5. Ask for additional information.

In view of the fact that we have to breathe oxygen in order to live, in addition to being more than half composed of it, we can safely call it the most important constituent of air. Its discovery as a separate gas is usually credited to the English chemist-clergyman, Joseph Priestley, who first prepared it in 1774 by heating mercuric oxide.

Oxygen in its gaseous form is slightly heavier than air and will dissolve to a small extent in water. It is this dissolved oxygen that fish employ in respiration. Oxygen is used as the standard in our scale of atomic weights, being assigned a value of 16. It liquefies at a temperature of -297⁰ F.

Oxygen occurs widely in nature making up some 21 per cent of the atmosphere, about 47 per cent of the earth's crust, and some 65 per cent of the human body. Except for the oxygen in the atmosphere, essentially all of it is chemically combined with other substances.

Note: Fahrenheit is the name of a thermometer scale with freezing point at 32⁰ and boiling point at 212⁰.

oxygen [ɔksɪʤən]

mercuric oxide

[mə:kjuərɪk ɔksaɪd]

gaseous [gæsɪəs]

atomic [ətɔmɪk]

liquefy [lɪkwɪfaɪ]

The discovery of radioactivity was due more or less to pure accident. Once during the year 1896 Henry Becquerel, professor of physics at the Sorbonne obtained a preparation of uranium bisulphate for the purpose of studying the phosphorescence of this substance. But his interests were drawn in some other direction, and he threw the material into one of the drawers of his work-table.

Now it happened that in this drawer was a box containing some unexposed photographic plates, and the ampoule of uranium bisulphate fell right on top of that box, remaining there undisturbed for several weeks. Intending to take some photographs, Becquerel finally opened the drawer, pushed aside the ampoule with the forgotten preparation, and took out the box with the plates. But when he developed his photographs he found that the plates were badly spoiled, as if they had been previously exposed to light. This was very strange, since the plates had been carefully wrapped in thick black paper and never yet opened. The only object in the drawer that might have been responsible for the damage was the preparation of uranium bisulphate, which had for so long rested so close to the plates. He repeated the experiment with some new plates. But this time he deliberately placed an iron key from one of the drawers between the photographic plate and the hypothetical source of the mysterious radiation.

A few days later a diffuse silhouette of the key began to appear slowly against the darkening background of the negative. Yes, it definitely was a new kind of radiation coming from the atoms of uranium, a radiation that easily penetrated materials nontransparent to ordinary light, but was still unable to pass through the thickness of an iron key.

Becquerel [bek(ə)rəl]

Sorbonne [ˏsɔ:bɔn]

uranium [ju(ə)reɪnɪəm]

bisulphate [baɪs٨lfeɪt]

phosphorescence

[ˏfɔsfəres(ə)ns]

ampoule [æmpu:l]

iron [aɪən]

silhouette [ˏsɪluet]

After scientists had begun to study the way light acts, they worked out two different ideas about the nature of light. At first these ideas did not agree. But as scientists have studied light still further, they have come to realize that both are right. One portrait of light is given by the theory, advanced by Newton, that light consists of a stream of particles or corpuscles traveling in straight lines or rays. By means of this theory it is possible to explain many things including the laws of mirrors and lenses, and the formation of shadows. The other view of light, advanced by the Dutch scientist, Christian Huygens, at about the same time, is that light is a train of waves traveling through space. This theory also explains the laws of mirrors and lenses very well, and even the formation of shadows. Nevertheless, Newton's theory was favored for a long time and Huygens' was almost forgotten. Then, in 1801, an English scientist named Thomas Young did an experiment that could not be explained at all by Newton's corpuscular theory!

Huygens [haɪgənz]

corpuscular [kɔ:p٨skjulə]