of them. Oxygen, hydrogen, and nitrogen are three different kinds of air. Here are three bottles containing these different kinds of air. There is no apparent difference in point of color, nor have they any smell; you cannot distinguish them by these senses. But a very simple implement enables us to do so. This little taper serves the purpose of a new sense to us. I do not know which of these contains hydrogen, which oxygen, or which nitrogen; but this lighted taper will soon tell me.

Prof. J. here put the taper into the bottle containing nitrogen, and it was extinguished; he then re-lighted it, and put it into another, that containing oxygen, and it burned brightly; next, he put it into the third containing hydrogen, and there was a slight explosion, which put out the taper, leaving, however, the snuff, which ignited again, when it was withdrawn-the gas itself burning—and which ignited also when placed in the oxygen. This, then, said he, enables me to distinguish these three gases. This, in which the taper kindles and burns brightly and re-kindles, is oxygen; this, which takes fire itself, is hydrogen; this, which extinguishes the taper, is nitrogen. These three substances, hydrogen, oxygen and nitrogen exist in plants in different proportions, not in the shape of air, but in a solid form. We cannot imitate it; but they do assume this form naturally. Sulphur, you know exists in small quantities in plants, and phosphorus in a still smaller quantity. Now, these substances compose the organic part of plants, or that part which burns away. But where does the plant get these things of which it consists so largely the carbon comes partly from the air, and partly from the soil; oxygen partly from the air and soil; hydrogen mostly from the soil; nitrogen altogether from the soil; sulphur and phosphorus, altogether from the soil. Oxygen and hydrogen compose water, and the plant gets them either from the rain, or from the water in the soil; carbon it gets partly from the air and partly from the soil. Now that you may understand how it is that plants derive these things from the air and soil, I must make you acquainted with another substance.

If I take a piece of limestone, reduce it to powder, put it into a vessel, pour on it first a little water, and then an acid, as nitric acid, it will boil up, or effervesce. This boiling up or effervescence is produced by the evolution of a kind of air which produces these bubbles. In this kind of air, the taper will be extinguished. It therefore corresponds in this particular with the gas called nitrogen, in one of these bottles. How are we to distinguish between these two gases? It is in this way: If I undertake to pour the nitrogen into this glass, I cannot do it; if I undertake to pour it on this candle, it has no effect upon it; but if I take the gas which produces this effervescence and pour it into the glass, I

can fill it, and though the glass appears to be empty, it will be found to be full of it, for if I put the taper into it, the blaze will be immediately extinguished. There is, therefore, this marked difference between the two gases: the one, the carbon, can be poured out into another vessel, because it is heavier than common air; but the nitrogen which is lighter, cannot be poured out; but it will rise. Hence the extinguishment of the taper is no test of the presence of carbonic acid or nitrogen; but they are distinguished altogether by their comparative weight. Common air is composed of 79 parts of nitrogen to 21 parts of oxygen, or nearly-carbonic acid constituting about 4-100 of it. This small quantity of carbonic acid exists in the air, and from this small quantity, plants derive all the carbonic acid which they get from the air.

How do they take it in? I showed you in a former lecture, that the under surface of the leaves of plants is covered with an immense number of minute pores, and that these pores vary according to the circumstances under which the plants live. They draw in through these pores, carbonic acid during the day, but not during the night. The very great number of leaves and surfaces thus presented to the air, enables the plant to draw from it the minute portion of carbonic acid necessary to its growth. This is one of the wonderful things, of which nature is full. You cannot but be astonished to find, that this never ceasing operation is going forward, and that the countless leaves of plants, which seem to us as intended merely for the ornament of trees, and to gratify the eye, by their perpetual motion, as the winds pass through them, are actually necessary to enable the plants to extract from the air, or to drink in the element so necessary to their growth and maturity.

I shall, at the next meeting, draw your attention to the substances existing in plants; that is to say, I shall show you that wood contains these elementary substances-carbon, oxygen, hydrogen and nitrogenbut that it does not contain them in the states in which I have exhibited them here, but in a different form, and I shall also show you that the plant consists of other substances, which are necessary to its existence as such. For instance, this piece of wood, (holding up a rod) consists of what we call, woody fibre, mostly. The stalk or straw of wheat, and grain, contains more than one kind of matter; so the seeds of plants, such as linseed, contain oil, among other things; so that we have all these things, growing in plants, in the wood, in the seeds, &c.

At the next meeting, I shall again call your attention to the functiona of the leaf, and the manner in which the leaf acquires carbon from the [Assembly, No. 175.]

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atmosphere, in order to explain the functions of animal life, and to show how these functions are related. I cannot do this now, because I must introduce new names and things, and because the subject comes in more strictly in connection with the next lecture. But I may make one observation here in relation to these substances, nitrogen, carbon, derived from carbonic acid, oxygen and hydrogen, which compose water; that nitrogen is obtained from the soil in various forms, and that that is one form in which it is taken in by plants, but not so universally as some have supposed. There is one form in which nitrogen exists, and that is in ammonia or common hartshorn. The nitrogen, which is necessary to the growth of plants, is often taken in, in this form, though not universally; and though it exists in plants, in small quantities, yet it is of the greatest possible consequence to the existence of human and animal life. Thus much of the organic parts of plants.

I pass on to the inorganic parts of plants; and here I shall show you the necessity of those mineral substances of which I spoke at our last meeting. If you take the ash of wood, or of any plant, and submit it to the same chemical examination to which I submitted that part of the soil remaining after being burnt, you will find what the chemist tells you, that this ash consists, not of one or two substances, but of eight or ten. It will be found that the soil and the plant contain the same substances; the only one not in the plant being alumina. What is the function of alumina in the soil? Its mechanical function is to anchor the plant. Tenacity is necessary for this purpose. Some plants grow in mere sand, but the great majority of them require a certain degree of tenacity in the soil, which is obtained by mixing silica with clay. This alumina being clay, explains why it is that it is not in the plant, but only in the soil. It does not enter into the plant but gives tenacity to the soil, which is necessary to retain the plant. Take any plant, and it will be found to contain this ash, and this ash you will find contains all these substances, some in larger, some in smaller quantities. To show the composition of the ash of different plants, Prof. J. referred to the tables exhibiting the composition of the ash and straw of different plants.

There you will see that all these different substances present in the soil are also present in the plant, but the proportions differ. You will observe that in the ash of wheat, oats, barley and rye, potash exists in the proportion of about 23-100, whereas, in the soil, the same ingredient is present in but a comparatively small proportion. So with phosphoric acid; it constitutes nearly half the ash of the grains, whereas in the soil it is exceedingly small. Now, this phosphoric acid, though present in small quantities in the soil, is so necessary to the growth of plants that they are found to contain a large proportion of it. Now, [pointing to the table exhibiting the composition of the ash of straw,] it will be seen that the straw contains but a small quantity of phosphoric acid. Potash in Indian corn is very like that in wheat. The straw of wheat contains a large proportion of silica; the ash of grain a large proportion of phosphoric acid. This acid rises as the plants grow, while the siliceous matter comes in by the roots and lodges itself in the straw. We see similar differences if we look at the composition of the ash of our green crops, as the turnip and potato; the potato is more than half potash, while the phosphoric acid is small compared with that in the grains. In short, every plant, taken as a whole, contains these things in proportions different from any other plant; and plants of different kinds or families differ materially. So different parts of the same plant contain these substances in different proportions. What is the inference from all this? Suppose a plant to be growing; it must get from the soil those substances which it most requires. If, in forming the flower and perfecting the seed, these substances must flow up readily, and the soil must furnish them in sufficient quantities, or the plant must cease to grow rapidly, this shows the practical applications or results that we shall arrive at—to which practical men have not yet done—but which when we shall have reached a system of refined Agriculture will enable us more intelligently to adapt cur modes of cultivation to the growth of plants; and to that we shall come bye-and-bye. But to the practical application of these facts. First, you see what plants grow better in some soils than in others; that if plants grow well on a given soil, it must be because that soil supplies the wants of the plant. Now, some soils contain very little phosphoric acid; if the soil contains much potash and you put upon it a plant requiring little, it will not grow well, whereas, if you put upon it another plant requiring a great deal, it will grow well.

When speaking of the relations of Geology to Agriculture, I showed you that the kind of trees growing upon different tracts of land indicated differences of soil-differences arising from the geological confor