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Taxonomy is the science or practice of categorising things in an orderly fashion. That usually means hierarchically, so that a top-level grouping might contain several smaller groups, each of which contains several sub-groups. There's a great economy to this approach, and it meshes well with the nature of many of the things we might need to classify, notably life. The term was first used in 1735 by the great taxonomist Carolus Linnaeus in his grand work on classification, 'Systema Naturæ'.

This kind of hierarchical categorisation results in a tree-like structure, with limbs, branches, twigs and leaves spreading out from a single point of origin. That's handy, because if you know what high-level category something fits into, you might already know a lot about it - if I tell you I found a Devil's coach-horse last night, that really won't tell you much at all unless you happen to be familiar with such beasts. If I tell you that it's a kind of insect, you should immediately know it has six legs and an exoskeleton, and you might guess (correctly) that it probably has wings. If I tell you it's a beetle, you'd probably also have some idea of its body shape, and maybe you would infer that it has hardened wing-covers.

There are many pit-falls to describing things this way, though. Perhaps most importantly, the general rules describing members of a high-level category often have many exceptions, and it is not always easy to know whether to assume a tendency is shared by any given member of a class. It's tempting to say that mammals are hairy creatures which give birth to live young, for example, but platypuses lay eggs and many adult whales are hairless. There may also be things that we would like to place in more than one higher-level class, which some kinds of taxonomical approach make impossible; among living things, it is not unheard-of for individuals from two evolutionary branches to cross-breed and produce something new. Such hybrids are more common in the broader sense of taxonomy - there are a number of sciences which overlap several larger fields of study, for example, like physical chemistry, and any categorisation of cooking styles is going to need to account for multiple geographical origins.

There are several variations on the details of biological taxonomy, but the classic hierarchy goes as follows. A mnemonic for this, should you need one, is 'King Philip Calls Out For Good Soup'. You could also have 'Kangaroos Prefer Chilled Outback For Grooving Soulfully'.


The Five Kingdoms are Monera, Fungi, Protista, Plantae and Animalia.


We're in chordata, along with all the other vertebrates, plus hagfish, lampreys and a couple of other sort-of-fishes.


Mammalia is one of the five traditional classes of vertebrates.


We're in the order of primates; cats, dogs, bears, mustelids and hundreds of other land mammals are in Carnivora, although a few of them, like the giant panda, are mostly herbivorous.
We're in the family hominidae, along with all the other great apes. Most family names end in -ae, often attached to the name of an emblematic order within that family - Corvidae, for example, includes the crows and other birds of the genus Corvus, but also magpies, nutcrackers and so on. Similarly Brassicaceae includes the true brassicas like cabbage and turnip, but also rocket (arugula) and thousands of other plants distributed among hundreds of other genera.
Humans belong to the genus Homo, of which we are the only extant species. All the other hominins, like the neanderthals, australopithecus, Homo floresiensis and so on are dead now, although we may have inherited some of their genes. The genus gives the first, capitalised part of the Latin name of a species, which is followed by the specific species name and subspecies if necessary, which should be written in lower-case. The whole name is conventionally written in Italics.


A species is sometimes defined as a population of organisms capable of interbreeding and producing fertile offspring, but there's a neatness implied by that definition which does not correspond with reality. When lions and tigers breed, for example, some of their liger offspring turn out to be fertile, and there are many cases of supposedly distinct species interbreeding in nature to produce fertile offspring. It is not always clear whether two species are capable of interbreeding, if only they had the chance and they got in the mood. One fun difficulty with the idea of a species is a ring species1, where two related animals might not be able to interbreed, but they could breed quite successfully with intermediate animals who breed with each other. The definition is also quite useless for bacteria and other life-forms that only reproduce asexually.


Modern humans all count as Homo sapiens sapiens; there was once also Homo sapiens idaltu, and there's a case for counting neanderthals too, given there is some evidence that they interbred with our Homo sapiens sapiens ancestors. That would make them Homo sapiens neanderthalensis.


Usually only plants are described in terms of varieties. Tea can be either Camellia sinensis var. sinensis or Camellia sinensis var. assamica, for example. There are many other ways to describe sub-groupings below the subspecies level - forms, subforms or cultivars in botany, breeds of domesticated animals, and so on. We also talk about races of humans, of course, but the idea has little scientific validity.

Some other ways of categorising

Taxonomic ranks not mentioned above include subfamily, tribe, subtribe, infraorder, infrafamily, infraclass, superorder, superfamily, subphylum and various others - perhaps most importantly, the three 'empires' imposed above the kingdoms, namely the Eukaryotes (anything with a cell nucleus), Prokaryotes (bacteria) and Archaea (ancient micro-organisms once classed with the bacteria, but which turned out to be far too weird for that to make sense). Apparently the extra levels are usually the result of trying to minimise the disruption to existing classifications when a recognised grouping needs to be moved up or down a level. Also, botanists prefer to talk about divisions rather than phyla. I can only assume that most of this stuff makes some sense to someone.

There is some debate about whether biological taxonomy should always be strictly phylogenetic - that is, classifications should always assume the existence of shared ancestors, an approach known as cladistics. For the most part, evolutionary trees fit in well with trees of characteristics, but there have been a large number of cases where groupings of organisms based on shared characteristics turned out not to be closely related at all - such groupings are said not to be monophyletic, if they include organisms from a separate evolutionary branch. For example, although the butterflies are monophyletic, the moths are not - while we might be able to make some generalisations about moths, they are really just those members of Lepidoptera that are not butterflies2. On a much grander scale, the reptiles turn out to be a surprisingly messy grouping; in order to use only strictly monophyletic categories, we would need to count birds (and hence dinosaurs) as reptiles3, or else put crocodilians and birds in another class together, since they are more closely related to each other than they are to the likes of lizards. We might also divorce the turtles and a few others from the rest of the reptiles. This is all very inconvenient, given how many textbooks and so on have been written on the assumption that it is legitimate to talk about 'reptiles', and this question is not likely to be settled for a while yet.

In the computing world, tags are sometimes used instead of, or alongside taxonomies. The main difference is that tags are not usually hierarchical in any sense. Relatedly, the only limit on what tags something can have is usually numerical - you might be restricted to ten tags per photo, for example, but in purely taxonomic classification you can usually only belong to one of each level of category (one phylum, one order, one species and so on). Tags tend to be very informal, with users allowed to tag their contributions with whatever they fancy, but they are sometimes used very specifically so that computers can make sense of them.

In many contexts, it probably makes sense to give up on categorisation altogether and just think of things in terms of adjectives. This may be at the expense of being able to make inferences which are likely to be broadly correct in any particular case based on only a few words, but it has the considerable advantage of acknowledging how many things (and people) don't fit neatly in any of the available categories, and avoiding the misconceptions that arise from expecting them to.


1Mark Ridley's Evolution on ring species
2Bug Guide on order Lepidoptera
3Tree of Life on Amniota

Electrolysis is the separation of chemicals using an electrical field. 'Lysis' literally means 'breaking down', and 'electro' means it is done with electricity. The process has enormous practical importance, as well a huge role in the history of chemistry - in the space of less than two years in the first decade of the 19th century, Sir Humphry Davy used electrolysis to isolate six new chemical elements1 - first sodium, then potassium, calcium, barium, magnesium and strontium.

Electrolysis works because many chemicals decompose into ions - charged particles - when they dissolve in water, or they melt. Substances like this are known as electrolytes. In an electric field, the two kinds of ions will travel in opposite directions, each building up on one or the other electrode. Ionic compounds, like salts, always split up into positive and negative ions when they enter a liquid state, since they are made of ions in the first place. Acids also split into ions in water. Every acid includes at least one hydrogen atom, which is prone to detaching from whatever molecule it's part of and attaching itself to a nearby water molecule instead. The hydrogen leaves behind its electron when it does that, so the rest of the original molecule is left with the electron's negative charge, while the water molecule becomes a hydronium ion, with the positive charge of an added proton. In an electric field, the two ions then go their separate ways.

When the positive ions (cations) arrive at the negative electrode (the cathode) they pick up electrons and return to their uncharged state. Similarly, when the negative ions (anions) arrive at the the other electrode (the anode) they deliver their spare electrons, completing an electric circuit. This is how solutions of electrolytes conduct electricity. The newly uncharged atoms are left free to form new bonds - atoms of gas will join up into small molecules which build up to make bubbles, while solids will accumulate around the electrode.

Electrolysis is used in industry whenever electrolytes need to be separated into their component parts, and that includes a lot of different circumstances. Chlorine is extracted from sea water in large quantities, for example, to be used as a bleach and a component in products like PVC. Aluminium is extracted from ore by electrolysis, as are all of the most reactive metals, the ones that can't be extracted just by heating them with carbon. Submarines can stay underwater for months at a time by using electrolysis to separate water into hydrogen and oxygen2, and since both chemicals have numerous other uses, this process also has some industrial importance. Electroplating and anodizing are also based on electrolysis - one usually used to deposit a layer of pure metal onto a surface, the other used to deposit a layer of oxides.


2Edexcel GCSE Science

Limestone is one of the most common kinds of sedimentary rock - about a tenth of all sedimentary rock is limestone. It's just as well that it's common, because it's incredibly useful not just as a building material in its unprocessed form, but also as the main raw ingredient of cement (hence also concrete) and quicklime, which among other things is a standard ingredient in glass production. It's also crushed up and used as a foundation to build roads on, ground up finely to add to toothpaste or excessively acidic soil, and used in a wide range of other industrial processes. Britain alone digs up 76 million tonnes of limestone every year1 - that's more than a tonne for every person on the island!

Limestone is also known for its fossils - something it has in common with some other sedimentary rocks, notably shale. Many of the best-preserved fossils found have been discovered in limestone, and in fact limestone itself is largely composed of the fossilised remains of coral and other marine invertebrates. Limestone is susceptible to erosion by acid and to a lesser extent plain old water. This is usually a very slow process, though - most cave systems have been carved out of limestone bedrock by rainwater over the course of millions of years, but the Great Pyramid of Giza is still in pretty good shape. However, acid rain accelerates the process hugely, and conserving buildings and monuments made of limestone has become a much bigger concern since the rise of mass industry.

Limestone often contains some silica, but it is mainly composed of calcium carbonate (CaCO3), as are chalk, marble (which is limestone that has been transformed by extreme heat and pressure) and limescale (which is largely limestone that has been dissolved in the water supply and then deposited on the side of your kettle). That is why limestone is so vulnerable to acid, including the carbonic acid you get when carbon dioxide (CO2) dissolves in water - the calcium carbonate reacts quite strongly with many acids, turning into chemicals which then dissolve readily. That in turn is one reason why the rising concentration of CO2 in our atmosphere is a cause for concern: as the oceans absorb CO2, they become more acidic, making it impossible for many marine organisms to form the shells and skeletons they rely on.

Those same shells and skeletons would eventually become limestone if they got the chance, and part of the reason they won't is that the processing of limestone into cement contributes enormously to the rise in global CO2. It takes high temperatures to bring about the necessary chemical reactions, which also release CO2 from the rock. It has been estimated that between 2%2 and 5%3 of global carbon emissions or more come from the manufacture of cement, and that figure is still rising. There are several possible ways of reducing these emissions4, many of them already widely adopted in rich countries but less so in developing countries where cement production is growing rapidly. None of them are used universally, and development of some of the more radical possibilities for emissions reduction has been slow. After all, the costs of emitting CO2 are almost always borne by someone else.

The chemistry of cement production is quite straightforward, and in its basics it was known to the ancient Romans and others. Limestone or another source of calcium carbonate is heated to around 800°C, causing it to glow brightly and decompose into CO2 and calcium oxide, also known as quicklime. This is a powerful alkali, with many uses. Portland cement is made from a mix of this and powdered clay; add in sand and pebbles, and you have concrete. The ancient Romans used volcanic ash rather than clay, which may have something to do with why the Colosseum has stood up so well5. When quicklime is mixed with water, it reacts to form calcium hydroxide (slaked lime - see also 'a small green vial in a Bengali supermarket'), letting off a great deal of heat in the process. Civilisations in many parts of the world used slaked lime cement as a building material, as much as 5000 years ago - often holding together blocks of limestone.


1 Edexcel GCSE Science
2 National Cement Production Estimates: 1950 - 2010
3 Carbon Dioxide Emissions from the Global Cement Industry
4 Decoupling carbon emissions from cement production
5 The Secrets of Ancient Rome’s Buildings

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