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Sentences for "chemists "
26 But all chemists are not agreed as to these proportions.
" As our story proceeds, we shall see that the chemists of the middle ages, the alchemists, founded their theory of material changes on the difference between a supposed essential substratum of things, and their qualities which could be taken off, they said, and put on, as clothes are removed and replaced.
Certain facts make this supposition tenable; and some chemists expect that the supposition will be proved to be correct.
How greatly must this phenomenon have affected the imagination of the chemists of ancient times, always so ready to be affected by everything that seemed supernatural! Black and red were the symbols of darkness and light, of the evil and the good principle; and the union of these two principles represented the moral order.
He seems to have been able to describe the results of his experiments only in the language of the phlogistic theory; just as the results of most of the experiments made to-day on the changes of compounds of the element carbon cannot be described by chemists except by making use of the conceptions and the language of the atomic and molecular theory.
Although Boyle7 had stated very lucidly what he meant by the composition of a definite substance, about a century before Lavoisier's work on combustion, nevertheless the views of chemists concerning composition remained very vague and incapable of definite expression, until the experimental investigations of Lavoisier enabled him to form a clear mental picture of chemical changes as interactions between definite quantities of distinct substances.
Not only did Lavoisier realise and act on this principle, he also measured quantities of substances by the one practical method, namely, by weighing; and by doing this he showed chemists the only road along which they could advance towards a genuine knowledge of material changes.
Since Lavoisier realised, for himself, and those who were to come after him, the meaning of the terms element and compound, we may say that chemists have been able to form a mental picture of the change from one definite substance to another, which is clear, suggestive, and consistent, because it is an approximately accurate description of the facts discovered by careful and penetrative investigations.
Since chemists realised the meaning of Dalton's book, published in 1808, and entitled, A New System of Chemical Philosophy, elements have been regarded as distinct and definite substances, which have not been divided into parts different from themselves, and unite with each other in definite quantities by weight which can be accurately expressed as whole multiples of certain fixed quantities; and compounds have been regarded as distinct and definite substances which are formed by the union of, and can be separated into, quantities of various elements which are expressible by certain fixed numbers or whole multiples thereof.
They enable chemists to state the compositions of all the compounds which are, or can be, formed by the union of any elements.
These descriptions of elements and compounds also enable chemists to form a clear picture to themselves of any chemical change.
Some chemists speak of the electrons, which are the ß-rays from radium, and the kathode rays produced in almost vacuous tubes, as non-material particles of electricity.
As the facts about kathode rays, and the facts of radio-activity are (at present) inexplicable except on the supposition that these phenomena are exhibited by particles of extraordinary minuteness, and as the smallest particles with which chemists are concerned in their everyday work are the atoms of the elements, we seem obliged to think of many kinds of atoms as structures, not as homogeneous bodies.
For many centuries chemists have worked with a conceptual machinery based on the notion that matter has a grained structure.
As the facts of radio-activity led to the conclusion that some of the minute particles of radio-active substances are constantly disintegrating, and as these substances had been labelled elements, it seemed probable, or at least possible, that the other bodies which chemists have long called elements are not true elements, but are merely more stable collocations of particles than the substances which are classed as compounds.
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