Saltpeter is a salt from which may be drawn by analysis a fixed alkali similar to the salt itself, and also a volatile acid, which forms the principal constituent. From this acid are derived those properties which distinguish saltpeter from any other salt. These properties are to crystallize in needles, to excite a sensation of freshness when touched by the tongue, and to decompose from contact with a burning phlogistic, to which its acid unites with a report.
This salt forms on the surface of the ground, in caves, cellars, stables, and other covered places, impregnated with vegetable and animal substances, to which the air has access. Old walls formed of materials that have resisted the action of fire, such as plaster and lime, also contain large quantities of saltpeter.
The air is the principal agent in the formation of the salt. This is not because it contains the salt in itself, but because it develops saltpeter by a sort of fermentation which it excites in certain materials, thus deriving it from the latent principle of niter therein contained. There is an analogy in connection with the fermentation of the juice of grapes. The resulting spirits are in no way derived from the air itself, but the action of the air is absolutely necessary for the development of the spirits from the juice, and no artificial means can secure the result, apart from this operation by the air.
It is possible to increase the quantity of saltpeter produced naturally in the soil, by soaking it with liquids derived from the putrefaction of animal and vegetable substances. It is necessary, however, that the soil should be covered against rain, which would dissolve and carry off the saltpeter as fast as formed; and also that the place should be supplied with free air, in order that the salt may condense and take shape. It is required in addition that the earth should be spaded often, for the sake of a free circulation of the air, which, as it penetrates, develops the nitrous principles. The more the soil is stirred, the more it will produce saltepeter. Where it is left undisturbed, the salt forms only on the surface. A good soil, properly treated, becomes freshly charged with a full quantity of the salt in a period of three years. After the removal of the deposit, it will begin to show traces of the new supply at the end of two months.
Common salt, whatever its source, has the property of producing saltpeter when it is spread over the ground in a covered place that is exposed to the air. The product will be more abundant if the soil contains vegetable and animal matter. There is, indeed, much evidence to prove that saltpeter and common salt are the same in essence, and that the only difference is in the quantity of the volatile acid contained by each, which is larger in saltpeter, due to the more active fermentation.
Two observations tend to establish the truth of this conjecture. The first is that saltpeter resembles common salt, more or less closely, according to the measure of its acid content. When the acid is entirely removed, the saltpeter is almost identical with common salt. On the other hand, common salt nitrifies in proportion to the fermentation set up by this acid gas. The second observation has to do with the fact that saltpeter is never formed without the presence of common salt, even in earth from which all trace of both salts has been previously removed. It is, therefore, reasonable to believe that common salt is merely an imperfect niter.
Saltpeter is removed from the soil by means of a cold washing in lye. To facilitate the flow of water, and to prevent the earth from clogging the vent of the vat, a piece of barrel bottom is placed within, just in front of the vent, crosswise, and the interval is filled with small stones or fragments of old plaster. Ashes are placed in the vat to almost one-sixth of its height. At the same time as these serve to remove the fats from the saltpeter, they also supply to the acid portion that fixed alkali of which it may be in need. Care must be taken that the quantity of ashes be not too large, since an excess supply would act as an absorbent. When duly arranged, the vat is filled with saltpetrous soil, or plaster, which has been crushed and passed through a screen. When the material is earth, it is essential that it should have been well enriched. It is also necessary that it should be placed in the vat very carefully, since, if compressed to any extent, it will prevent the water from flowing, or cause it to flow with extreme slowness. The earth is covered with straw in order to avoid compression when water is poured in. For the same reason, the water is introduced very gently, until the quantity is sufficient for dissolving the saltpeter. In order to charge the water fully with niter, it is let flow from the first vat into the second, from the second into the third, from the third into the fourth. This series is sufficient to charge the water to' the utmost with saltpeter, if the soil be of good quality. The water is drawn out from the fourth vat into a boiler over the fire. While being boiled, the water is care-fully skimmed, until it takes on sufficient consistence to congeal when a drop is let fall on a plate. It is then transferred to another vessel, where it is left for a half hour in order to deposit its impurities. Then, before growing cold, it is poured into basins, where the saltpeter forms in crystals as the liquid cools. The water is drained from the basins on the fifth day. It is still impregnated with saltpeter, and to it is given the name mother water. This mother water, together with the skimmings, is poured over other soil that is to be washed in the lye, thus adding to its richness. The saltpeter that results from this series of washings is called the first batch. This batch produces always a certain quantity of common salt, which settles to the bottom of the boiler, and is removed by the skimmer before the water is drawn out. It should be noted that this common salt, when it is found in a considerable quantity, as in the first batch, is deposited always before the saltpeter. But, when it is found in a small quantity, as in the second and third batches, it is the saltpeter that forms first, while the common salt remains dissolved in the mother water. If it is found that the common salt is constantly formed first, the fact shows that a larger quantity of water is required in order to hold it in solution than is required in order to hold the saltpeter in solution, for the reason that the common salt does not dissolve more readily in boiling water than in cold water, while saltpeter is dissolved twice as quickly by heating the water. Since this is the case where the quantity of common salt is considerable, why should it not be the case when the quantity is small ? Can it be that the smaller quantity of common salt, being distributed through a large quantity of saltpeter, the particles find themselves too widely separated among those of the saltpeter to unite themselves and thus to crystallize?
The saltpeter is purified by melting it in water and boiling it until a film is formed. A little alum is thrown in while it is thickening, both in the first vat and in the others, and from this comes a thick scum, which is skimmed off. This is the best procedure for removing the fatty materials and for purifying the liquid. Glue is sometimes used instead of alum, but with less satisfactory effect. The water is drained off on the third day and thrown over other earth.
The third batch, or second purification, is made in the same manner. Care should be taken, before using the vats for a new supply of earth, to pass pure water through them in order to complete the removal of any saltpeter. This water, called the washing (lavage), is used for the next washing with lye.
Saltpetrous earth gives commonly a gros 1 of saltpeter to the pound. Earth of the very best sort gives sometimes a gros and one-half.
The vessels used for the purifying of the saltpeter should be deep rather than wide. Much of the water is dissipated in boiling, and this wastage is proportioned to the surface of the water.
Saltpeter should be of the third batch, when it is to be employed in the making of powder, or any of the mixtures used in fireworks. When it is desired for fireworks, it must be ground in a mortar, or crushed on a wooden table with a wooden pestle, and afterward passed through a silk screen. The finer and drier it is, the better its effect.
Saltpeter by itself is not combustible. When it is kindled and burns, this is by reason of the material with which it is brought in contact, as, for example, when it is placed on a plank or on charcoal. Then, the inner air which it contains is developed by the action of the fire, and thus excites the particles of sulphur contained in the material, penetrating the pores so that they burst into flame, carrying with them the particles of saltpeter separated by their action. If, on the contrary, saltpeter is placed on something incombustible, and denuded of its sulphur, as upon a shovel or a tile heated in the fire, it simply melts without becoming kindled, and is reduced to a liquid form. It takes body again on cooling and forms a salt that is hard and more solid even than the original. The name given to it in such form is rock saltpeter. This is prepared sometimes to be used as a powder for hunting, by melting it over the fire without the addition of water. A little sulphur is thrown over it during the melting in order to remove any fatty material. The sulphur burns along with whatever remains of a fatty sort while the saltpeter itself is not kindled. But it must be noted that this operation can not be repeated without weakening the salt. This is because, when no more fatty material remains, the acid gases have too great facility in escaping.
1 One-eighth of an ounce.