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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. luisigilobits

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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 ammated 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 ex

plained in the following way. Caloric has the power of expanding nearly all bodies with which it unites, by insinuating itself among their par ticles* 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 acce lerated.

In this country, there seems 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

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

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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.

The second mode of preserving timber, by the total exclusion of atmospheric air, &c. is fully proved by the trunks of large fir-trees being found, in many places in Scotland, several feet deep in moss, in so high a state of preser vation, 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 proved by large quadrupeds having been recently found incased in ice, in Siberia; and, secondly, by excessive heat, providing there is

*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.

+ 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.

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 enquire, 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

* "We observe, (says Captain Lyons, 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."

have taken place, and from the great diversity of opinion there exists amongst men on the subject. of 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 inten tion, to follow, in an Essay, the different individuals through their various opinions 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.*

*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