Crystals (1958)

I suppose most of you have seen a crystal at one time or another probably in a natural history museum or in a chemical bottle here's an unusually big Crystal dug out of a Clay Pit in Brazil here's a smaller one broken from the side of a Clift in the Rock in Arkansas but crystals aren't always big beautiful things like this as a matter of fact crystals are everywhere the salt you use to flavor your food for example it consists of little crystals of sodium chloride you can see their shapes by looking at them through a magnifying glass little cubicle crystals of sodium chloride snow here are some enlarged pictures of single snowflakes each is a single Crystal of water here's a crystal I like especially well probably because I made it myself it's a crystal of Alum as a matter of fact you can make a crystal just like it I put some directions in the film notes in case you want to do it yourself you buy the Alum you need at a drugstore and grow the crystal in a mason jar if you do do this I'm sure you'll be very much impressed by one thing the crystal keeps growing maybe for a week it keeps getting bigger but all the time it has plain faces meeting in sharp edges you can see I haven't ground the faces on this Crystal or polished them at all this is just the way the Beast grows it almost seems as if you were getting something out of nothing something with a shape all its own which it decides all by itself throughout its career in order to grow a single perfect Crystal as big as this you have to take pains and let everything happen slowly but you can see what's actually happening by letting things happen a little bit faster and looking at the business under the microscope I'm going to take a solution of Alan which is just itching to deposit crystals and make a little pool of it on the microscope slide and then add a few little crystals of Alum to give the Alum something to deposit on and then get myself in focus and let her rip there they are you can actually see them grow if I turn now to a higher magnification the little crystals keep pretty much the same shape as they grow just getting bigger naturally they all sit down on the microscope slide on one of their biggest faces so you see the shape of the crystal depends mostly on what the crystal is made of and not on its size so long as it's free to grow in all directions now crystals don't only grow out of solutions they grow out of gases snowflakes water crystals grow that way and one of the places one of the most important places where crystals grow is out of melted materials when you cool them here I've got another powder you can get at the drugstore called SEO which melts to convenient temperature I'll put a little of it on a microscope slide then I'll carry it over here to the hot plate and melt it and now let's go over here to the microscope there's one growing turning very slow slowly in the liquid as it grows because it hasn't quite sat down on the slide yet it seems almost to push those flat faces ahead through the part that's still melted of course you know that the whole Crystal isn't moving and shoving its front along the liquid next to the face is becoming solid and adding itself to the face but it's doing this in such a way that the stuff added on also forms a flat face so the crystal keeps its shape while it grows now I'll melt this back carefully on the hot plate so as to leave just a little rounded blob of Crystal that rounded blob will be a single Crystal but it won't have flat faces and I want you to see what happens when this cools down again under the microscope there it goes growing the faces it wants to have filling in the hollows and squaring off the rounded Corners so that its boundaries are all flat again notice now what happens when two crystals bump into each other so to speak they don't push each other around they just stop growing where they meet because they've used up all the stuff to grow with there so the boundary between two crystals isn't necessarily one of the natural faces on either of them the boundary line just depends on how the crystals happen to come together when the whole thing is finally Frozen you have a dense mass of crystals maybe no single one of them shows any of its natural faces but the whole mass is made up of crystals just the same almost all rocks are made up this way granite's a good example here's a hunk of it let's take a close look at it you can see grains of the different colored minerals which make up Granite here's a grayish grain of quartz here's a grain of pink Fel Spar and the little black Flex are grains of a kind of mica and each of these grains will be a single Crystal notice that the three things don't mix together in the solid they crystallize out in separate grains before I get through you'll begin to see why when you came in I was melting some bismo in a crucible over here and it's been Cooling and crystals ought to have formed on the sides of The Crucible so that when I pour off the part of the bisou still melted you can see them sometimes you can show up the crystals in a finished piece of metal by polishing the surface and then etching it carefully with something that will eat away the metal here's a cast brass door handle which the manufacturer polished and then over the years the sweat on people's hands etched it and now you can see the boundaries between some of the crystals which make up that door handle in most pieces of metal the crystals are much smaller than these crystals because they've been broken up when the metal was drawn or rolled or hammered into shape but the metal is crystallin just the same well so almost all solid things are made of crystals in fact if you ask a physicist these days to define a solid he'll probably say a solid is a bit of matter that is crystallin of course that doesn't seem to get him very far because somebody is sure to ask what do you mean by crystallin but he's got an answer to that one the answer comes in two parts the first part says all matter is made of atoms the second part says in Crystal of matter the atoms are arranged in regular order I want to talk a little more about what he means by this and show you a little of the evidence for it the idea is that the crystal is a pattern of atoms a regularly repeated pattern of atoms like the regularly repeated pattern of a wallpaper if the atoms were flat and made a two-dimensional Crystal they might be taking up an orderly arrangement like this arrangement of pennies on the table in three dimensions they might be making an arrangement like these big bearing balls closely packed together of course these close packed Arrangements aren't the only Arrangements the atoms might take they're just simple examples of what I mean now already perhaps you can see what these two ideas the idea that a crystal is made of little units atoms or molecules all alike and that those little units are arranged in the crystal in a regular array you can see how those two ideas would explain what you've been looking at under the microscope for instance those Alum crystals kept the symmetrical shape as they grew now suppose you had a two-dimensional Crystal like with a pattern of atoms like the arrangement of pennies there are more atoms and solution all around it but since the arrangement is symmetrical in the crystal you expect pennies to add themselves on at the same rate here and here and here and so on because conditions are the same at all those places the crystal will grow symmetrically and keep its shape so long as nothing gets in the way and prevents more atoms from reaching the surfaces let me show you another kind of evidence for these two ideas it's a kind of evidence which doesn't come from the process of growth it comes from the properties of the finished Crystal this is the evidence of cleavage if you've ever handled Micah you're acquainted with cleavage the mic crystals you find in nature come apart easily in One Direction and not in any other so that you can split them into extremely thin tough sheets the mic acts as if it was made like a book with a lot of leaves of tough paper but if you actually try it you'll get the feeling that there's no limit to the thinness of those leaves of paper you'll feel that if you were skillful enough you could split each Leaf of the mica in two and then each of those in two again and so on it makes you guess that the atoms in the mic are arranged in great sheets that the atoms are bonded together very tightly in the sheets and the bonding forces between the sheets are very small just the arrangement of atoms in sheets like that is a kind of orderliness there aren't many crystals that cleave as well as m al for example doesn't cleave at all but a great many crystals do cleave amazingly well let me show you the cleavage in this Crystal it's a crystal of nickel sulfate hexahydrate it Cleaves parallel to this face I'll put the thing down on a little plastic seam to hold it in place I'll take a single-edge razor blade and direct the blade parallel to the face and give the back of the blade a smart tap she comes apart and now look at that as flat as you please you may say oh you picked a special place in the Crystal but look I can CLE it again parallel to that same face and get a thin plate out of the thing so it isn't a special place in the crystal it's a special Direction in the crystal now maybe you will say oh that isn't any special Direction you've got a sharp razor blade and you're cutting the thing apart with it all right if that's what you think let me try the same stunt in a different direction I turn my razor blade in a different direction and I whack it and nothing much happens until I whack it a lot harder and finally of course the thing busts and there's no nice flat face it's just broken apart like anything else now let me show you the cleavage in another Crystal a crystal of sodium nitrate here's a crystal that Cleaves in three directions not just one I can CLE it here like this and I can CLE it here like this and here like this by the way you can grow both these crystals nickel sulfate and sodium nitrate by recipes given in the film notes in case you want to play around with the stuff yourself sodium nitrate has a lot of interesting properties it has most of the properties of the mineral calide when big crystals of calite were first discovered about 300 years ago in Aquarian Iceland the scientists of that day got hold of some and their studies of the stuff made the first big start in our understanding of crystals as I say sodium nitrate and calite have many properties in common in particular they both cleav the same way when those earlier scientists saw the way calite Cleaves they had the idea that maybe calite was made of tiny building blocks all alike and all having the shape of little cleavage blocks of calite you may be inclined to laugh at this and say that calide like all matter must be made of atoms and atoms don't look like building blocks okay you're telling me what do the atoms look like little hard round iron pellets don't let me fool you with those pennies and bearing balls I've been showing you they just show where the atoms are not what the atoms look like for all I know the atoms look like fourleaf clovers I want to emphasize that those early scientists produced both of the main ideas which go into the picture of a crystal that we have today first they were saying crystals have an ultimate fine structure the building blocks we would call them small groups of atoms instead of blocks but the difference is really more a matter of words than of anything else second they were saying that structure has a regular array the blocks are stacked up in a regular order and they keep stacking up in regular Arrangement as the crystal grows it turns out that you can think of any Crystal not only crystals which show cleavage but any Crystal as made of little blocks stacked up in regular array without overlapping on top of one another and side by side if you choose the right shape of block in the case of Alum the right shape of block is a cube and you can think of the Alum Crystal as made of little cubes all stacked up on top of one another without overlapping this way notice that I've made the main faces on the model of the Alum Crystal by stepping back the cubes as I piled them up at first you might think that would mean that the face would be pretty rough but you got to remember that these little cubes are really only a few atoms wide their sides are only about 110 millionth of a cm long and you wouldn't feel that roughness or even see it under the microscope one of the things that gave early support to the building block idea was the fact that all crystals of the same stuff have the same angles between corresponding faces the crystals might be very different in size they might have grown at different rates on different faces so that they have different overall shapes like these two Alum crystals nevertheless the angles between corresponding faces of all crystals are the same so long as the crystals are made of the same material it all suggests very strongly that each material forms its own building block which gets repeated again and again as the crystal grows now turn back for a moment to the microscope and remind yourself of what is happening while a crystal grows atoms are constantly arriving at the surfaces and adding themselves onto those surfaces in an orderly way and they're doing this at an amazing rate of speed the crystals you're looking at are growing fairly fast suppose a crystal grew at the rate of only a couple of millim a day which is fairly slow you can calculate that about a thousand layers of molecules would have to be laid down per second on the surface of that Crystal and all the molecules have to be laid down in the right sort of order I want to borrow a trick from Sir Lawrence Bragg to give you just a rough idea of some of the things that we think must go on as a crystal grows sir Lawrence blows soap bubbles he blows them in a pan of soapy water Tiny Bubbles which rise to the surface and collect in rafts the bubbles behave a little like Adams because when they're floating on the water they attract one another a little and when they touch they stick to one another quite strongly as atams do but again like the atams they have a squashy sort of size so that they take up space watch how the bubbles collect together into an orderly arrangement it's the same as the arrangement of pennies I showed you the so-called close-packed Arrangement notice that the rafts are made of large Parts all of the parts have the same sort of order but the orderliness of the parts is turned in different directions so that there are grain boundaries between the parts this is like a solid made of lots of crystals which started growing in different places and finally joined it's like the salol after it is all solidified or that brass door handle when you think of how fast the atoms have to get into order much faster than this the more astounding their achievement seems to be but this will give you some little idea of what a hustle and bustle must really have to go on on an atomic scale sometimes when the molecules are very big and have an irregular shape they're too sluggish to get themselves arranged into order then they conal into a glass a glass is very much like a liquid but it's like a liquid in which the molecules are no longer able to move past each other it has patches of order in the midst of disorder the disorder is Frozen in place so to speak and can't get it self rearranged into order liquids and glasses are pretty disorderly Arrangements of molecules and gases are the most disorderly Arrangements of all the only order in gases is the arrangement of atoms grouped in each molecule of the gas later in this course you'll learn more about gases but already you can see why gases are so very much alike in a great many ways whereas there are so many different kinds of solids the only way gases can differ is in the the kinds of molecules they are made of but solids can differ also in the kind of orderliness which those molecules take up in the crystals which comprise the solid I can show you this selectiveness actually happening in some of the materials you've already looked at I'll melt some Salo on the hot plate over here there and bring it over to the microscope and let it cool down so that it wants to crystallize then I'll seed the Salo with a tiny Crystal of Alum nothing happens no Sor will crystallize on it now I'll seed the MK with a few little crystals of Sailor and you can see the seol crystals grow leaving the Alum Crystal unaffected the orderliness of Alum is not the right kind of orderliness for salal and the salal simply Waits until the right kind of orderliness is presented to it and I think now you can see what I promised you would see about Granite why it is that the three different ingredients quartz felbar and mic form three separate kinds of crystals in the granite and don't form just a single crystal in which the molecules of quartz felspar and mic all take part together it's because those three kinds of molecules normally form crystals with three different kinds of order when the three kinds of molecules are all together they can't find a kind of order which is common to all three of them so they crystallize out separately each in its preferred way you know after 20 years of growing crystals I still find the whole business nearly miraculous you go to the microscope to watch it and it happens again and you know that the atoms are at work bily almost uniring constructing something something regular something orderly constructing something which from an Adam's point of view is simply enormous