, or that which prevents it from becoming ice.

We have already seen, that transparent aqueous vapour, or gas, in becoming water, it being a chemical change, developes, simultaneously with its latent caloric, a definite portion of electricity, obeying the law of all chemical change.

Bearing, then, this in view, and proceeding but one step farther, we at once perceive the necessary sequence of the electrical phenomena that accompany hailstorms.

When a cloud is deprived of its electricity, no matter by what cause, as already stated, the small globules of water condense into large ones, and form rain. Under ordinary circumstances, this rain

would be carried to the earth in the fluid state; but when hail is formed, the rain, instead of descending, is carried upwards by an upmoving current of atmosphere, as we have already seen explained, and losing, as we know it does, 1° of caloric for every one hundred yards it ascends from the level of the ocean, it soon arrives at a temperature sufficiently low to deprive it of its caloric of fluidity. When this takes place, the globules of rain are at once converted into ice. When, however, the 140° of caloric of fluidity is eliminated, if (as before laid down) it is true that electricity is developed in the free