A paper bearing this title was lately read before the French Academy of Sciences, by its author,
Dr. Boucherie, and I propose, as an appropriate sequel to the foregoing pages upon the woods, to attempt to convey a general notion of the numerous experiments referred to.
He contrasts the increasing consumption and the rapid decay of timber, with its slow rate of production, which make it necessary to economise its employment He adverts to the many projects for its preservation, enumerated by our countryman Mr. John Knowles, (see Note, p. 22,) and the methods subsequently proposed, to many of which he objects from their uselessness; to others from the slow and superficial manner in which timbers pert with their contained fluids, or absorb new ones by simple immersion, (circumstances long since proved by Duhamel;) and to all from their expense, which is of course the ultimate test of general application.
Dr. Boucherie argues that all the changes in woods are attributable to the soluble parts they contain, which either give rise to fermentation or decay, or serve as food for the worms that so rapidly penetrate even the hardest woods. As the results of analysis he says, that sound timbers contain from three to seven per cent of soluble matters, and the decayed and worm-eaten rarely two, commonly less than one, per cent; he therefore concludes that "since the soluble matters of the wood were the causes of the changes it undergoes, it is necessary to its preservation, either to abstract the soluble parts in any way, or to render them insoluble by introducing substances which should render them infermentable or inalimentery;" which he considers may be done by many of the metallic salts and earthy chlorides.
Dr. Boucherie shows, by parallel experiments upon "vegetable matters very susceptible of decomposition, as flour, the pulps of carrot and beet-root the melon, etc., (which only differ from wood, of which they possess the origin and constitution, by the greater proportion of soluble matter which they contain,)" that in the natural states they rapidly alter, but are preserved by the pyrolignite of iron, (pyroliguite brut de fer.) a cheaper material than the corrosive sublimate commonly used, and one very desirable in several respects. He presumed that by immersing the end of a tree immediately after it was felled into a liquid, the vital energies not having ceased, the tree would then absorb such fluid through all its pores, by a process which he calls aspiration; and in this fortunate surmise he was entirely successful. This led step by step to numerous practical results, which their inventor enumerates as follows, and describes in separate chapters.
1st. "For protecting the woods from the dry or wet rot." 2nd. " For augmenting their hardness,"
3rd. " For preserving and developing their flexibility and their elasticity." 4th. "For rendering impossible the changes of form (jeu) they undergo, and the splits (disjunctions) which take place when they are brought into use, or are submitted to atmospheric changes."
5th. "For greatly reducing their inflammability and combustibility." 6th. "For giving them various and lasting colours and odours."
I shall endeavour to convey a general notion of the methods in the same order.
1. Durability. He took a poplar tree, measuring 28 mitres in height and 40 centimetres diameter, simply divided from its root with its branches and leaves undisturbed, and immersed it erect to the depth of 20 centimetres in a vessel containing pyrolignite of iron; in six days it was entirely impregnated even to the leaves, and had absorbed the large quantity of three hectolitres (p. 132). This method required powerful lifting apparatus, and a support for the tree to lean against, and was therefore objectionable.
He repeatedly operated upon trees lying on the ground, by attaching to their bases waterproof bags containing the liquid: the experiments were varied in many ways; sometimes portions of the branches were lopped off, but the crown or tuft was always left upon the principal stem; at other times the aspiration was effected by boring detached holes near the earth supplied with different fluids, which gave rise to all kinds of diversities in the result; and other trees were pierced entirely through, and a horizontal cut extending to within an inch or so of each side was made with a thick saw, leaving only sufficient wood for the support of the trees.
For fear of losing the trees upon which he had the opportunity of experimenting, the process was not deferred beyond 24, 36, or 48 hours after they were felled, as the vigour of the absorption was found to abate rapidly after the first day, and that at about the tenth day it was scarcely perceptible: it was also found the aspiration entirely failed in dead wood, whether occurring at the heart of old trees, or at parts of others from any accidental interruption of the flow of the sap during the growth: and also that resinous trees absorbed the fluids leas rapidly than others.
Observations were also made of the quantities of the liquids taken up: these fluids, when of a neutral kind, as the chloride of soda, often equalled in bulk that of the wood itself. without causing any addition to its weight; the acid and alkaline fluids were leas abundantly absorbed, apparently from contracting the vassels by their astringent action. It is stated that the pyrolignite of iron effected the preservation of the substance when equal to less than a fiftieth of the weight of the green wood. These points are all separately treated in the original paper.
2. The hardness of the wood was considered by various workmen to be more titan doubled by the action of the pyrolignite.
S. The flexibility, (due to a certain presence of moisture,) was increased in a remarkable manner by the chloride of lime and other deliquescent salts, the degree of elasticity depending upon their greater or less concentration. As a cheap substitute for the above, the stagnant water of salt marshes was adopted, with a fifth of the pyrolignite, for the greater certainty of preservation. Pieces of prepared deal, 3 millimetres thick and 60 centimetres long, were capable of being twisted and bent in all directions, as into screws, also into throe circular coils; the wood immediately regained its figure when released: this condition lasted eighteen months, that is, until the time his paper was road.
4. The warping and splitting, principally due to the continual effect of the atmosphere in abstracting and restoring the moisture, was stayed by impregnating the wood with a weak infusion of the chloride, so as always to retain it to a certain degree moist; one-fifth of pyrolignite was also added in this case. The seasoning of the wood was also considered to be expedited by the process, and which was not found to interfere with the ordinary use of oil-paint, etc Large boards of the prepared wood, some of which were painted on one or both sides, and similar boards of unprepared wood, were compared; at the end of twelve months, the former were perfect as to form, the latter were warped and twisted as usual.
5. The inflammability and combustibility of the woods were also prevented by the earthy chlorides, which fuse on their surfaces by the application of heat, and render them difficult of ignition. Two similar cabins were built of prepared and of ordinary wood respectively, and similar fires were lighted in each; the latter was entirely burned, the other was barely blackened.
6. In respect to colours infused by the aspiratory process, the vegetable colours were found to answer less perfectly than the mineral, and the latter succeeded best when the colour was introduced at two processes, so that the chemical change, (that of ordinary dyeing,) occurred in the pores of the wood itself Odorous matters, could only be infused in weak alcoholic solutions, or essential oils: they were considered to be equally durable with those supplied by the hand of nature. Resins, similarly introduced, were found to increase amatingly the inflammability of the woods, and to render them impervious to water.
On the whole the method is considered to promise the means of working almost any desired change in the constitution and properties of woods, when the fluids are presented to them before the vitality of the tree has ceased. It is true we have as yet only two years' trial of these experiments, but they have been scientifically deduced, and their inventor is still engaged in prosecuting them. It is to be hoped, and also expected, that these interesting and flattering promises of success will be realised, and even extended, when tried by that most severe of all tests, time.*
* Since I collected the above particulars from the number for June, 1840, of Let Annales de Chimie el de Physique, pp. 113 to 157,1 have been favoured by J. E. Puddock, Esq., with a printed copy of the English translation of the original paper, preceded by the report of Messrs. De Mirbel, Arago, Poncelet, Audonin, Gambey, Boussingault, and Dumas, on the part of l'Academie des Sciences, confirming the value of the invention. In France, Dr. Boucherie has relinquished his brevet, and thrown the process open to the public in consideration of a national reward; and immense preparations have been there made for the employment of the preservative process for the French navy. In England Dr. Boucherie and Company have obtained two patents, and Mr. Puddock, their agent, has specimens of pine, plane-tree, etc., variously prepared and coloured, with the pyrolignite of iron, the prussiate of iron, the prussiate of copper, and various other metallic salts, etc.
See Appendix, Note B. page 459; and also Appendix, Vol. II. Note H, page 953.