dies may be united, in all possible proportions. Equal measures of water and alcohol may be united; or one drop of the former with a gallon of the latter; or a drop of the latter with a gallon of the former, or in any intermediate proportions; and, in every case, the union will be perfect, uniform, and permanent. But, in most solutions, the liquid cannot combine with more than a certain definite quantity of any solid or aeriform body. Thus, water can only take up a certain known weight of common salt; or alcohol, of camphor. The point, at which the dissolving power of the liquid ceases, is called the point of saturation. The liquid itself is then said to be saturated. When solutions are made, as in the arts, on a large scale, it is evidently of the utmost importance that we should know where this point of saturation stands, that we may not waste time and material in attempting to push the process beyond a limit fixed by the inviolable laws of Nature.

(c.) Again. It is worthy of remark, that, when a liquid has been saturated with one substance, it is often capable of combining, at the same time, with a second and a third. Thus, water, which has taken up its full proportion of common salt, will dissolve a further quantity of Glauber's salt, and a yet further quantity of Epsom salt. On this fact is founded a very convenient process for obtaining these salts from sea-water, (which always contains them,) by gradual evaporation. When a given quantity of the water is evaporated down near to the point at which the least soluble salt saturates the liquid, that salt will begin to crystallize. Thus we obtain common salt. A still further evaporation gives Glauber's salt; and the remaining liquid holds dissolved a quantity of Epsom salt. It is obvious, that such a process can never be carried on, with economy and success, without a knowledge of the relative solubility of these substances.

(d.) Having noticed the case, in which one substance has different degrees of affinity for two or more others,

we take this opportunity of presenting a striking principle, which applies in many such cases. The difference in the affinity may be so great, that it will occasion both decomposition and the forming of a new compound. For example: if we take spirits of camphor, (a solution of camphor in pure alcohol,) and pour a little water to it, we shall find the camphor precipitated, in a solid form, to the bottom. The reason is, that the alcohol has a much greater affinity for the water than it has for the camphor; in consequence of which, the latter is separated, and resumes its solid state, and a new combination is formed of the water and alcohol. Here, as a substance seems to make choice or election of one substance rather than another, with which to unite, we apply the term elective, and call it elective affinity; and since there is but one decomposition and one new compound formed, it is called, single elective affinity.

There is another case, in which double decomposition and composition will take place; that is, the two original bodies being both compound, will each be decomposed, and two new compounds produced, from a mutual exchange of ingredients. For example: take sugar of lead (which is a compound of vinegar and lead) and white vitriol, (which is also a compound, formed by uniting sulphuric acid with a metal called zinc.) If these substances, in a state of solution, be mixed; the vinegar in the sugar of lead, having a stronger affinity for the zinc of the other compound than it has for the lead, will forsake the latter and unite with the zinc; while, on the other hand, the sulphuric acid, having a stronger affinity for the lead than for zinc, will quit the latter and unite with the former. Thus, we shall have two new compounds; the one composed of vinegar. (or acetic acid) and zinc, and hence called acetate* of

* The chemical names of compounds are so arranged, as to indicate the simple substances of which they are composed, and also the proportion in which the ingredients combine. Thus, where an acid so combines with an oxide, or other base, as to neutralize it and be itself neutralized, the name of the compound is formed by changing the last syllable of the acid from ic into ate, or from ous into ite.

zinc; the other, of sulphuric acid and lead, and hence called sulphate of lead. This is called double elective affinity. This beautiful principle affords to the artist, who understands it, a ready mode of separating a solid from a solution; of purifying mixtures, &c. ; and is of most extensive application in the useful arts.

II. (a.) We come now to another modification of affinity, still more curious and interesting. In the compounds of which we have spoken, the affinity exerted is comparatively weak. The compound retains the properties of its ingredients, which seem unchanged by the combination, and takes a character intermediate between theirs,—a circumstance which contributes greatly to the usefulness of solutions, and without which, indeed, they could have none of their present value. In the mode of action which we shall now consider, and which may be termed, by way of eminence, chemical composition, the affinity acts with more energy, the union effected is more intimate, and is generally attended with such an entire change of properties, that we cannot discover in the compound any trace of the ingredients. Thus, two gases, oxygen and hydrogen, being combined, in certain proportions, form water, a fluid substance, entirely destitute of the characteristic properties of either of its constituents. So, if mercury be united with a certain proportion of chlorine, which is a gas, it forms a solid, well known in medicine by the name of calomel, and which differs entirely from its ingredients, in form, appearance, and taste, and in its effects on the animal system. So, again, if we unite an acid and an alkali, as oil of vitriol and soda, a compound results, which has neither the intense acidity or

Thus, acetic acid, combined with zinc, gives acetate of zinc ; acetous acid, combined with zinc, would give acetite of zinc. An acid takes ic, when it contains a larger proportion of the acidifying principle, that is, oxygen: ous, when it contains a smaller portion. Thus, four proportions of oxygen give nitrous acid, five proportions, nitric acid, &c. So sulphate of lead is thus called, because it is a compound of sulphuric acid and lead. In this compound, there are, of course, three simple substances, oxygen, sulphur, and lead.

corrosive power of the vitriol, nor the acrid bitterness and power over color of the alkali. The active properties of each substance are destroyed or neutralized, and a compound, distinguished for its mildness, is obtained; just as, in the preceding case, a compound of very active properties resulted from the union of ingredients comparatively inert.

(b.) These compounds are distinguished by another remarkable characteristic. Wherever or in whatever quantity found, they are not only composed of the same ingredients, but of these ingredients combined in exactly the same proportion. Thus, if water be found in any instance to consist (as it always does, if pure) of eight parts of oxygen and one of hydrogen, it may be assumed, as a permanent and universal law, that all other pure water, wherever obtained, will contain the same ingredients, in the same proportions. Were these ingredients in any other proportions, the resulting compound would not be water, but some substance of very different properties; and this conducts us to another very singular law, which is, that the same ingredients, combining in different proportions, produce compounds differing essentially, not merely from the ingredients themselves, but also from one another. The union of mercury and chlorine, in one proportion, gives calomel, a useful medicine; while, united in a different proportion, they give corrosive sublimate, a deadly poison. How important is it, for all who prepare medicines, or any compound in the arts, to understand this law. If ignorant of it, they have no security, either that they will obtain the compound that they desire, or that some other one, fatal to their designs, may not be produced. The following example will illustrate the importance of this knowledge to the medical practitioner. Of two medicines, either might be administered separately, without injury, and perhaps with benefit; whereas, if both of them should be given in conjunction, or at nearly the same time, the most dreadful consequences might ensue; since the product

of such a combination might be poisonous. And, on the other hand, two useful medicines might be so related, that, if administered together, the one would completely neutralize the other.

(c.) But we have not yet unfolded all the wonders of this wonderful principle. We have seen, that, in this class of chemical compositions, ingredients may unite in different proportions. But the range of combination is vastly more limited, here, than in the case of solutions. There, the constituents might combine, in all proportions, within a certain limit. But here, they must always combine in the proportion of certain numbers, which are called their proportional numbers. Thus, in whatever compound we meet with oxygen, we shall find, that its quantity may be expressed by eight, or by some multiple of eight; as sixteen, twenty-four, thirtytwo, forty, &c. Hydrogen always unites in the proportion of one, or of some multiple of one; sulphur, in the proportion of sixteen; chlorine, of thirty-six, &c.*

Where the compound is formed by the union of two substances, one or both of which are already compounded, the proportional number of that substance will be expressed by the sum of the proportional numbers of its ingredients. For example: if water, a compound of oxygen and hydrogen, unites with lime, it will be in the proportion of 8+1=9† (which is the sum of hydrogen and oxygen) to 20+8=28 which represent the proportions of calcium and oxygen in lime. It is also a law of chemical combination, that the quantities of any two substances, which combine with the same quantities of a third, will also combine with one another : that is, if eight parts of oxygen will combine with one part of hydrogen, and with sixteen parts of sulphur,

*This is called the law or principle of definite proportions.

It may be well to mention, here, for the benefit of those readers who are not familiar with mathematical signs, that+means plus, more, or added to ;-minus, less ;Xmultiplied by ;÷divided by ;= equal to. As 8+1-9 means, 8 plus (or more) I (or 1 added to 8) equals 9; 9-1-8 means, 9 minus (or less) 1 is equal to 8; 3×4 12 means, 3 multiplied by 4 is equal to 12; 12÷4-3 means, 12 divided by 4 is equal to 3.