[7] The following are the symbols employed by Dalton:—
which represent in order, hydrogen, nitrogen, carbon, oxygen, phosphorus, sulphur, magnesia, lime, soda, potash, strontia, baryta, mercury; iron, zinc, copper, lead, silver, platinum, and gold were represented by circles enclosing the initial letter of the element.
[8] Approximate values of the atomic weights are employed here.
[9] The definite distinction between potash and soda was first established by Duhamel de Monceau (1700-1781).
[10] The reader is specially referred to the articles ALIZARIN; INDIGO; PURIN and TERPENES for illustrations of the manner in which chemists have artificially prepared important animal and vegetable products.
[11] These observations were generalized by J.B. Dumas and Polydore Boullay (1806-1835) in their “etherin theory” (vide infra).
[12] This must not be confused with the modern acetyl, CH3·CO, which at that time was known as acetoxyl.
[13] It is now established that ortho compounds do exist in isomeric forms, instances being provided by chlor-, brom-, and amino-toluene, chlorphenol, and chloraniline; but arguments, e.g. E. Knoevenagel’s theory of “motoisomerism,” have been brought forward to cause these facts to support Kekulé.
[14] Victor Meyer and G. Heyl (Ber., 1895, 28, p. 2776) attempted a solution from the following data. It is well known that di-ortho-substituted benzoic acids are esterified with difficulty. Two acids corresponding to the formula of Kekulé and Claus are triphenyl acrylic acid, (C6H5)2C:C(COOH)·C6H5, and triphenyl acetic acid, (C6H5)3C·COOH. Experiments showed that the second acid was much more difficult to esterify than the first, pointing to the conclusion that Claus’ formula for benzene was more probable than Kekulé’s.