Michael Faraday

“ You see we can not look into the middle of a body this piece of glass. We perceive the outside form and the inside form, and we look through it, but we can not well find out how these forms become so, and I want you, therefore, to take a lesson in the way in which we use a ray of light for the purpose of seeing what is in the interior of bodies. Light is a thing which is, so to say, attracted by every substance that gravitates (and we done not know any thing that does not) . ”

“ How beautiful that is! how it makes the light come through some parts and leaves it dark in others, and all because we weaken the force of cohesion between particle and particle. Whether you have this mechanical power of straining, or whether we take other means, we get the same result; and, indeed, I will show you by another experiment that if we heat the glass in one part, it will alter its internal structure and produce a similar effect. ”

“ You sit there upright, and I stand upright here, because we keep our centres of gravity properly balanced with respect to the earth; and I need not tell you that on the other side of this world the people are standing and moving about with their feet toward our feet, in a reversed position as compared with us, and all by means of this power of gravitation to the centre of the earth. ”

“ Let us now leave this subject which I have written upon the board under the word FORCE—GRAVITATION—and go a step father. All bodies attract each other at sensible distances. I showed you the electric attraction on the last occasion (through I did not call it so) ; that attracts at a distance; and in order to make our progress a little more gradual, suppose I take a few iron particles [dropping some small fragments of iron on the table] . There! I have already told you that in all cases where bodies fall it is the particles that are attracted. ”

“ You know that there is a planet, Uranus, revolving round the sun with us, but eighteen hundred millions of miles off, and because there is another planet as far off as three thousand millions of miles, this law attraction, or gravitation, still holds good, and philosophers actually discovered this latter planet, Neptune, by reason of the effects of its attraction at this overwhelming distance. ”

“ I have here a body in which I believe you will see a change taking place in its condition of cohesion at the moment it is made. It is at first yellow; it then becomes a fine crimson red. Just watch when I pour these two liquids together—both colorless as water. [The lecturer here mixed together solutions of perchloride of mercury and iodide of potassium, when a yellow precipitate of biniodide of mercury fell down, which almost immediately became crimson red.] Now there is a substance which is very beautiful, but see how it is changing color. ”

“ It will be impossible for us to consider the Laws of Nature, and what they effect, unless we now and then give our sole attention, so as to obtain a clear idea upon the subject. Give me now that attention, and then I trust we shall not part without our knowing something about those laws, and the manner in which they act. ”

“ There is some alum crystallized very beautifully by nature (for all things are far more beautiful in their natural than their artificial form) , and here I have some of the same alum broken into fine powder. In it I have destroyed that force of which I have placed the name of this board—COHESION, or the attraction exerted between the particles of bodies to hold them together. Now I am going to show you that if we take this powdered alum and some hot water, and mix them together, I shall dissolve the alum ”

“ That bar of iron one inch square has such power of attraction in its particles—giving to it such strength—that it will hold up twenty tons’ weight before the little set of particles in the small space equal to one division across which it can be pulled apart will separate. In this manner suspension bridges and chains are held together by the attraction of their particles, and I am going to make an experiment which will show how strong is this attraction of the particles. ”

“ I have put into that basin a few things which those who are in the body of the theatre will not be able to see, and I am going to make use of this power which matter possesses of attracting a ray of light. If Mr. Anderson pours some water, gently and steadily, into the basin, the water will attract the rays of light downward, and the piece of silver and the sealing-wax will appear to rise up into the sight of those who were before not high enough to see over the side of the basin to its bottom. ”

“ Here is a piece of common glass, and if I insert this in the path of the polarized ray, I believe it will do nothing. There is the common glass [introducing it] . No light passes through; the screen remains quite dark; but I am going to warm this glass in the lamp, and you know yourselves that when you pour warm water upon glass you put a strain upon it sufficient to break it sometimes—something like there was in the case of the Prince Rupert’s drops. [The glass was warmed in the spirit lamp, and again placed across the ray of light.] ”

“ I have shown you this experiment first, so that you might understand how glass attracts light, and might then see how other substances like rock-salt and calcareous spar, mica, and other stones, would affect the light; and, if Dr. Tyndall will be good enough to let us use his light again, we will first of all show you how it may be bent by a piece of glass. [The electric lamp was again lit, and the beam of parallel rays of light which it emitted was bent about and decomposed by means of the prism.] ”

“ It is very manifest that this is owing to the attraction of the particles one for the other being less in the direction in which they give way than in other directions. I have on the table before me a number of little bits of calcareous spar, and I recommend each of you to take a piece home, and then you can take a knife and try to divide it in the direction of any of the surfaces already existing. ”