The New Student's Reference Work/Atom
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When a homogenous body is broken up into very small parts, even the smallest parts which we can produce by any instrument or see by any microscope, these parts appear to be all exactly alike in structure. But we may fairly ask whether, if it were possible to continue the process of division yet further, we should still find the body made up of parts exactly alike. The facts indicate that this question would have to be answered in the negative. For there are other methods, besides the use of mechanical instruments, for separating a body into its parts. Thus, for instance, if we place zinc in dilute sulphuric acid, the zinc will set free hydrogen, which formerly constituted a part of the sulphuric acid. The sulphuric acid has thus been divided into parts. Again, if we confine a mixture of gases in a vessel with porous walls, we find that some of these gases pass through the walls more rapidly than others; and that by collecting those that come through first and those that come through last, we can separate the mixture into parts that are very different from one another. From considerations of this kind we are led to think that matter is made up of particles far transcending in smallness the reach of the most powerful microscope. The smallest mass of any substance in which the properties of the substance will remain is called a molecule. But all substances, except about ninety which are called elements, have been decomposed into other substances having different properties. The smallest mass of each of these elementary substances is defined as an atom. Atom is a Greek word meaning “indivisible;” it acquired its present English signification about the beginning of the 19th century.
An atom is defined as a portion of matter which is indivisible by chemical methods: but there are excellent reasons for thinking that this atom which is indivisible by chemical methods is made up of still smaller parts. One of these reasons is that a single type of atom–say hydrogen–is capable of emitting light of many different wave lengths, just as a piano or an orchestra can emit many different wave lengths of sound. And just as a piano is a complex instrument, so we are led to think that an atom of hydrogen–or indeed an atom of any other substance–is probably a very complex mechanism. A still stronger reason for thinking that the chemical atom is divisible is the fact that Sir Joseph Thomson has recently succeeded in splitting off, from the hydrogen atom, parts which are called “electrons” and which have a mass of approximately one two-thousandth that of the hydrogen atom. These electrons may be obtained from matter by other processes also, such as X-rays, ultra-violet light, and high temperatures. One investigator has estimated that the size of an electron bears to the size of an atom about the same relation as the size of a pinhead bears to the size of the dome on St. Paul’s Cathedral.
In all, there are about ninety kinds of atoms: there are the “elements” of the chemist. Professor Millikan of Chicago University has found the mass, in grams, of a hydrogen atom to be 1.62XIO-24.
Experiments indicate that it would require about 100,000,000 average molecules laid side by side in a straight line to cover a distance of one centimeter.
See Sir Joseph Thomson’s Corpuscular Theory of Matter (Scribners), Soddy’s Interpretation of Radium, Cameron’s Radium and Radioactivity (Romance of Science Series), Kimball’s Properties of Gases (Houghton Mifflin Company).