The Saltpetre Controversy

Since the introduction of gunpowder in the Middle Ages, saltpetre had become a material of prime military as well as industrial significance, for which supplies continued to depend on the natural formation of nitrate deposits on certain stones and earths (hence the name “salt of stone”). Such formations occurred abundantly in tropical climates and rather less readily in the temperate zones of Europe, where saltpetre makers exploited the nitrates that accumulated in the earthen floors of stables and such-like places, or which grew as an efflorescence on the walls of buildings. Whereas England, with her colonial possessions, could rely on imports from India, France was concerned to be self-sufficient and in the late eighteenth century made great efforts to develop production.

At the same time, advances in gas chemistry cast important new light on the manner in which saltpetre was formed. Hitherto, it had variously been imagined to derive from a supposed generation of nitric acid in the air, which then deposited to form nitrates; or from a transformation of vitriolic acid by an inflammable principle yielded by the decay of organic matter; or to be a vegetable product, formed by plants, from which it passed to the animals that ate them, finally to be released when the plant and animal materials decomposed (7). These vague old notions gave way to a much more precise interpretation when the discovery of oxygen and nitrogen led Lavoisier to recognise that nitric acid (the anhydride) is a compound of these gases. The formation of saltpetre was now revealed to depend fundamentally on an oxidation of nitrogen, and since this gas in its elemental form was known to be highly inert it seemed plain that the nitrogen involved must derive from organic compounds rather than from the atmosphere: organic decay was conceived to release nitrogen which in its nascent state joined with oxygen from the air. The longmysterious process of nitrification thus seemed at last to find a simple explanation as a process closely analogous to the slow oxidation of sulphurous peats, by which sulphuric acid and sulphates were formed.

This appealing interpretation was not to go unchallenged for long, however. It came to be vigorously disputed in the 1820s by a certain Longchamp, who offered instead a theory involving fixation of atmospheric nitrogen by a chemical mechanism in the soil (8). Although his ideas were dismissed by Gay-Lussac and others, Longchamp reopened a debate on nitrification that was to continue for another half century, and it was this that provided the immediate context for Kuhlmann’s researches.

Kuhlmann’s own interest in saltpetre dated from 1825, when he was consulted by the local gunpowder authority regarding some experiments to be conducted at the Lille refinery (9). In the work that followed, he recognised the presence of ammonia in the nitrifying materials, a discovery that provided the germ for the key idea he was to elaborate in the years to come: that ammonia is in some way involved in the nitrification process. Kuhlmann went on to develop a mechanism that accounted for the promotional rôle of ammonia in terms of what we would now call a catalytic reaction. His inspiration here came from the highly original theory of the lead-chamber process advanced in 1806 by Clément and Desormes (10). This had attributed the formation of sulphuric acid in the chambers to the oxidising action of cyclically regenerated nitrous gases—in effect the first intermediate-compound theory of catalysis—and Kuhlmann conjectured that in nitrification ammonia might play an analogous rôle. He suggested that carbonate of ammonia (from organic decay), by its alkaline action induced the oxidation of atmospheric nitrogen to form nitric acid and hence nitrate of ammonia; this then yielded up its acid to such other bases as chalk (carbonate of lime), thereby itself reverting to carbonate of ammonia ready for further action. The ammonia in this way served as an intermediary or vehicle (or to use an analogy Kuhlmann himself became fond of, as a shuttle) in the nitrification of the chalk, in a manner that explained how although present only in small amounts it could nevertheless produce unlimited effects. Characteristically, Kuhlmann appealed to experiment to demonstrate the plausibility of his scheme, observing a reversible reaction when he brought together carbonate of lime and nitrate of ammonia in solution. Cyclical reactions of this kind were to hold a deep fascination for Kuhlmann, and he came to see them as important “levers” underlying many of the processes of nature as well as a number of industrial operations (11).

A Platinum Catalyst

By the time he came to publish his researches in 1838 Kuhlmann had come to see a further rôle for ammonia, however, and it is in this that the historical significance of his work primarily lies. He now believed that ammonia, formed by organic decay (and perhaps in other ways), not only served as a promotional agent but was also itself the immediate substrate from whose oxidation the nitric acid resulted. In this way he offered a third and original theory of nitrification, distinct both from Longchamp’s fixation of nitrogen and from the Lavoisian direct oxidation of organic matter. It was to find support for such a theory that Kuhlmann turned to experiments on the laboratory production of nitric acid under the influence of a platinum catalyst. And he now made his discovery that ammonia could indeed be oxidised to nitric acid by passage with air over heated platinum sponge (or even under the influence of a red hot porcelain tube), similar reactions also occurring with volatile ammonium compounds and with cyanogen. On the other hand nitrogen gas gave no reaction under such conditions, nor was it oxidised by nascent oxygen from manganese dioxide, points supporting the greater likelihood of his own theory over Longchamp’s.

How Kuhlmann came to think of experimenting with platinum is uncertain. It is likely, though, that the immediate inspiration owed less to contemporary studies of platinum, on which Kuhlmann seems to have been incompletely informed, than to a discussion of nitrification by Dumas, in his “Traité de chimie”, which Kuhlmann acknowledged as an important influence on his own nitrification studies. A further factor that Dumas had there pointed to as promoting the natural oxidation of nitrogen was a supposed “condensation” of the gases by the action of porous bodies. (Saltpetre makers had long known that nitrification required materials in porous form—chalk, for example, rather than marble.) Dumas had cited a striking example of the powerful effects of porosity in the case of charcoal, which, if first impregnated with hydrogen sulphide gas and then immersed in oxygen, condensed the oxygen with such energy as to induce reaction of the gases even in the cold. And he added, “It is also known that almost all porous bodies determine the combination of hydrogen and oxygen in the manner of platinum sponge” (12). Kuhlmann similarly believed porous bodies to have a significant influence on nitrification, and he seems to have regarded platinum sponge simply as a porous body active in particularly high degree, making it a useful laboratory analogue of natural conditions.

One remaining question led Kuhlmann to pursue further his researches with platinum. He wanted to know how extensive might be the formation of ammonia itself in nature, as a basis for nitrification, and for guidance on this he turned again to laboratory experiment. His attempts to form ammonia by direct combination of nitrogen and hydrogen over a platinum catalyst met with no success (only with the much later work of Haber was such a reaction achieved with significant yield). But he found that under the influence of platinum, ammonia could indeed be formed by the reaction of hydrogen with nitric acid or with the various nitrogen oxides. From these and other reactions he concluded that ammonia was formed whenever hydrogen encountered nascent nitrogen in suitable conditions, and was encouraged to believe that the formation of ammonia in nature was widespread.

Kuhlmann’s theories seem to have quickly become common currency. He claimed in 1840 that “My theoretical ideas on this point [the formation of nitrates from ammonia] are today generally adopted by chemists” (13), and he was particularly gratified that they had found the sanction of Liebig. Of course there were possible objections, which Kuhlmann did not hide. In particular, even platinum produced no detectable conversion of ammonia at the temperatures of natural nitrification. To this he could only answer by appealing to slow reaction over time. We now know, of course, that the answer to this and to other puzzles was to lie not simply in time but in microbiology. Nevertheless, by calling attention to the rôle of ammonia Kuhlmann did make a significant contribution to the elucidation of the modern nitrogen cycle, according to which nitrogen passes through the various major forms of elemental gas, ammonia, nitrates and organic materials, in a complex series of interlocking transformations. It is a cycle that would have delighted Kuhlmann!

Industrial Applications

Although Kuhlmann presented his researches with platinum as subsidiary to his larger theoretical concerns, he also clearly recognised their interest in their own right:

Kuhlmann here underestimated earlier progress, but his work did provide an impressive demonstration of the versatility and potential of platinum and he was congratulated on the range of his researches by a commission of the Academy of Sciences (15). Kuhlmann was naturally alert to possible industrial applications. The catalytic formation of hydrogen cyanide in some of his experiments suggested one such manufacture (developed a century later to supply the plastics industry (16)). In other experiments he had found that vinegar could be catalytically converted to ethyl acetate, which he thought might open up a cheaper route to the production of alcohol. But the applications he evidently took most seriously, since for these he secured patents, were the manufacture of nitric and sulphuric acids (17).

On nitric acid manufacture Kuhlmann wrote prophetically in 1838:

His patent of 22nd December 1838 briefly indicated the passage of ammonia, air and steam over a heated bed of “porous bodies”, preferably platinum sponge. Whether he made any attempt to develop such a process industrially we do not know. If so it seems to have been short-lived, for when in 1846 we find him again speaking of the national importance of producing nitric acid from ammonia, there is no longer any mention of platinum. Instead his attention had now turned to manganese dioxide, which his subsequent researches had shown could provide the basis for another of those cyclical mechanisms in which he took such pleasure, serving as a vehicle for the progressive oxidation of ammonia by cycling between higher and lower oxidation states (19). As is well known, the industrial development of the platinum process had to await the further researches of Ostwald in Germany in the early 1900s and the stimulus of World War I.

Kuhlmann did, however, attempt to exploit the patent he simultaneously secured for the manufacture of sulphuric acid by the catalytic oxidation of sulphur dioxide. This process had, of course, been discovered and patented in England seven years earlier by the obscure Peregrine Phillips, though whether Kuhlmann knew this is uncertain. At all events Kuhlmann’s patent reveals a more sophisticated plant and a deeper awareness of the problems, and whereas Phillips planned to produce only ordinary acid, Kuhlmann perceptively saw that a chief virtue of the catalytic process was its ability to furnish the fuming acid, oleum (it was primarily as a source of oleum that it was to come into general use half a century later). Kuhlmann’s plant clearly borrowed from existing lead-chamber and Leblanc soda technology. Sulphur was burned in a furnace evidently similar to those he used to feed his chambers. After passing through flues to deposit dust, the mixture of sulphur dioxide and air was directed through red-hot pipes — made of platinum, cast iron, or stoneware — containing platinum sponge suitably dispersed among fragments of glass or other inert material. (Alternatively, a heated reaction vessel could be used, with the platinum arranged in layers on a series of perforated plates.) The gases, now charged with sulphur trioxide, were next passed through a series of large water-cooled lead pipes, in which at points nearest the furnace fuming acid was collected and a little further on acid of lower strength. To facilitate the condensation and promote the draught through the system a current of steam was here introduced. Condensation was completed by passage through a long series of demi-johns part-filled with water, terminating in a chimney to create the draught (an arrangement commonly used in France to capture the acid fumes from soda furnaces).

A supplementary patent that closely followed affords further insight into Kuhlmann’s work. He here emphasised that his process could burn materials other than sulphur — such as pyrites or hydrogen sulphide — which he hoped might result in easier reaction, by avoiding the dust in the gases from commercial sulphur. He also now extended his patent to cover the use of oxygen gas as an alternative to air, for he had noted that pure oxygen gave a faster reaction and more complete condensation. In the event Kuhlmann was not to succeed (20). We are told that his large-scale trials failed, partly because of the great difficulty he encountered in satisfactorily condensing the acid gas when mixed with a large excess of air, and partly because he found that the platinum rapidly lost its power. Moreover, to obtain fuming acid it proved necessary to dry the air, creating an added complication. Only from the late nineteenth century was the modern contact process successfully developed, when ways were found to obtain sufficiently pure sulphur dioxide to avoid catalyst poisoning, and when the proportions of the gases were properly regulated for optimum reaction.

Other Fields of Research

Besides providing the context for his work on catalysis, Kuhlmann’s investigations of nitrification were also the starting point for some of his other principal fields of research. It was a natural progression to examine the role of nitrogen in crop nutrition, and in a series of papers beginning in 1843 Kuhlmann became an early contributor to the grand debate on the nitrogen question that so preoccupied agricultural chemistry in the mid-nineteenth century (21). Contrary to Liebig, he believed in the importance of nitrogenous as well as mineral fertilisers, supporting his position with comparative field trials near Lille in the early 1840s. He considered that plants assimilated their nitrogen as carbonate of ammonia, and to explain how such a volatile compound could be held available in the soil he conceived another of his ingenious cyclical reaction schemes. Although he was mistaken in the details of his agricultural theories, his work perceptively highlighted some of the principal questions that were later taken further in the well-known researches of Boussingault and Ville (22). Another of Kuhlmann’s major interests, the chemistry of building materials, had its origin in the efflorescences on walls that were commonly ascribed to nitrification (Kuhlmann in 1840 showed that they were often in fact carbonates and sulphates).

Much of Kuhlmann’s research was closely linked with the problems and opportunities of his locality, but he kept in touch with the larger world of science through his contacts with the Academy of Sciences — of which he was elected a corresponding member in 1847 (23) — and through the friendships he enjoyed with prominent scientists in France and abroad. The most illustrious of these was Liebig, whom Kuhlmann no doubt met when they were both students in Paris. Kuhlmann published German versions of many of his papers in Liebig’s Annalen, and Liebig dedicated to Kuhlmann one of his agricultural writings. Kuhlmann’s house near Lille was described as something of a scientific gathering place. Dumas, conveying news of Kuhlmann’s death to the Academy in 1881, recalled how:

It was at such a gathering in 1850 that Dumas, then Minister of Trade and Agriculture, had presented to Liebig the decoration of the Legion of Honour (25). Dumas similarly spoke of Kuhlmann’s factories as a magnet for industrialists. From an early date, he tells us,

One can only regret that a man whose career illuminates so instructively the chemical world of his day has never been the subject of a full biography.