1. Pure water is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 8 parts by weight of the former; and pure sulphur-dioxide, to take another example, is found always to consist of sulphur and oxygen combined in the ratio of 8·02 parts by weight of sulphur to 8 parts by weight of oxygen. (The Law of Constant Proportion.) 2. Another compound is known consisting only of oxygen and hydrogen, which, however, differs entirely in its properties from water. It is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 16 parts by weight of the former, _i.e._, in it a definite weight of hydrogen is combined with an amount of oxygen _exactly twice_ that which is combined with the same weight of hydrogen in water. No definite compound has been discovered with a constitution intermediate between these two. Other compounds consisting only of sulphur and oxygen are also known. One of these (viz., sulphur-trioxide, or sulphuric anhydride) is found always to consist of sulphur and oxygen combined in the ratio of 5·35 parts by weight of sulphur to 8 parts by weight of oxygen. We see, therefore, that the weights of sulphur combined with a definite weight of oxygen in the two compounds called respectively “sulphur-dioxide” and “sulphur-trioxide,” are in the proportion of 8·02 to 5·35, _i.e._, 3 : 2. Similar simple ratios are obtained in the case of all the other compounds. (The Law of Multiple Proportions.) 3. From the data given in (1) above we can fix the combining number of hydrogen as 1·008, that of sulphur as 8·02. Now, compounds are known containing sulphur and hydrogen, and, in each case, the weight of sulphur combined with 1·008 grammes of hydrogen is found always to be either 8·02 grammes or some multiple or submultiple of this quantity. Thus, in the simplest compound of this sort, containing only hydrogen and sulphur (viz., sulphuretted-hydrogen or hydrogen sulphide), 1·008 grammes of hydrogen is found always to be combined with 16·04 grammes of sulphur, _i.e._, exactly twice the above quantity. (The Law of Combining Weights.) Berthollet (1748-1822) denied the truth of the law of constant proportion, and a controversy ensued between this chemist and Proust (1755-1826), who undertook a research to settle the question, the results of which were in entire agreement with the law, and were regarded as completely substantiating it. [Illustration: PLATE 16. [by Worthington, after Allen] PORTRAIT OF JOHN DALTON. _To face page 100_]] Dalton’s Atomic Theory.