On comparing the two systems, there cannot for one moment be a doubt as to the immense advantages of the latter over the former. As an example, in the refinery where this new process is carried on, the result (that is to say, pure saltpetre) is obtained in one day instead of six, with less than one half the amount of labour and coals.
On reflection, the reason of the great gain of time by this process will suggest itself. In the former method, when allowed to remain quiet, the crystals formed are very large, and the spaces left in them always contain a certain amount of mother-water, which necessitates its being crystallized three times to perfectly free it from the liquor. In the latter, the crystals are so minute that there is practically no space for the mother-water to collect; consequently, by careful washing, the saltpetre is obtained perfectly pure.
Wood charcoal is the woody fibre that remains after the liquid and more volatile parts have been driven off by the fire in the process of charring. The temperature resulting from the combustion of charcoal is much higher than that from burning wood, in consequence of the absence of the large quantity of water which wood contains, amounting to between 50 and 60 per cent.; the object, therefore, of charring wood is the removal of moisture, and also, what is of great importance, the expulsion of those matters contained in it which become volatile before they are burned, thus rendering a large amount of heat latent. The woods generally used in this country in making charcoal for gunpowder are the alder, willow, and dogwood. There are about 60 acres of wood grown for charcoal at Waltham Abbey. The alder is cut every eight years, and the willow in six years. It is used after one year. Other woods are sometimes used by English and foreign manufacturers, but none produce a powder of such quality as obtained from the above. It is usually considered that better charcoal is distilled when the wood is allowed to season for a time; but recent experience has shown that wood only lately cut and peeled, after being desiccated in a hot chamber, will make equally good charcoal with that which has been seasoning for three or four years.