Imágenes de páginas

subjects as water, the atmosphere, and combustion, to which it is desirable that he should be introduced at an early stage in his studies, are thus brought much more forward than would otherwise be the case.

In Part III. the elements are treated systematically, according to the periodic classification. In this manner, while avoiding a sharp separation of the elements into the two arbitrary classes of metals and non-metals, it has been possible to so far conform to the prevailing methods of instruction, that all those elements which are usually regarded as non-metals (with the two exceptions of boron and silicon) are treated in the earlier portion of the book.

The science of chemistry has of recent years developed and become extended to such a degree, that the difficulty of giving a fairly balanced treatment of the subject, within the limits of a small text-book, is an ever-increasing one, and it necessarily resolves itself into a question of the judicious selection of matter. In making such a selection, I have endeavoured, as far as possible, to keep in view the requirements of students at the present time, without, however, following any examination syllabus.

Acting upon this principle, I have omitted all detailed description of the rare elements and their compounds, confining myself merely to a short mention of them in a few general remarks at the commencement of the various chapters.

Although from a purely scientific standpoint many of these rare substances are of the greatest interest and importance, it must be admitted that they stand quite outside the range of all the customary courses of chemical instruction; and so far as the wants of the ordinary student are concerned, the space which would be occupied by an account of these elements is more advantageously devoted to such matters

as are discussed in the Introductory Outlines. Moreover, it is a matter of common observation that text-books, even upon the shelves of reference libraries, and which bear unmistakable evidence of much use, are frequently uncut in those portions which treat of these elements.

Details of metallurgical processes, also, are out of place in a text-book of chemistry, and must be sought in metallurgical text-books. Only such condensed outlines, therefore, have been given as are sufficient to explain the chemical changes that are involved in these operations.

The great importance to the student, of himself performing experiments illustrating the preparation and properties of many of the substances treated of in his text-book, cannot well be over-estimated. If he be in attendance upon a course of chemical lectures, opportunity should be given to him for repeating the simpler experiments he may see performed upon the lecture table: if he be not attending lectures, the necessity for this practical work on his part is greater still. Instead of burdening this text-book with specific directions for carrying out such elementary experiments, frequent references have been made to my “Chemical Lecture Experiments," where minute directions are given for carrying out a large number of experiments, many of which may be easily performed, and with the very simplest of apparatus.

Several of the woodcuts have been borrowed from existing modern works, such as Thorpe’s “ Dictionary of Applied Chemistry,” Mendelejeff's “Principles of Chemistry,” Ostwald's “Solutions,” and others. Care has been taken, however, to exclude all antiquated cuts, and a large number of the illustrations are from original drawings and photographs.




In preparing this edition I have availed myself of the opportunity, which the necessity to reset the book in type has afforded me, of making some more extensive changes and additions than the exigencies of stereotype plates would have allowed, with a view to bringing the matter well into line with the most recent advances of the science. The general plan of the book remains the same, the alterations consisting almost entirely of extensions and additions, amounting in all to about fifty pages. Some of the subjects dealt with in Part I will be found considerably amplified, without, however, any departure from the original intention of making these chapters introductory outlines only. A new section in the text of the book has been devoted to a description of the five new elements of the argon group, and these elements have been included in the periodic scheme of classification. For many of the most recent facts relating to these elements I am indebted to Travers' “Experimental Study of Gases."

Descriptions have also been included of a number of recent manufacturing processes (several being illustrated by new cuts), which are now being carried on by the modern applications of electricity-such as the manufacture of phosphorus, graphite, caustic soda, sodium carbonate, potassium chlorate, aluminium, and others.

The atomic weights which in previous editions were given in a separate column of the table of weights, under the head of more exact values, have now been replaced by the so-called international atomic weights, which are published annually in the Berichte. Not that it is of any moment to the student whether the atomic weight of, say nickel, is 58.6 or 58.7: but it is obviously desirable that as far as possible there shall be some uniformity, so that he shall not find a different value given in every different book he may refer to.

G. S. N.

July 1902

« AnteriorContinuar »