**chapter is being re-written**
Dear Tristan and Lauren,
It is autumn in Europe. I still working in the curing department of Uncle Jeppe’s bacon factory. I have found in Andreas not just someone to learn from, but his curiosity makes him the ideal partner on my quest of discovery.
On the other hand I also know, as another good friend that I met in Denmark has told me, if I have 5 years left on earth and I have to do something new, it will be best if I spend the first four years preparing for it.
Working through the complexities of the matters at hand will have a reward in my life, but also in yours if you would choose to follow on this exciting path. It really seems like the most complicated industry in the world.
Every day is spent on solving a giant mathematical equation.
The friends name is Martin Sauer. His dad has been in the pork business all his life and has traveled extensively through Africa. I was telling Martin one day all that Jeppe and Andreas has been teaching me. Martin laughed and said that I will spend a lifetime on these matters and must not try and remember everything. When I get back home I will have ample time to go over my notes. More than this, I may learn many new things that may seem to contradict some of the things that I’ve learned. I am looking forward to meet his father because I heard that he met Livingston.
It was a strange thing that Martin told me. The thing about learning things that may seem contrary to what I was taught at first. What was even stranger was that the following Monday, Jeppe told me that this was true when it comes to saltpetre and nitrite. Remember that I told you that it is the key ingredient in curing bacon?
This statement is not entirely accurate. The real magical ingredient in bacon is salt!
So opened up to me another vast world. The world of salt.
At night, after supper, we still read Foods by Edward Smith, written in 1867. He writes, “the oldest and best known preserving agent is salt, with or without saltpetre.” (Smith, E, 1867: 34) (1)
Remember the quote from the American Encyclopedia of 1858. It said that “Very excellent bacon may be made with common salt alone, provided it is well rubbed in, and changed sufficiently often. Six weeks in moderate weather, will be sufficient for the curing of a hog of 12 score.” (Governor Emerson . 1858: 1031) (1)
So Jeppe started last Monday, during my lunch time lessons to discuss the matter of salt. As was the case with saltpetre, a world started to open up for me that I did not know existed.
That white substance that I used so many times back in Cape Town and now, here in Denmark, without giving a second thought as to the nature and the power inherent in it.
As I could have guessed, the story of the use of salt goes back much further even than the story of humanity.
A study has shown that sodium chloride was probably collected and stored by one of the oldest species of the genus Homo, Homo Habilis who loved between 1.4 and 2.4 million years ago. (Munas, F.; 2014 :213) Evidence suggest that our closest extinct relative, Neanderthal who lived between 40 000 and 400 000 years ago, dried meat as a way to preserve it. (anthropology.net) It is easy to imagine them learning this very early on by observing meat scraps that remained at a killing or slaughtering site and freeze-dried or simply dried out and lasted longer than the fresh meat that was removed from the carcass. Whether they used salt is not known, but if Homo Habilis did and if they dried their meat in order to preserve it, it is easy to think that Neanderthal used salt also. Linking salt with nutrition and preservation of meat is easily identified by simple observation of nature. A society living next to the sea or any other salt source such as a salt pan or a salt spring, would have seen this and have incorporated it into their culture.
There is clear evidence that using salt to preserve has been practiced since before the last ice age, some 12 000 years ago. Salt deposits in the hills of Austria and Poland, the shores of the Mediterranean and Dead Sea, the salt springs and sea marches across Europe and Asia (Bitterman, M, 2010: 16) and on the vast plains of Africa would have provided salt to cultures across the world. It seems as if there is not a time known to humans, when salt was not used to amend our diet and quite possibly to preserve meat. Humans dried their meat and salted it and this salting was called curing (2). Adding salt to meat evolved into an art from the earliest time known to us.
As our way of life evolved, we domesticated our food sources. We started with the fig, probably many years before we did the same to grain. Archaeologists found domesticated figs dating back to 9400 BCE. Sheep were domesticated around 8000 BCE, cattle and pigs around 7000 BCE. (Bitterman, M, 2010: 17)
In general, we can say that sometime between 15 000 and 5000 BCE, human society’s need for salt increased rapidly as we needed salt for ourselves and our domesticated livestock. The livestock had to supplement their diet with salt and we needed it for curing and preserving foods, tanning hides, producing dyes and other chemicals and for medicine. “We evolved with a physiological requirement for salt; our culture was born from it. Access to salt became essential to survive. Salt localized groups of people.” (Bitterman, M, 2010: 17) Curing took meat which we culled from nature and brought it into culture. (Laszlo, P, 1998: 14) It turned the art of preserving into an expression of community and “togetherness” by transforming “preservation of food” into culinary delights of great enjoyment.
There is evidence that by 1,200 BCE, another great traders civilization of ages past, the Phoenicians, were trading salted fish in the Eastern Mediterranean region. (Binkerd, E. F.; Kolari, O. E. 1975: 655–661) Saltworks were one of the main features of their settlements in Labanon, Tuniaia, Egypt, Turkey, Cyprus, Crete and Sicily.
By 900 BCE, salt was being produced in ‘salt gardens’ in Greece and dry salt curing and smoking of meat were practiced and documented. (Binkerd, E. F.; Kolari, O. E. 1975: 655–661)
Ancient records of 200 BCE tell us that the Romans learned how to cure meat from the Greeks and further developed methods to “pickle” various kinds of meats in a brine marinade. Salting had the effect of reddening the meat and the report of this observation became the first recorded record of the colour effect of saltpeter. (Binkerd, E. F.; Kolari, O. E. 1975: 655–661)
Marcus Porcius Cato (234 BCE – 149 BCE) or Cato the Elder was a Roman statesman, who devoted himself to agriculture when he was not engaged in military service. He recorded careful instructions in dry curing of hams. (Hui, YH, et al, 2001: 505)
In his Latin work, De Agricultura (On Farming), written in 160 BCE, this Roman statesman and farmer, gives an ancient recipe for curing pork with salt.
“After buying legs of pork, cut off the `feet. One-half peck ground Roman salt per ham. Spread the salt in the base of a vat or jar, then place a ham with the skin facing downwards. Cover completely with salt. After standing in salt for five days, take all hams out with the salt. Put those that were above below, and so rearrange and replace. After a total of 12 days take out the hams, clean off the salt and hang in the fresh air for two days. On the third day take down, rub all over with oil, hang in smoke for two days…take down, rub all over with a mixture of oil and vinegar and hang in the meat store. Neither moths nor worms will attack it.” (economist.com)
Cato may have imitated a process whereby hams are smoked over juniper and beech wood. The process was probably imported by the Roman gourmets from Germania. (economist.com)
Phoenician ships spread the technology of salt making across the Atlantic, to Spain and as far north as England. India, China, Japan, and Africa developed their own salt industries.
Hardly a region on earth or a civilisation could be found who did not produce salt. Salt was taxed, traded, used as currency and consumed on a global scale. (Bitterman, M, 2010: 17 – 25)
A Dutch legend says that the curing of herring was invented by Willem Beukelsz around the early 1300’s. Whether this is entirely true or not, we know that the Cossack’s produced cured caviar. The Romans used a sauce called garum on their food. Garum was made among other with brine (salt solution). (Laszlo, P, 1998: 5, 7, 11)
The Danes are great traders and Copenhagen is a key center for trading Saltpeter.
The domestication of our food sources, the need for preservation and the technology to produce salt developed hand in hand as features of the spread of human culture and civilization.
What was the mechanism that made salt such an effective preservative? What exactly is a salt and how did we unravel its composition? In order to understand the mechanism of salts preservative power, we must first know what a salt is. Initially, only common salt was known and a handful of others, including niter. (Leicester, H. et al.; 1952: 75)
THE PRINCIPLES OF PARACELSUS
The multi-talented physician, Paracelsus (1493 – 1541), saw salt as one of the three principles namely salt, sulfur, and mercury. This contemporary of Copernicus, Leonardo da Vinci and Martin Luther, widely regarded as the founder of toxicology, imagined that in every object the principle responsible for its solid state is salt; a second principle (sulfur) is responsible for its inflammability or “fatty” state; and a third (mercury) is responsible for its smoky (vaporous) or fluid state. (www.scs.illinois.edu)
A “fire analysis” was done on a body to isolate the salt. Many cultures, across the world developed the concept which in the West became popular by the teachings of Empedocle (c. 490 – c. 430 BCE) that all matter comprise off earth, air, fire and water. Earth being the passive element which remains after the fire analysis. By the late 16th and early 17th century, it was realized that the solid which remained after the fire analysis, could further be divided into SALT, which could be removed by diluting it in water and what remained was called EARTH. EARTH was the non-volatile residue left after burning and SALT diluted in water and had a saline taste. While Luther’s reformation gained a foothold in Central Europe and Da Vinci was painting the Last Supper in the Convent of Santa Maria delle Grazie, in the city of Milan, Paracelsus taught that salt, thus extracted, demonstrated the presence of the universal salt principle and that it was this principle that was behind any body’s solid state and its resistance to fire. The criteria for SALT being that it is diluted by water and have a saline taste is the reason why liquid acids also came to be known as salts. (Siegfried, R.; 2002: 76, 80, 81)
The Bavarian alchemist and chemist, Johann Glauber (1604 – 1670) reported in the mid-1600’s on the mutual destruction of acids and alkali’s. Hailing from a poor background, he traveled and learn alchemy from various laboratories and teachers. At age 20 he was afflicted with stomach problems, probably after contracting spotted typhus which, for some time after contracting it, causes nausea and vomiting. His travels across Europe brought him to the city of Vienna where the residents recommended that he drinks from a local miracle spring which will restore him to health again. Glauber was skeptical about the idea but did it in any event. He drank from “Hungarian Spring.” (5) His appetite returned and soon he was in good health again. (www.thechemicalengineer.com and Siegfried, R.; 2002: 77)
This unlikely outcome pricked his curiosity. Locals believed the healing power to be due to the presence of saltpeter in the water. He spent the winter, evaporating water from the spring and analyzing the salt. What he found was not saltpeter, but sodium sulfate. (www.thechemicalengineer.com) Sodium sulfate is a mild laxative and he would later market it as Glauber’s salt. The study of niter would preoccupy him for years to come, but in his mid-fifties, sodium sulfate would take front and center stage in his research work. He got carried away a bit when naming it “Sal Mirabilis Glauberi.” He overstated its healing power so dramatically that he received considerable opposition from his contemporaries who regarded him as a dreamer and charlatan. (The Guardian, 1934) An interesting fact to note for our continued interest in the study of saltpeter is the ease with which he was able to test for saltpeter by the mmid-1600s. He was able to produce nitric (HNO3) by applying sulfuric acid to saltpeter. Later he made potassium carbonate (K2CO3, which is produced by burning saltperter with charcoal) and nitric acid (HNO3, which they made by distilling satlpeter with fullers earth) from saltpeter and was able to combine these two, potassium carbonate and nitric acid to yield saltpeter, showing his thorough grasp of the acid-base composition of salts. (www.encyclopedia.com) Nitric acid or spirit of niter is a volatile acid and fixed niter (potassium carbonate) is a solid caustic. From this, Glauber concluded that twofold substance, containing both and acid and an alkali.
It was the fact that he could reconstitute saltpeter with fixed potassium carbonate and nitric acid that showed him that saltpeter was not the ultimate universal solvent which alchemists were looking for and that he claimed to have found. It was this disillusionment with niter that caused him to turn his attention back to his Sal Mirabilis Glauberi or sodium sulfate. The formulation of Glaubers salt is Na2SO4·10H2O.
Glauber’s contribution to the study of salts was considerable. He eventually expanded the list of known salts far beyond common salt and niter. His work on the mutual destruction of acids and alkali’s was done with Otto Tachenius. (The Age, 1975) The effervescence that was observed when acids and alkali’s are mixed became the standard way of judging the alkali or acid quality of a body. The idea was simple. If a known acid is added to something and it effervesced, the other body is an alkali and vice versa. The early chemists did not see this effervescence as gas being liberated, but as some kind of a vigorous strife. Acid-base reactions usually produce heat and it is easy to see how the bubbling was seen as “boiling.” (Siegfried, R.; 2002: 76)
Glauber introduced the idea that acids could combine with metals or alkali’s to form a salt. The mechanism behind the combinations is seen by him as a certain associative principle which he called “Gemeinschaft.” In his work he insisted on an accurate description of the technical operation at work. (todayinsci.com) (6)
The medical chemist, Van Helmont, created a model in the 1600’s and he postulated that this acid-alkali reaction is part of animal digestion. One of his students by the name of Sylvius, progressed this idea and thought of all bodily functions as acid-alkali reactions and bodily fluids are either acid or alkali. (Siegfried, R.; 2002: 76) Robert Boyle (1627 – 1691) refuted this suggestion by showing some of the many exceptions, yet the reaction between acids and alkali remained “the most familiar among among real laboratory material.” By the end of the 1600’s there were three well-known mineral acids, namely, spirit of niter (nitric acid), spirit of salt (hydrochloric acid), and vitriolic acid or spirit of sulphur (sulfuric acid). (Siegfried, R.; 2002: 77)
When exactly the acid-alkali reaction was first studied or who identified it has not been uncovered. It is one of the great untold stories of science. What we know is that during the 1600’s, the term alkali was first used in the Arab world and referred to a vegetable alkali. Books from that time refer to a plant that was called kali and contained potassium carbonate. The ancients obtained it from leaching ashes from the burned remains of the plant. (Siegfried, R.; 2002: 77)
By the 1600’s, they recognized another common alkali namely alkali of tartar. This was obtained from the residue in wine barrels and today we know that it was also potassium carbonate. In the early days, they retained the particular name to link it to where it was found. (Siegfried, R.; 2002: 77)
The saturation point of an acid-alkali reaction was believed to be the point where the effervescence stops. At this time, air was not a recognized chemical element and nobody had the idea that the effervescence could be due to the liberation of air. Robert Boyle developed an alternative way to test for acidity or alkalinity and to determine the saturation point of acid-alkali reactions. In his time it was already well known that acids turn green vegetable colour, red. Boyle was probably the first to observe that an alkali would turn the blue to green. It took until 1750 before the use of colours to identify acids and alkali’s became commonplace in the scientific community. (Siegfried, R.; 2002: 77)
By the beginning of the 1700’s, a third class of salts was well established namely neutral salts, containing both acid and alkali. These salts did not effervesce with either acids or alkalies. This category was soon expanded to include the combination of an “acid with earths and metals as well as with alkalies.” (Siegfried, R.; 2002: 77)
Earth is what remains at the end in a distillation vessel and can not be dissolved in water. There has been a debate whether different earths exist as was the case with different salts and sulfurs (or oils). Scientists were able to, for example, identify a certain kind of earth derived from stones, coral or seashell. It is able to dissolve in acid and when strongly heated, it forms a powdery residue that absorbs water. These were known as absorbent earths. (Heilbron, J. L.; 2003: 226)
The great French chemist, Guillaume- François Rouelle (1703-1770), under whom Lavoisier studied, published some of his most important work between 1744 and 1754 on the subject of salt. At this time, salt was defined as water-soluble, saline-tasting solid. He suggested what is essentially, the modern understanding of salt (sel). He was the first to distinguish between acid, neutral, and basic salts. That is, anything that would fix the acid into a solid state whether an alkali, an earth or a metal. (Siegfried, R.; 2002: 79)
Another scientist of this time who devoted much of effort to the study of neutral salts was the German scientist, Wilhelm Homberg (1652 – 1713). He spent his adult career in Paris. By the time of his most important work, the following has been established during the previous century. The neutrilization of an acid by and alkali. The mutual destruction of properties. “The available salts were the vitriolic or spirit of sulphur, spirit of niter, spirit of sea salt, and acid of vinegar. The only alkali was salt of tartar (potassium carbonate). ” (Siegfried, R.; 2002: 86)
From the wok of Homberg, it is clear the implicit principle behind his experimental work was the conservation of weight. An example is an experiment he did in 1699 where he set out to determine the amount of volatile acid salts contained in its solution. “He added acid to weight quantity of alkali until the alkali was saturated, presumably judging that point by the cessation of effervescence. The resultant salt was then dried as thoroughly as possible and weighed. The increase in weight Homburg took to be the weight of the real acid in the solution used. He carried out this procedure using spirit of niter, oil of vitriol, aqua regia, and distilled vinegar, compiling tables of his results. He took no account of the loss of carbon dioxide that escaped from the alkali, of course, for he knew nothing of it.” We see “how futile these attempts were until glasses were recognized as part of chemistry and techniques were developed for measuring and isolating the different kinds of air.” (Siegfried, R.; 2002: 88)
It was about this time when the analysis/synthesis cycle was established as a way to confirm the qualitative composition of a body. If elements obtained from an analysis by fire could be reformed again into the original body, it would prove that the analysis was done correctly and the elements that constitute it, identified. “The permanent secretary of the Academy (of science in France), whose duties included the writing of summaries of the worthy articles in the Mémoires commented that “One is never so sure of having decomposed a mix into its true principles as when with the same principles one can recompose it.”” (Siegfried, R.; 2002: 90)
Before the 1700’s, scientist could not distinguish between the different alkali metals. Sodium and potassium were often confused. Potassium was produced artificially by slowly pouring water over wood ashes and then drying the crystal deposits. Some of these metals were also found naturally on the edges of dried lake beds and mines and sometimes at the surface of the ground.
Henri-Louis Duhamel (1700 – 1782) realised that certain metals had similar characteristics. He studied samples of salts found in nature and produced by people artificially. This included the study of saltpetre (potassium nitrite), table salt, Glauber’s salt, sea salt and borax. (Krebs, RE, 2006: 51) He discovered sodium carbonate and hydrochloric acid, a solution with a salty taste, in 1736. (Brian Clegg, rsc, chemistryworld)
Humphry Davy, an English Chemist, was the uniquely talented young man who changed history when he isolated sodium and potassium in 1807.
He had the first direct electric current generator at his disposal, the electric battery that Alessandro Volta had invented in Paris in 1800. Davy ran an electric current through caustic soda (sodium hydroxide) and was able to isolate sodium from it. He did the same for potassium, isolating it from potash.
Chlorine was already being produced through electrolysis by the decomposition of sea salt by the electric current. Caustic Soda and chlorine had many applications by the end of the 1700’s.
Fats were processed with caustic soda to produce soap. Fabrics were being bleached with chlorine, a process discovered by Berthollet. (Laszlo, P, 1998: 50)
In 1807, Humphry Davy found that the “muriate of soda” produced by burning sodium in a vessel full of chlorine was chemically identical to salt. (Brian Clegg, rsc, chemistryworld)
Humphry wrote in 1840, “Sodium has a much stronger attraction for chlorine than oxygen; and soda or hydrate of soda is decomposed by chlorine, oxygen being expelled from the first, and oxygen and water from the second.”
“Potassium has a stronger attraction for chlorine than sodium has; and one mode of procuring sodium easily, is by heating together to redness common salt and potassium. The compound of sodium and chloride has been called muriate of soda, in the French nomenclature; for it was falsely supposed to be composed of muriatic acid and soda; and it is a curious circumstance that the progress of discovery should have shewn that it is a less compounded body than hydrate of soda, which 6 years ago was considered as a simple substance, and one of its elements. According to the nomenclature which I have ventured to propose, the chemical name for common salt will be sodane.”
“Common salt consists of one proportion of sodium, 88, and two of chlorine 134; and the number representing it is 222” (Davy, H. 1840: 247)
The importance of this is that the knowledge that the salt used for preserving food is mainly sodium chloride, existed from the early 1800’s.
It was now possible to analyse the nature of sodium chloride and the other kind of salts that exist. The nature of the composition of salt that has been dissolved in water and the interaction between salt and meat and between salt and microorganisms such as bacteria that are present in meat.
It is possible to look at everything that make up sea salt and salt from inland springs and dry salt beds and we can begin to understand and appreciate the effect of salting meat and how it happens that it preserves the meat.
It was found that salt had other metals and compounds of a diverse, but consistent nature. These other elements present in salt that we find naturally on earth, do they impact on the curing process at all? And if so, how? (4)
As I have learned, answering these questions would be very important in order to improve the consistency and the quality of the bacon we cure.
It has been a very busy week-end. Martin took me around the old city. I am excited to learn more about Livingston from his dad since I have heard that Livingston has seen many of the great salt beds and natural salt springs in Africa. So much work has been done by scientists in Europe and America, in India and China. Has there been any discovery in Africa that can help enhance our understanding of the effect of salt on curing in order to improve our processes and procedures and ultimately our products?
I am excited for the new week. Martin agreed to take me along when he is meeting with ship owners who buy their bacon. I hope to learn much from him. He has been trading with many of the Europeans who have moved into the north and central parts of Africa.
I continue to miss you guys. Keep my letters. Read them often. Work hard in school. Help Ava around the house.
Warm greetings, with love!
(c) eben van tonder
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(1) We have seen how pervasive the occurrence of nitrate is on earth. One expect to find it in every natural salt spring, salt marsh, dry salt lake and in sea water. “Some curing” will take place with almost any natural salt. However, it has been shown that bacon that was produced with either no nitrites or nitrite levels of 15 ppm, “off-flavours were high and increase rapidly. A significant reduction in off-flavours in pork during storage was observed when nitrites were added > 50 ppm.” (Rahman, SM, 2007: 307)
Salt springs, analysed in South Africa contained as little as < 1 mg/ L of Nitrate (H)
This does not correlate with the statement by Smith and the American Encyclopedia about the fact that normal salt was equally successful in curing meat.
Adding salt enhance the flavour, but it also accelerate lipid oxidation, even at low levels of addition. Lipid oxidation leads to off flavour development in meat that does not contain any nitrites. Even a 0.5% addition of sodium chloride significantly increase lipid oxidation when added to restructured pork chops and pork sausage patties following freezer storage. (Pearson, AM, et al, 1997: 269)
(2) ‘The binomial name Homo neanderthalensis – extending the name “Neanderthal man” from the individual type specimen to the entire species – was first proposed by the Anglo-Irish geologist William King in 1864 and this had priority over the proposal put forward in 1866 by Ernst Haeckel, Homo stupidus. The practice of referring to “the Neanderthals” and “a Neanderthal” emerged in the popular literature of the 1920.” (Wikipedia. Neanderthal)
(3). Meat curing can be defined as the addition of salt to meat for the purpose of preservation. (Hui, YH, et al, 2001: 505)
(4) It turns out that “food-grade salt of the highest purity should be used in meat curing practices. Impurities such as metals (copper, iron, and chromium) found in natural salt beds, salt produced from salt springs or sea salt accelerate the development of lipid oxidation and concomitant rancidity in cured meats. Although salt may be of very high purity, it nonetheless contributes to meat lipid oxidation. Nitrite and phosphates, help retard this effect.” (Hui, YH, Wai-Kit Nip, Rogers, R. 2001: 492)
(5) One source says that this took place in Naples and not Vienna. There could have been been a spring in Naples, called Hungarian Spring. Drug Discovery: A History by Walter Sneader, 2005, John Wiley and Sons Ltd., p. 64 puts the place where he became ill as Vienna, which fits the Hungarian Spring designation much better.
(6) There is at least one source that puts his invention of Glaubers salt at 1659. However, from The Renaissance of Science: The Story of the Cell and Biology, by Albert Martini, 2015, Abbott Communications Group, Glauber discovered a simple method of manufacturing hydrocloric acid in 1625, when he was 21. He did this by combining sulfuric acid with table salt (sodium chloride). Sodium sulfate salt was produced by this reaction which is Glauber’s salt, a mild laxative. On the other hand, the 1659 date for the invention of Glaubers salt may refer to the publication in 1658 of his Tractatus de natura salium. In 1660 a second part was added to the Miraculum mundi. Is is apparently only here when Glauber started to make “sal mirabile” (Glauber’s salt) his main focus of his work, replacing niter. (www.encyclopedia.com).
The Age, Melbourne, Australia, 3 June 1975, Page 1
Davy, H. 1840. The Collected Works of Sir Humphry Davy …: Elements of chemical philosophy. Smith, Elder & Co.
Gouverneur Emerson . 1858. The American Farmer’s Encyclopedia. A O Moore.
Heilbron, J. L.. 2003. The Oxford Companion to the History of Modern Science. Oxford University Press.
Hui, YH, Wai-Kit Nip, Rogers, R. 2001. Meat Science and Applications. Marcel Dekker, Inc.
Krebs, RE. 2006. The History and Use of Earths Chemical Elements. Greenwood Press.
Laszlo, P. 1998. Salt, Grain of Life. Columbia University Press.
Leicester, H. M., Klickstein, H. S.. 1952. A Source Book in Chemistry, 1400-1900. Harvard University Press.
Munas, F.. 2014. Mission To Earth. New Authors Press.
Pearson, AM, et al. 1997. Healthy Production and Processing of Meat, Poultry and Fish Products, Volume 11. Chapman & Hall
Rahman, SM. 2007. Handbook of Food Preservation. Second edition. CRC Press.
Siegfried, R. 2002. From Elements to Atoms: A History of Chemical Composition, Volume 92, Issues 4-6. American Philosophical Society
The Guardian, London, Greater London, 13 Oct 1934, page 13
Figure 1: http://www.livingstoneonline.ucl.ac.uk/biog/dl/bio.html
Figure 2: http://www.france24.com/en/20121031-bulgaria-oldest-prehistoric-town-discovered-europe-provadia-solnitsata-ancient-salt-site-archaeology/
Figure 3: http://www.culturaitalia.it/opencms/museid/article.jsp?language=en&article=/en/contenuti/percorsi/percorso197/capitolo_0004.html&tematica=&selected=
Figure 4: http://www.engineerswalk.co.uk/hd_walk.html