reduced by assuming the atomic number of the elements which gives the smallest quotient in the calculation 1, and dividing the other quotients by this smallest quotient. Thus in the present example 1-852: 9.260 = 15; consequently 1 At. nitrogen is combined with 5 At. oxygen. Felspar contains in 100 parts, 16.65 potash, 18.14 alumina, and 65.21 silica: how many atoms of these three substances does it contain? Here we have to calculate the atomic weights of the three proximate elements: thus Now 3528 3528 : 21172 = 1:16. Consequently felspar contains 1 At. potash, 1 At. alumina, and 6 At. silica, probably combined as follows: (K O, 3Si 02) + (Al2 03 3Si O2). In many compounds, especially of the organic class, all the elements occur in more than 1 atom, and the division of the larger quotients by the smallest gives improper fractions, which must be got rid of by sup posing the number of atoms of the element which gives the smallest quotient to be 2, 3, 4, &c. Dry benzoic acid, for example, contains in 100 parts, 74:34 carbon, 4:42 hydrogen, and 21.24 oxygen. 2-655: 4·425: 12·390 = 1 : 1·66... 4'66, or multiplying by 3, = 3: 5:14. Hence dry benzoic acid contains 3 At. oxygen, 5 At. hydrogen, and 14 At. carbon. To save the trouble of making such calculations by the rule of three, Wollaston (Ann. Phil. 4, 176; comp. also Schw. 14, 126) introduced his Scale of Chemical Equivalents constructed on the principle of the ordinary Sliding Rule. A slide moveable up and down the middle of a narrow board is marked with the numbers 10.... 500 placed at distances corresponding to their logarithms, so that, for example, the distance between 10 and 11 is as great as that between 100 and 110. On the right and left of the slide the names of the elementary substances and their more important compounds are marked on the board in places corresponding to their atomic weights. When the slide is set right in the board, that is to say, not drawn either up or down-the number 10 on the slide stands opposite 1 At. oxygen on the board; 11.25 on the slide opposite to 1 At. water on the board; so likewise 12.5 corresponds to 10 At. hydrogen; 20 to 2 At. oxygen, and 1 At. sulphur; 30 to 3 At. oxygen; 40 to 4 At. oxygen; 50 to 5 At. oxygen and 1 At. sulphuric acid; 130 to lead; 140 to oxide of lead; 190 to sulphate of lead, and so on. Consequently, in this position of the slide it may be seen that 190 parts of sulphate of lead contain 140 oxide of lead and 50 sulphuric acid; or 130 lead, 20 sulphur and 40 oxygen, it being supposed that the numbers of atoms of the con stituents are previously known. If now it be required to find what quantities of these several elements are contained in 100 parts of sulphate of lead, the slide is to be so placed that the number 100 shall stand exactly opposite to sulphate of lead, when the required quantities will be found. opposite the names of the several elements. In this and other ways the Equivalent-scale (which may also be made with a moveable circle instead of a slide) may be employed. However, the fractional parts of the numbers cannot be marked on the divisions of the slide with a degree of accuracy equal to that which may be obtained by calculation, and moreover the number of the elements and their compounds is so great that their names cannot all be marked on the board, so that searching for them in this manner often takes a longer time than calculation; hence the use of these Equivalent-scales has not become very general. D. Qualitative alterations of elements caused by chemical combination. Since the chemical combination of heterogeneous substances produces a homogeneous mass, it must necessarily be accompanied by a change in the properties of the elements. In the less intimate combinations this change is very unimportant, no more in many cases than is necessary to cause the different properties of the elements to merge into one another. Thus common salt when dissolved in water loses its solid form and imparts its saline taste to the water, while the specific gravity of the solution is nearly a mean between that of the salt and that of the water. The properties of the more intimate compounds on the other hand are in almost all cases totally different from those of their elements. The two tasteless substances oxygen and sulphur produce by combination the intensely sour and corrosive sulphuric acid; the solid substances carbon and sulphur form when combined the volatile sulphuret of carbon: grey mercury and yellow sulphur form the bright red compound vermillion; &c. &c. Although all the elements of a compound body exert some influence in determining its properties, this influence is nevertheless exerted in very different degrees; it often happens that one element is much more effective in determining the physical and chemical characters of a compound than another; the former may be said to possess more formative power, while the latter serves either as a base or groundwork. Thus the metals may be said to act as bases, while the non-metallic elements, particularly oxygen, hydrogen, chlorine, fluorine, bromine, iodine, selenium and sulphur may be regarded rather as formative principles. The former, when they possess considerable specific gravity, impart this property more especially to the compounds, while the latter generally destroy the metallic lustre, opacity, and great conductive power of heat and electricity by which metals are distinguished, and impart to the compounds peculiar chemical characters, e. g., those of acids or salifiable bases. There is more resemblance between the compounds of oxygen with different metals, as also between the several metallic chlorides, sulphurets, &c. than between the compounds of one and the same metal with oxygen, chlorine, sulphur, &c. a. Density. In most instances, the compound occupies a smaller space than its elements taken together before combination; condensation generally takes place, less frequently expansion, or neither. a. Relation between the Density of Gaseous Compounds and that of their Gaseous Elements. Most combinations of this class are attended with condensation, and always according to simple relations of volume; many however take place without alteration of bulk so that the compound takes up the same space as the sum of the elements before combination: we know of but one combination of gases which is accompanied by expansion. These relations are exhibited in the following table. The first division includes the cases in which no change of volume takes place; the second those attended with condensation; the third the single case accompanied by expansion. But few organic compounds are mentioned in this table, as their relations of volume will be fully treated of in another place. Column A. Names of the compounds. B. Their chemical formulæ. C. The number of measures in which the bodies combine. D. The sum of these measures or the volume of the elements before combination. E. The number of measures occupied by these elements after combination. F. The atomic weight of the compound. G. Spec. grav. of the compound, that of air being assumed = 1. H. Quotient obtained by dividing the sp. gr. by the atomic weight, i.e. the atomic number. I. Reduced atomic number, obtained on the supposition that 1 vol. hydrogen gas contains 1. x atoms of hydrogen. The columns F, G, H, I are comparable with columns B, C, D, E of the table, page 55. Since the specific gravities of many simple substances occurring in the compounds here enumerated is not known from observation, we are obliged in regard to them to calculate the sp. gr. and thence the relation by volume hypothetically. Thus from analogy, the vapours of selenium and tellurium have been assumed to be 6-atomic, that of antimony 2-atomic, and fluorine gas together with the vapours of carbon, boron, silicium, titanium, tin and bismuth as 1-atomic. Since for example the atomic weight of tin is 59 times as great as that of hydrogen, and the sp. gr. of hydrogen gas = 0-0693, the hypothetical sp. gr. of vapour of tin 59.0.0693. The atomic weight of antimony is 129, and its vapour is supposed to be 2-atomic, hence the hypothetical sp. gr. of its vapour is 2.129.0.0693; to find that of selenium we multiply by 6. 40, and so on. The hypothetical parts of the calculation are denoted in the table by notes of interrogation in most cases the hypothetical element is the number of volumes; in that of nitric acid it is the sp. gr of the vapour VOL. I. F From this table the following results may be deduced :— 1. Compound bodies in the gaseous state are more expanded, and contain fewer atoms in a given volume than simple substances in the same state. While the simple gases are 6, 2, and 1-atomic, the compound gases are never 6-atomic, but a. 2-atomic; arsenious acid; b. 1-atomic; water, nitrous oxide, most oxygen acids, the weaker hydrogen acids, such as sulphuretted hydrogen and several metallic iodides, bromides and chlorides, some containing one and some two atoms of the salt-radical; c. atomic, sulphuret of mercury and glacial acetic acid; d. -atomic: oxide of chlorine, hyponitric acid, the stronger hydracids, such as hydrochloric acid, ammonia, compounds of 3 At. hydrogen, iodine, bromine or chlorine with 1 At. boron, phosphorus or metals, and those of 1 At. bromine or chlorine with 2 At. mercury; e. -atomic; anhydrous carbonate of ammonia (NH3, CO), perchloride of phosphorus ; f. atomic; compounds of hydracids with ammonia and with phosphuretted hydrogen. It is scarcely necessary to remark that by,,, or atomic gases, we are not to understand gases really containing two-thirds, one-half, one-third or one-fourth parts of atoms, but gases of which 1 volume contains only,,, or as many atoms as an equal volume of hydrogen gas. It appears as if the greater number of atoms, which together form a compound atom, attach to themselves a greater quantity of heat, so that the compound atom becomes surrounded with a larger calorific envelope. 2. The volume of the constituents bears to that of the compound the following relations: 1 vol. A with 1 vol. B forms 2 vol. AB, therefore 1 + 1 : 2 = 2: 2. To this case belong the first 12 examples in the table. 1 + 1 : 1 = 2 1, Carbonic acid, phosgene, iodide, bromide, and chloride of mercury. 1+ 2:3= 33, Neutral carbonate of ammonia. 12:23: 2, Water and the 6 following compounds. 1 + 2:1 3:1, Cyanogen and the 4 following compounds, supposing the hypothetical relations of volume to be correct. 1+ 3:3= 43, Sulphuret of carbon, with the same limitation. 1+ 3:2 = 4: 2, Ammonia, with the 2 following hypothetical cases. 1+ 3:1= 4 1, Dichloride of sulphur, arsenious acid. 1 + 6:9 7:9, Sulphuret of mercury. 1 + 6:67: 6, Sulphurous acid and the 4 following compounds. 1 + 6:47: 4, Phosphuretted hydrogen and the 5 following compounds. B. Relations between the Density of Solid or Liquid Compounds, and that of their Solid or Liquid Constituents. There is no known instance in which the combination of solid or |