The lecturer then proceeded to criticise a statement commonly found in text books, that chemical combination suppresses altogether the properties of the combining bodies. The reverse of this statement is probably true. To take the case commonly given of the combination of copper and sulphur when heated; this is good as far as it goes, but there are numerous instances, as ClI, SSe, etc., where the original properties and characters of the combining elements do not completely disappear. The real statement is that the original properties of the elements disappear more or less, and least when the combination is weak and attended with the evolution of a slight amount of heat, and in every case some properties are left which can be recognized. So with reference to the question of atomic and molecular combination, as atomic combination does not necessarily produce change, it does not differ in this respect from what is usually called molecular combination. The lecturer then referred to an important difference in the adicity of chlorine and oxygen. Chlorine can combine with methyl or ethyl singly. Oxygen can combine with both and hold them together in one molecule. The recognition of this fundamental difference between chlorine and oxygen, this formation of double oxides as opposed to single chlorides, marks an epoch in scientific chemistry. The lecturer then considered the subject of chemical formulæ; it is the bounden duty of every formula to express clearly the number of atoms of each kind of elementary matter which enters into the constitution of the molecule of the substance. A formula may do much more than this. If we attempt to express too much by a complex formula we may veil the number of atoms contained in it. This difficulty may be avoided by using two formulæ, a synoptic formula giving the number of atoms present, and a complex formula perhaps covering half a page, giving the constitution of the molecule. But between the purely synoptic formula and the very elaborate formula there are others--contracted formulæ--which labor under the disadvantage, as a rule, of being one-sided, and so create a false impression as to the nature of the substance. Thus, for instance, to take the formula of sulphuric acid, H_{2}SO_{4}. This suggests that all the oxygen is united to the S; (HO)_{2}SO_{2} suggests that two atoms of hydroxyl exist in the molecule; then, again, we might write the formula HSO_{2}OH, or H_{2}OSO_{3}. All of these are justifiable, and each might be useful to explain certain reactions of sulphuric acid, but to use one only creates a false impression. The only plan is to use them variously and capriciously, according to the reaction to be explained. Again, ethyl acetate may be written-- H_{3}C\ H_{2}C/ \ O / OC\ H_{3}C/ Or condensed-- H_{5}C_{2} \ }O H_{3}C_{2}O/