A few words will suffice to explain the meaning of the terms which head the names, letters, and numbers of the Table of Elements. [Page 87] The names of the elements have very interesting derivations, which it is not the object of this work to go into; the symbols are abbreviations, ciphers of the simplest kind, to save time and trouble in the frequent repetition of long words, just as the signs + plus, and - minus, are used in algebraic formulæ. For instance--the constant recurrence of water in chemical combinations must be named, and would involve the most tedious repetition; water consists of oxygen and hydrogen, and by taking the first letter of each word we have an instructive symbol, which not only gives us an abbreviated term for water, but also imparts at once a knowledge of its composition by the use of the letters, HO. Again, to take a more complex example, such as would occur in the study of organic chemistry--a sentence such as _the hydrated oxide of acetule_, is written at once by C_{4}H_{4}O_{2}, the figures referring to the number of equivalents of each element--viz., 4 equivalents of C, the symbol for carbon, 4 of H (hydrogen), and 2 of O (oxygen). The long word paranaphthaline, a substance contained in coal tar, is disposed of at once with the symbols and figures C_{30}H_{12}. The figures in the third column are, however, the most interesting to the precise and mathematically exact chemist. They represent the united labours of the most painstaking and learned chemists, and are the exact quantities in which the various elements unite. To quote one example: if 8 parts by weight of oxygen--viz., the combining proportions of that element--are united with 1 part by weight of hydrogen, also its combining number, the result will be 9 parts by weight of water; but if 8 parts of oxygen and 2 parts of hydrogen were used, one only of the latter could unite with the former, and the result would be the formation again of 9 parts of water, with an overplus of 1 equivalent of hydrogen. It is useless to multiply examples, and it is sufficient to know that with this table of numbers the figures of analysis are obtained. Supposing a substance contained 27 parts of water, and the oxygen in this had to be determined, the rule of proportion would give it at once, 9: 27:: 8: 24. 9 parts of water are to 27 parts as 8 of oxygen (the quantity contained in 9 parts of water) are to the answer required--viz., 24 of oxygen. The names, symbols, and combining proportions being understood, we may now proceed with the performance of many interesting CHEMICAL EXPERIMENTS.