whereby the combustible material at the bottom of the chimney may be reached when the kiln is ready to be lighted. The passage is easily made by laying a straight pole on the ground and arranging the billets on each side of it in the way that a house is built up with cards, the pole being finally withdrawn.

The wood placed immediately against the chimney should consist of thin split pieces, dry enough to take fire readily. The best material to use, if obtainable, is the half-charred wood from a previously-burnt kiln. The packing near the chimney should be specially close, all insterstices being filled up with chips and shavings.

As split wood takes fire most readily on the split side, such wood should be placed with this side facing the chimney or downwards, as the case may be. This position of the pieces also helps the wood to be packed with greater ease and closer together.

The thickest pieces should be placed where the heat will be strongest and steadiest, that is to say, about midway between the chimney and the periphery.

4.-Covering the kiln.

In order to prevent the unchecked entry of air amongst the wood and to regulate the indraught during the carbonization, the covering put over the kiln should be such that, while it is easy to put on and take off or increase and diminish in thickness at any point, it should subside evenly as the kiln subsides, without falling away or opening out in rents and fissures. Experience has shown that it should always consist of two parts, (1) an inner layer composed of moss, sods of turf, green weeds, leafy twigs or green grass, and (2) an outer one of wet earth plastered or thrown over the first.

The inner covering must obviously be formed with some green, yielding fibrous material that does not take fire too easily and is at the same time able to hold together, however much the kiln may subside. Moss and close turf are the best for the purpose, and grass the worst. When grass is used, it ought to be short, soft, and fine. Whatever the material is, it should be the same throughout, otherwise the covering will both lie and subside unevenly.

For the outer covering we require a soft earth which will not form a too stiff and impermeable mass when moistened, will not harden and become full of cracks with the great internal heat of the kiln, and will not conduct heat too rapidly, but which will at the same time not lie so loosely as to fall away too easily and not be so porous as to be too freely permeable to air. Hence the best natural

material is loam containing a large proportion of vegetable remains, and the best material of all is the earth obtained from an old kiln. with its large admixture of ashes and fine cinders.

The first covering should be laid on beginning from the top, so that every portion of it may be supported and prevented from slipping downwards by the overlapping portion immediately below. It should be thick enough to prevent the earth of the outer covering from falling through amongst the wood and thus retarding and even preventing carbonization. In order to obtain a good indraught of air while the kiln is taking fire, the covering should not at first be put on too thick near the ground, and may even be left open at a few points there, such openings being stopped only when the carbonization is in full progress. Similarly, the vent of the chimney should also be left open until then.

The earth for the second covering should be freed of stones and other large fragments, which would destroy its even texture and let in unequal draughts of air. All clods should be broken up fine and the whole mass of material thoroughly well worked up until it is of uniform texture throughout. For the top of the kiln and those portions which have a gentle slope, the earth need only be moistened just sufficiently to keep the particles together, and then it is best thrown on with a shovel, so that it may get evenly distributed and ultimately rest safely at the proper angle of repose. For the steep portions, especially when grass is used inside, the earth should be made into a sort of thick mud and plastered over the grass. To prevent the earth from slipping off the steeper portions, it has to be propped up, especially near the ground. Two simple and effective modes of propping are shown in Fig. 67.

5.-Firing of the kiln.

If the kiln is to be fired from below, a torch is formed at the end of a long pole with grass and some highly inflammable chips. of wood. The lighted torch is inserted into the open passage left along the ground and pushed home against the bottom of the chimney, the pole being at once withdrawn. The draught along the ground and up the chimney carries the fire into the latter, from which then as centre it is able to spread outwards amongst the wood to be carbonized. The chips and other small fragments of wood placed in the chimney are quickly consumed; as they subside, fresh pieces must hence be gradually stoked in from the top. Ultimately, when the fire has become established and has begun to spread outside the chimney, this latter is filled up tight to the top with short billets of wood. If this last operation is not properly done, the chimney will soon become empty and cause the wood from the sides to fall in, thus leading to unequal subsidence and to the breaking up of the kiln. After the chimney is full, and even earlier if the wood is very dry or a strong wind is blowing, the tunnel along the ground is filled up with short straight billets well packed together. When the combustion inside the kiln is in full progress, the covering is completed over the open extremities of the chimney and tunnel. It requires some experience and judgment to close these openings at the proper time.

If the firing is to take place from above, a dishful of live coal is dropped into the chimney and the fire worked into the chips below with a thin bar of iron or even a green sapling. The fire is stoked from time to time with small pieces of dry wood, and, finally, when the wood in the chimney is in full combustion and the fire has reached the bottom, the chimney is filled up and closed in the same manner as in the method of firing from below, already described.

Firing from below is always troublesome, and the necessity of leaving a passage open along the ground breaks up the regularity of the stacking and renders the kiln liable to excessive subsidence on one side during the process of carbonization. To compensate for these drawbacks, it is more certain in its results, as unless the chimney is properly constructed and the fire skilfully stoked, fire lighted from above may fail to reach the bottom of the chimney, thereby rendering the carbonization of the lowest tier of wood a difficult matter, or at any rate entailing the overburning of the wood in the upper tiers after it has already become carbonized.

6.-The process of carbonization.

Whether the kiln is lit from below or from above, the whole of the wood in the chimney must be on fire before the burning is allowed to extend into the wood beyond.

Assuming that the wood in the chimney is fully ablaze first, the fire spreads thence outwards in the form of an inverted cone with an ever-widening base, until the whole of the kiln is on fire. This mode of progression of the fire is explained by the principle of the parallelogram of forces. The heated air and other gases tend to rise vertically, while lateral contact of the wood to be carbonized creates a tendency for the fire to extend horizontally. The resultant of these two tendencies is at first an oblique line not far removed from the vertical, and since the height to which the fire can extend is limited, and the temperature inside the kiln is constantly rising, the horizontal spread of the fire becomes more and more conspicuous until the whole of the wood at the bottom is carbonized. Thus the carbonization proceeds progressively from the top downwards.

During the process of carbonization large quantities of various. gases are given off. The whole of these gases being unable to leave the kiln, what remains behind condenses inside and trickles. down through the lower tiers of wood to the ground, where it is absorbed or from which it flows away through the foot of the kiln. While any piece of wood is being carbonized, first of all steam, the characteristic colour of which is a bluish-grey, issues forth. This is followed by russet-coloured vapours, which would, if condensed, yield pyroligneous acid, tar, wood-spirit, &c. When the carbonization is complete, if the burning is still continued, a clear blue flame proceeds from the carbonized wood, proving that only charcoal is left and is being burnt away.

Some of the gases given out by the carbonizing wood form explosive mixtures with the oxygen of the air; if they are not given a free vent, explosions will take place inside the kiln, disarranging the wood and causing the covering to burst.

7.—Conduct of the carbonizing operations.

If the formation and expansion of the fire-cone took place uniformly in every direction, all that would be required would be to keep the covering sufficiently pervious to air along the edge of the expanding cone (that is to say, at the level at which carbonization was going on) and to maintain it air-tight elsewhere, especially

over those portions of the kiln where carbonization was completed. Hence, the first thing that would be done after closing the flue of the chimney would be to pierce small vent holes, 1 to 2 inches in diameter and about 2 feet apart, all round the kiln a foot or so below the apex. The object of these holes, which could be easily made with a bamboo or sapling pointed with iron, would be to admit the necessary amount of air for carbonizing the wood at the top of the kila and allow the vapours and other gases of distillation to pass out freely. When the carbonization at this level was complete, which fact would be recognized by the pale blue colour and transparency of the smoke, the holes would be closed and a new line of them opened 1 to 2 feet lower down. In this way the charcoal-burner would gradually effect the carbonization of the entire kiln, the natural spread of the fire-cone being aided and regulated by means of the holes. He would then cover up the kiln as thickly as possible in order to stop all combustion, and in a few days the kiln would have cooled down enough for the covering to be taken off and the charcoal removed.

Under actual conditions such extreme uniformity is unattainable, owing to several causes of irregularity, the principal of which are the following:

(i). Inevitable defects in the packing of the wood, in consequence of which unequal draughts are produced, lead

ing to more rapid carbonization and, therefore, more sinking at some points than at others.

(ii). Differences in the amount of moisture contained in different pieces of wood.

(iii). Difference of density, even when only a single species is

used.

(iv). Movements of the atmosphere, from which the kiln can never be effectually screened.

(v). Unequal nature of the site.

(vi). Unavoidable errors of judgment, to which the most skilful are liable.

To overcome these various causes of irregularity requires no little skill and experience and unremitting care and watchfulness on the part of the charcoal-burner. To gain his end he must have recourse to one or more of the four following measures, which constitute the whole of his duties at the present stage of his work:

:

I. ERECTION OF A SCREEN ON THE WINDWARD SIDE OF THE KILN.— The cheapest form of screen is one of thatch supported against upright posts firmly fixed in the ground. But the first precaution