COMPOSITION OF WOOD.

M. Chevandier, in 1844, published the results of analysis of five woods,— beech, oak, birch, poplar, and willow. The woods were reduced to powder, and desiccated at a temperature of 140° C., or 284° F., before being submitted to analysis. The results of analysis are given in table No. 148.

There is a remarkable nearness to identity in the composition of these woods, and also in the composition of the trunk and the branches.

The results show that the composition of woods is practically as follows:

Showing that there is only 56 per cent. of combustible matter, that there is a large quantity of oxygen, nearly sufficient to neutralize the whole of the hydrogen, and that there is only 2 per cent. of ash.

The above-mentioned analysis is corroborated by the analysis of M. Violette, who ascertained the composition of different parts of the same tree, desiccated at a temperature of 80° C. or 176° F., with the results given in the following table No. 149:

Here it appears that the composition of the wood is about the same throughout the tree, and that of the bark also; that the wood and the bark have about the same proportion of carbon, 49 per cent., but that the bark has more ash than the wood. The leaves and the small roots have less carbon than the wood,-only 45 per cent.; and more ash,-7 and 5 per cent.

The leaves when dried at 100° C. lost 60 per cent. of water, and the branches 45 per cent.

COMPOSITION OF ORDINARY FIREWOOD.

The respective percentages of the constituent elements of stacked wood in its ordinary state are, of course, reduced in amount when the water is taken into account. Thus, in the following analysis of ordinary firewood, containing 25 per cent. of moisture, the carbon constitutes only 37.5 per cent. of the fuel:

The density of a large number of woods has already been given in table No. 65, page 208. These values can, in most instances, only be given as approximate, for the density changes with the hygrometric condition of the wood. The specific gravity varies from 1.35, that of pomegranate, to 0.24, that of cork wood.

The density of the ligneous fibre of which wood is formed, has been ascertained by M. Violette, from a great number of observations. The samples of wood were reduced to powder in a mortar, and dried at a temperature of 100° C. He found that the fibre of all woods had the same density, and that its specific gravity was 1.50.

It is said that the quantity of intersticial space in a closely-packed pile of

Table No. 150.-OF THE WEIGHT AND BULK OF WOODS IN FRANCE.

wood, consisting of uncloven stems, is 30 per cent. of the gross bulk; for cloven stems, the intersticial space amounts to from 40 to 50 per cent.

A cord of pine wood,—that is, of pine wood cut up and piled,—in the United States, measures 4 feet by 4 feet by 8 feet, and has a volume of 128 cubic feet. Its weight, in ordinary condition, averages 2700 lbs.; or 21 lbs. per cubic foot.

A "corde" of wood, in France, has a volume of 4 cubic metres, or 141 cubic feet.

Firewood is measured, in France, by the voie, of which the volume is As the length of the billets is 1.14 metres, or 2 cubic metres, or 2 stères. 3.74 feet, the half-voie, or stère, measures 1.14 metres x 0.88 metre × 1 metre, equal to I cubic metre, or 35.3 cubic feet; and the voie is equal to 70.6 cubic feet in bulk. The weight of the voie of firewood, in Paris, is from 700 to 750 kilogrammes, or from 1544 to 1653 lbs., averaging 1600 lbs.

The voie of wood for making charcoal, in the forests of the Ardennes, weighs 1324 lbs. ; it consists of one-fourth oak and beech, one-fourth poplar and willow, and one-half elm. The hard wood for charring, of the forests of the Meuse, weighs 1653 lbs. per voie.

The above and other particulars given by M. Chevandier are collected and arranged in table No. 150, showing the weight and bulk of ordinarily dry wood.

QUANTITY OF AIR CHEMICALLY CONSUMED IN THE COMPLETE

COMBUSTION OF WOOD.

In terms of the average percentages of carbon, hydrogen, and oxygen, in wood, page 440, the quantity of air consumed is, by the rule 1, page 400,

1.52 (50+3 (6

or 80 ÷ 13.14= 6.09 lbs.

8

-)) = 1.52 × 52.625 = 80 cubic feet,

GASEOUS PRODUCTS OF THE COMBUSTION OF WOOD.

For one pound of dry wood the products are, by the expressions ( a ), (b), (c), page 401,

PRODUCTS.

Carbonic acid,...

Steam,

Pounds.

Per cent.

.50 ×.0366 = 1.83 or
6×.09

21.7

= 0.54 or

6.4

= 6.08 or

71.9

Nitrogen,.... (50 × .0893) + (6 × .268)+(1 x .01)

Total weight of products,

says 81⁄2 lbs. weight of products.

The volumes of the products at 62° are, by the expressions (e), (ƒ),

Total volume of products for 1 lb. of wood, 109.16 being about 13 cubic feet per lb. weight of gaseous products.

TOTAL HEAT OF COMBUSTION OF WOOD.

The total heat of combustion of dry wood is, by rule 4, page 406.

145 (50 +4.28 (6 − 41 ) ) = 145 × 53.745 = 7792 units,

8

which is a little more than half, or 541⁄2 per cent., of that of coal, and is equivalent, by rule 8, page 406, to the evaporation of 0.15 × 53.745 = 8.07 lbs. of water at 212°.

When the wood holds 25 per cent. of water, there is only 75 per cent. or three-quarter pound of wood-substance in one pound; and the total heat of combustion is 75 per cent. of 7792 units, or 5844 units, which is only 41 per cent. of that of average coal. Similarly, the equivalent evaporative power is reduced to 6.05 lbs. of water at 212°, of which the equivalent of a quarter of a pound is appropriated to the vaporizing of the contained moisture.

TEMPERATURE OF THE COMBUSTION Of Wood.

It is found, in the manner already shown, page 407, that 2.136 units raise the temperature of the products 1° F. The total heat of combustion, 7792 units ÷ 2.1363648° F.; and 3648 +62 37 10° F., is the temperature of combustion.

=

When the wood holds 25 per cent. of water, the weight of the direct products is 75 per cent. of 8.45 lbs., or 6.34 lbs.; and the total heat of combustion is 5844 units, of which 1116° (total heat of steam) ÷ 4 = 279 units, are appropriated to evaporate a quarter of a pound of water from 62°, leaving 5844-279 = 5565 units of heat available for raising the temperature of the gases. To raise the direct products one degree of temperature, there are required—

Then, 55651.721-3234° F., the temperature of combustion. It is only 88.6 per cent. of the temperature for perfectly dry wood.

In order to obtain the maximum heating power from wood as fuel, it is the practice, in some works on the Continent,-as glass works and porcelain works, where intensity of heat is required, to dry the wood-fuel thoroughly, even using stoves for the purpose, before using it.

WOOD-CHARCOAL.

When wood is exposed to heat it is at first desiccated and afterwards carbonized. Under temperatures up to 300° F., the wood is simply desiccated. Under temperatures over 300° the gaseous elements are driven off, until at 650° the wood yields a charcoal which is black, solid, and brittle. The gases are not completely driven off except under much higher temper

atures.

Wood charcoal, completely converted, is black, solid, brittle, and friable; it preserves the form and structure of the wood from which it is made. Though easily pulverized, it makes a very hard powder.