Living vegetables are the agents which have been employed by Nature for protecting us against the effluvia arising from dead ones, and clearing the atmosphere of the carbonic acid thrown off by animal respiration. For, by making what is noxious to animals, the natural food of vegetables, this most important office has been fulfilled.

Again, during the life of a vegetable, water is put under contribution for a large portion of its support, and after its death, water and heat hasten its dissolution, and set its elementary parts at liberty to enter into new combinations. And, in this manner, the elementary particles of all animated nature (whether in life or after death) are never suffered to be at rest; but perform their offices in the vegetable or animal to which they belong, only for a limited time, and after death, are again destined to occupy another place in the great circle of composition and decomposition.

But, a propos, it appears by observation, as well as, by every information I have been able to procure, that the alternate changes from heat to moisture, and again from moisture to aridity, are the most favorable circumstances for hastening the destruction of timber.

The modus operandi, I think, may be explained in the following way. Caloric has the power of expanding nearly all bodies with which it unites, by insinuating itself among their particles ;' and, during its operation on timber, the pores of the wood become dilated, by which means, moisture or rain is more completely admitted into its texture; and, after rain, the atmosphere will generally be found to have the greatest capacity for moisture, consequently, the evaporation from the woody fibre will then be most abundant, and, by a continuation of such vicissitudes, the decay of wood is greatly accelerated.

In this country, there seem to be only two modes by which wood may be preserved from decay, for a very long period of time: the first, by expelling the natural sap and humidity from wood before it is used, and keeping it continually dry afterwards; and, the second, by totally excluding atmospheric air under a low range of temperature, and the intervention of some dense

substance.

Thousands of examples of the first kind of preservation are to be met with in old houses, where fires have been constantly kept. In such houses, even those species of wood, which, under the usual changes from aridity to moisture, and again from moisture to heat, run most rapidly to decay, are preserved for a great length of time.

Clay, water, cast iron, and some saline substances, excepted.

The second mode of preserving timber, by the total exclusion of atmospheric air, &c. is fully proved by the trunks of large firtrees being found, in many places in Scotland, several feet deep in moss, in so high a state of preservation, that the wood is frequently split by the country people and used as a kind of rush-light.'

In certain climates, there are still other means of preventing the elementary substances of animal and vegetable bodies taking their primitive forms, viz. through the medium of eternal frost, as is proved by large quadrupeds having been recently found incased in ice, in Siberia; and, secondly, by excessive heat, providing there is little or no humidity in the air, as is sometimes the case in Africa. But, as neither of these means of preserving bodies can be reduced to any practical utility in this country, it is useless to follow them farther.

In Great Britain, the woods which resist the powers of the destructive agents longest, are those which are of the greatest specific gravity, and closest texture, as the oak, for example; while the most porous, and, consequently, that of the least gravity, falls the easiest prey to destruction.

These considerations naturally led me to inquire, what is the cause or causes of dry rot in ships, in order that we may be enabled to guard against it?

The answer to this most important question is involved in considerable difficulty, owing to the different circumstances under which it is said to have taken place, and from the great diversity of opinion there exists amongst men on the subject.

Its cause has been attempted to be traced to a vegetable substance, to moisture, insects, impure air, putrescent juices of timber, and to the vegetable juices of timber.

It would be departing from my original intention, to follow, in an Essay, the different individuals through their various opinions

'The remains of those fine trees afford us a miserable picture of the degenerated state of our climate, probably owing to the rapid growth and insidious advances of that vegetable substance, moss.

2 "An elephant was recently found by M. Adams, near the mouth of the Lena (a river in Siberia), the flesh of which was still in so high preservation, that it was eaten by dogs." It is certain, nothing but the eternal frost in those regions could have arrested the putrefactive process in so large a quadruped, for so many centuries.

3 "We observed (says Captain Lyon, in his Travels in Africa) many skeletons of animals which had died on the desert, and occasionally the grave of some human being; all these bodies were so dried by the extreme heat of the sun, that putrefaction did not appear to have taken place after death. In recently expired animals I could not perceive the least offensive smell. Such was the dryness of the air, that the horse-tail, in beating off the flies, the blanket, and other clothing, emitted electric sparks, and crackled on being rubbed."

on this subject. But I am not inclined to impute the decay of timber to any one of those causes, abstractedly considered, but to an alternate action of certain destructive agents, to be hereafter mentioned.

Owing to dry rot being accompanied by the vegetation of fungi, some individuals have been induced to consider this as its chief cause, but, I trust, I shall be able to show, that it is only a link in the chain of causes, or rather a consequence of a certain state of the ship's timbers.

Linnæus has placed the order of the vegetable substance which accompanies dry rot, under the 24th Class (Cryptogamia), and in the 4th Order of that Class: but Dr. Smith has added a 5th Order, in which he places fungi.'

Those individuals, who assert that vegetation takes place, sui generis, from the juices of the timber, have been forced to this conclusion, from not being able to account for the universal diffusion of the seeds of fungi in any other way. But it is well known to naturalists and botanists, that the seeds of the mushroom may be disseminated by the wind, like the pollen, or poussiére séminale, of many other plants; or they may be conveyed from the forest to the dock-yard, and again, from the yard on board a ship, by adhering to the timbers, provisions, stores, &c. and there remain in a quiescent state, until called into vegetable existence, by favorable circumstances, viz. the united influence of heat, atmospheric air, and humidity.

It has already been stated in this Essay, that the decomposition of vegetable and animal bodies is greatly retarded by any of the three following circumstances; 1st, the total exclusion of atmospheric air; 2dly, great aridity of the aerial fluid; 3dly, the eternal cold of a deep flow-moss and that of the arctic circle-the most powerful antiseptics with which we are acquainted.

It appears, therefore, from the above data, that the abstraction

This Order is determined by the plant "having no leaves, and the fructification is in a fleshy substance."

The vegetable nature of this order of plants was long doubted by some naturalists, who were disposed to ascribe to them an animal origin; but the labors of Dryander, Schaeffer, and Hedwig, have shown that they possess a vegetable character, by detecting their seeds, and explaining the parts of fructification.

In the Synopsis Methodica Fungorum of Persoon, the order of mushrooms is divided into such as produce their seeds internally, or in vessels, and such as have them exposed or imbedded in an appropriate membrane. Miller's Guide to Botany, p. 181.

2

L'erigeron du Canada, cultivé d'abord au Jardin des Plantes de Paris, s'est disséminé dans toute la France, a l'aide d'une aigrette soyeuse.Nouveaux Elémens de Botanique.

of air, and even a partial abstraction of heat and humidity, will arrest the decay of all substances for a great length of time.

These conclusions seem to be sanctioned by all I have seen, and by every sensible observation I have read or heard, on the subject of dry rot in houses and in ships.

In houses, we continually observe, that the decay commences first, where the change from one to the other of these states is most frequent. The ends of joists inserted into damp walls, for instance, and the upper and lower timbers of a house, are most liable to decay the latter, arising from the evaporation of moisture from the ground, and the former, by the breaking in of the elements from above.

The same laws are uniformly obeyed in the decay of ships: for, in their upper works, where moisture, heat, and evaporation, follow each other in excess, the timbers run sooner to destruction than in their holds: this is owing, 1st, to the almost constant application of water to the decks, by rain or washing; 2dly, to the higher range of temperature produced there by animal and solar heat; and, 3dly, to the greater subsequent evaporation. Whereas, in the hold of a ship, the exclusion of solar and animal heat, and the low temperature maintained there by the iron ballast and ambient salt water, usually, in a great measure, preserve her timbers from decay.

These observations are further corroborated, by the yet more rapid decay of ships in warm and tropical climates, where moisture is more abundant, and heat more powerful, than in the temperate

zone.

Much controversy has taken place, of late, respecting the causes and difference between the dry and wet rot in ships. I believe it consists simply in this; the former is accompanied by vegetation, and the latter is not; or, in other words, air, heat, and humidity (besides their usual destructive qualities in dry rot), call the dormant seeds of fungi into life: while, in the wet rot, the same agents only hasten the decomposition of the woody fibre.

But these distinctions are little necessary, owing to the remedy for both being the same; for, admitting this spongy vegetation to possess the power of absorbing moisture, and maintaining a higher range of temperature than the surrounding dead matter, still, as we have no means of preventing the diffusion of the seeds, all we can do, is to endeavor to starve them, by expelling the sap, before the timber is put into a ship, and keeping her as free from moisture as possible afterwards; by which means the pollen will remain in a dormant state, and by the same measures, the wet rot, occasioned by air, heat, and humidity, will also be arrested. Indeed, our means of preserving a ship from decay, will be in exact

proportion to the powers we possess of freeing the timber of its natural sap and moisture, and keeping it dry afterwards.

Is it not a little remarkable, that some of the most able writers on dry rot should have fastened on causes which are hardly hypothetical; while those laws which are well established, and continue in perpetual activity, in the vegetable kingdom, should have beenpassed unnoticed by them?

Morrison" attributes the production of this vegetation (dry rot) to the mixture of salt and sulphur, mixed with oils from the dung of quadrupeds." While Mr. Bowden, at page 82 of his Treatise, says, "The cause of dry rot is heat acting on the vegetable juices; by which," says he, at page 87, "they (the juices) will rise from their dormant state into life and action, and the timber will be consequently destroyed."

Had these gentlemen not been sufficiently acquainted with the physiology of the seeds of plants to know that all vegetables perpetuate their species through the medium of seeds, suckers, slips, &c. they ought at least to have known, that when the great Author of nature separated the sea and earth from the chaotic mass, and called animals and vegetables into existence, he set certain limits to their sphere of action, by giving them the faculty only to multiply" after their kind." And although man has dominion over, and is invested with power to kill, and drive back ferocious animals to the desert, and root out certain noxious weeds from the garden; yet he has never been able to exterminate a single species of either from the face of the earth. Power, therefore, has never been given to the most exalted in the class of animated existence to produce the vilest insect; nor is the stately oak, the king and pride of the forest, (during life or after death,) capable of generating even a mushroom. But, under decay, this wood yields a suitable nourishment to that species of fungi, whose seeds had been previously disseminated by the wind, or otherwise.

I have here to mention, by the way, that although vegetables have not the power of locomotion, yet the diffusion of many of them is not less certain, by wings, spines, hooks, and scales; for instance, the downy appendages by which the dandelion wafts itself through the air is familiar to every one. Moreover, Nature seems less tenacious in the preservation of animal and vegetable life themselves, than she is in giving to the one a strong desire to propagate its species, and to the other the power of retaining life until that object is accomplished.'

1

Ray rapporte qu'à la suite d'un incendie arrivé à Londres, peu de temps après, les murs furent couverts de sisymbrium irio. Il ajoute que cette plante étoit rare et éloignée de cette ville. Les graines s'étaient sans doute

VOL. XXVI.

Pam.

NO. LI.

N