would obviously have been impossible for them to marshal themselves in the manner we have described, two of one substance associating with one of the other in the resulting chemical process. Now, in a solid body, the molecules are to a great extent fixed, and hence no chemical action is possible between such substances, except to a limited extent. There are, in general, two ways by which the required freedom of motion can be obtained: One is to convert the substance into vapor, when, as we have seen, the molecules become completely isolated, and move with great velocity through space, their motion being only limited by the walls of the containing vessel; but this method is only applicable to volatile bodies. The second method is to dissolve the solid in some solvent, when the molecules, as before, become isolated, and move freely through the mass of the liquid. The last is the method generally used, and water, being such a universal solvent, is the common vehicle employed to bring substances together, and for that reason it enters into a very great number of chemical changes. Such was its office in the process we have been studying. We dissolved both the sodic carbonate and the hydrochloric acid in water, in order that their molecules might readily coalesce. An experiment will enforce the principle I have been enunciating:

There are a great many substances which will act on sodic carbonate like hydrochloric acid; for example, almost all the so-called acids or acid salts, and, among others, that white solid with which you are familiar under the name of cream-of-tartar. Here we have creamof-tartar and sodic carbonate, both in fine powder, and we have been carefully mixing them together in this mortar. You see, there is no action whatever; and, in a dry place, we can keep the mixture indefinitely with

.

CONDITION OF SOLUTION REPRESENTED.

147

́out change. If, however (placing the mixture in this glass vessel), we pour water over it, we have at once a brisk effervescence, and carbonic dioxide is evolved as before. It required the water to bring the molecules together.

Since, then, the water plays such an important part in the reaction, I prefer to indicate its presence, and this may be done by using the symbol Aq. as previously described.

(Na2CO3 + 2HCl + Aq.) = (2NaCl + H2O + Aq.) + CO2.

Solution of Sodic Carbonate

and Hydrochloric Acid.

Solution of Common Salt.

This indicates not only that both of the factors are in solution, but also that we have, as one of the products, a solution of common salt. That the second product, carbonic dioxide, is a gas, I sometimes indicate by a line drawn over the symbol, as above.

The second reaction is equally simple, but creamof-tartar has a vastly more complex molecule than HCl. Its symbol is HKCHO, that is, each molecule consists of four atoms of carbon, six atoms of oxygen, one atom of potassium, and five atoms of hydrogen. I write one of the atoms of hydrogen apart from the rest, because it has a very different relation to the molecule-a relation which I shall hereafter explain. The reaction would be written thus:

(Na2CO3+2HKC,H2O. + Aq.) =

(2NaKC,H,O+ H2O + Aq.) + CO2.

Solution of Rochelle Salts.

With this reaction many of my audience must be familiar, as a mode of raising dough in the process of making bread. The first member of the equation indicates that the two substances are used in solution. There is formed, as the product of the reaction, besides the carbonic dioxide gas, which puffs up the

dough, the solution of a salt, whose molecule has the complex constitution I have indicated, and which is a well-known medicine under the name of Rochelle-salts. When soda and cream-of-tartar are used in making bread, this salt remains in the loaf. The amount formed is too small to be injurious, but I cannot but think, although it may be a prejudice, that chemicals had better be kept out of the kitchen.

LECTURE VII.

CHEMICAL REACTIONS.

To master the symbolical language of chemistry, so as to understand fully what it expresses, is a great step toward mastering the science; and so important is this part of my subject that I propose to occupy the hour this evening with a number of illustrations of the use of symbols for expressing chemical changes.

First, I will recur to the experiment of the last lecture, for we have not yet learned all that it is calculated to teach.

Let us again write on the black-board the symbols which represent the chemical process:

(Na2CO3 + 2HCI + Aq.) = (2NaCl + H2O + Aq.) + CO2.

Sodic

Carbonate.

Hydrochloric
Acid.

Common
Salt.

Water.

Carbonic Dioxide Gas.

We bring together a solution of sodic carbonate and hydrochloric acid; and there are formed as products a solution of common salt, water, and carbonic dioxide gas. I need not refer again to the circumstance that the state of solution is an essential condition of the change, for this point was fully discussed at the time; but, before we pass on to another experiment, I wish to call your attention to the fact that the several terms in this equation stand for absolutely defi

nite weights of the quantities they represent. Each symbol stands for the known weights of the atoms which are tabulated in this diagram (table, page 112), and the weights of the molecules, which the several terms represent, are found by simply adding up the weights of the several atoms of which they consist. When the substance is capable of existing in the aëriform condition, its molecular weight can be found, as I have shown, from its specific gravity; but these symbols assume that either by this or by some other method the constitution of the molecule has been determined; and, now that the result is expressed in symbols, nothing is easier than to interpret what they have to tell us. To begin with the sodic carbonate, Na,CO. The weight of this molecule is 2×23+12+3 × 16 = 46+ 12+48 -106 m.c. The weight of the molecule HCl is 1+35.5 36.5, and two such molecules would weigh 73 m.c. Next, for the products, we have NaCl = 23+ 35.5 58.5, and 2NaCl = 117.0, also CO2 = 12+32 44, and H2O =2+16=18. Hence the terms of our equation stand for the weights written over them below:

=

117

18

44

(Na2CO3 + 2HCl + Aq.) = (2NaCl + H2O + Aq.) + CO3.

We leave out of the account the water represented by Aq., for this, being merely the medium of the reaction, is not changed. Now we can prove our work; because, if we have added correctly, the sum of the weights of the factors must exactly equal the sum of the weights of the products-and so it is 106+73= 179, and 117+18+44179. Besides the information. which the equation gives us in regard to the manner in which the chemical change takes place, the symbols also inform us that 106 parts by weight of sodic carbonate are acted upon by 73 parts by weight of hydro