nitre daily, but forty pounds may be considered as the average product of the labour of three men at those works which I had an opportunity of visiting.

The workmen being badly provided with tools and apparatus, desert a rock whenever its size or hardness renders it difficult for them to manage it, and go in quest of a new establishment. Several caves and rocks which these strolling chemists have deserted, still contain many thousand pounds of nitre. These men are continually searching for masses of pure nitre, or rich veins of ore, by which much of their time is unprofitably dissipated. Still however most of our saltpetre-makers find it their interest to work the sand rock rather than the calcareous caverns, which last yield a mixture of nitrate of pot-ash and nitrate of lime. The rock saltpetre is greatly preferred by our merchants and powder-makers, and commands a higher price.

Mr. Barrow, in his travels through the southern parts of the continent of Africa, discovered native nitre, which is probably similar to the rock saltpetre of Kentucky. But Bowles, Dillon and Townshend assure us that those districts in Spain, which afford nitre most abundantly, contain neither chalk, limestone, gypsum, nor any other calcareous substance. The nitrate of pot-ash is obtained there by filtrating a certain kind of black mould which will continue for ages to yield annual supplies of it, together with muriate of soda, sulphate of magnesia, nitrate and sulphate of lime. Here then appears to be such a relation existing between the different saline substances, both acids and alkalies, that the causes which produce one of them, owing to some yet undiscovered circumstance, regularly produce all the rest. According to these authors the same mould will continue forever to yield these salts annually. This observation if correct, would induce us to believe, that both acids and alkalies are wholly formed from atmospheric air and not from the soil; as the soil would certainly be exhausted if any considerable portion of it entered into the composition of either the acids or alkalies, and would soon lose its power of attracting from the air the other constituent principles of the salts. Both in Spain and India, we are informed, that the mould which for fifty years in succession has yielded nitre, still con

tinues to afford it in undiminished quantities. But how shall we reconcile this fact with that before related concerning the production of nitre in the cavities of calcareous mountains which are, in many instances, so closely filled up with clay, that the air can have no access, from which every ray of solar light is excluded, and where the temperature can never exceed 57° of Fahrenheit? Is it absolutely certain, that nitre formed by natural processes so very dissimilar, possesses no properties necessarily resulting from the circumstances attendant on its formation? That all the nitrates of pot-ash with which we are acquainted, have certain properties in which they agree, is unquestionable, but the same may be said of lime and barytes, of soda and pot-ash, and many other substances, which in the early ages of chemical science, were probably identified. Hoffman, long ago proved, that nitrate of pot-ash afforded an alkali very different from that of wood ashes or salt of tartar. The observations of so distinguished a philosopher deserve much attention, and his experiments if repeated by modern chemists could scarcely fail of affording important results: that the sand rock saltpetre differs from that procured from the calcareous caverns, in the form of the crystal, in hardness and dryness, is known to all who deal in that article, and every powder-maker affirms that it makes better gun-powder. Whether this superiority is owing merely to its greater purity or exemption from an admixture of nitrate of lime, or whether the constituent acid and alkali are modified in some unknown manner, is yet altogether problematical. Chaptal, Thouverel, Guyton, and indeed most of the modern chemists, suppose, that pot-ash is a compound of lime and hydrogen, and that lime itself is formed of carbon, azote and hydrogen, and consequently that pot-ash consists of hydrogen, carbon and azote. Mr. Guyton thinks that soda is composed of magnesia and hydrogen, and that magnesia is a compound of lime and azote, and therefore, that soda is made up of hydrogen, carbon and azote. He is then of opinion that pot-ash, soda, lime and magnesia are nothing more than varied forms and proportions of the same constituent ingredients, differing from each other in the quantities and forces of attraction. This opinion de

rives great probability from an experiment of Bishop Watson, by which it would appear, that soda was actually converted into pot-ash. It is likewise corroborated by the apparent conversion of lime and soda into pot-ash in our calcareous caverns, and by the change of what the workmen call cubic salts, into nitrate of pot-ash. Thouverel affirms that he witnessed the real conversion of washed chalk into pot-ash, in his experiments on nitrous vapours, and Chaptal observed the same phenomenon when exposing chalk to the vapours of putrid bullock's blood. Now as the nitric acid combines readily with lime, soda and magnesia, as well as with pot-ash, it may be easily conceived, that it still retains its affinity for those substances, in every form which they may assume, whilst changing into each other, and that the "tertium quid" formed by the union of nitric acid and lime in the intermediate stage between lime and pot-ash, may possess properties very different from nitrate of lime or nitrate of pot-ash. The same may be remarked with regard to soda and magnesia. Here every chemist will recollect the ingenious observations of Dr. Mitchel, concerning nitric acid and the essential differences between that substance and septic acid at the moment of its formation. No person can doubt of the possibility of charging nitrogene with different portions of oxegen. The explosive efficient property of nitre may depend on a certain dose of this principle. But even admitting that pot-ash and nitric acid never vary in their nature, it may still be contended, that powder-makers have no means of ascertaining what proportion of acid and alkali that nitre ought to contain, which would form the best gun-powder. And whilst this is confessed, it surely can avail us little, to be very scrupulous in the adjustment of the proportions of the nitre to the charcoal and sulphur. The consumers of pot-ash, in every part of the world have remarked varieties in the quality of the salt, for which no particular cause can be assigned. It is very much to be regretted, that a regular series of experiments has never been instituted, to discover what kind of ashes would yield an alkali most proper for the formation of nitre. Charcoal should be examined with a similar view. Mr. Coleman has published experiments and remarks

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on this subject, (Philosophical Magazine, V. IX. p. 355.) which appear to me very interesting. By his mode of distilling wood in iron cylinders, he deprives it completely, of all the volatile oil, hydrogenous gas and pyroligneous acid. The charcoal prepared in this way, possesses uniformly the same properties, and by the employment of it, the powder now used in the British ordnance, is increased in strength one third.

The gun-powder manufactured in the United States, is said to be defective, from a disposition either to effloresce or deliquesce. The salts most liable to effloresee are such as have soda for their base. In many of our saltpetre caves, small quantities of the sulphate of soda have been discovered, which for want of sufficient care or skill in refining, are suffered to remain with the nitre. The disposition to efflorescence appears to be directly opposite to that of deliquescence; as in the one case, the air has a stronger affinity for the water of combination of the salt than that which exists in the salt for the water; in the other case the salt attracts moisture from its combination with air. It would seem then, that, as the air is capable of depriving the sulphate of soda of its water of combination, and as nitrate of lime attracts moisture from the surrounding air, it is possible, that a mixture of these two salts may be so made with nitrate of pot-ash, that the nitrate of lime may deprive the sulphate of soda of its water of combi- . nation, and in consequence of this addition of water, deliquescence may ensue, even when the atmospheric air and moisture are excluded. If Count Rumford is correct in supposing that the explosive force of gun-powder depends not upon the evolution of permanently elastic fluids or gases; but upon the almost instantaneous conversion of the water of combination existing in the powder, into steam by the caloric resulting from its inflammation; this explosive force may be diminished for want of that water which might have escaped by efflorescence, or on account of the slow combustion of the powder consequent on deliquescence.

A concern for the glory and defence of our country should prompt such of our chemists as have talents and leisure to investigate this interesting subject. In 1776, at the request of

M. Turgot, the celebrated M. Lavoisier was appointed superintendant of the French national powder works, and with what success he executed the duties of his important commission the history of their subsequent naval campaigns have sufficiently evinced. The efforts of European chemists, seem to have been principally directed to the removal of the marine salt which the nitre of Spain and India contains in great quantities. In the nitre of Kentucky, I have never detected a particle of that salt, and I am confident, that if any is found in it, the proportion must be very inconsiderable indeed. The rock saltpetre I am persuaded, would, with very little refinement, make gun-powder capable of retaining its efficient properties during the longest voyages, as I have never discovered, in that species of nitre, the smallest tendency either to deliquescence or efflo

rescence.

It will be observed, that I have not in this paper, hazarded any opinion with regard to the formation of nitre in our sand rocks. I freely confess that I have no theory on that subject which is satisfactory to my own mind, I am even disposed to suspect, that our greatest chemists have still much to learn with regard to this salt, so valuable in time of peace, so indispensable in time of war.

No. XL.

An Essay on the vermilion colour of the blood, and on the different colours of the metallic oxides, with an application of these principles to the arts. By Samuel F. Conover M. D.

Read June 20th, 1806,

On the Vermilion colour of the blood.

THESE subjects have excited the attention of some of the most eminent philosophers of the last and present century, though little progress was made in the explanation of these phenomena, previously to the institution of the pneumatic philosophy, when truth burst forth upon mankind, dispelled the

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