It is considered as a simple body, both because it is not capable of being resolved into other ingredients; and because it is itself capable of combining with other bodies, in a manner analogous to oxygen and chlorine. The most curious of these combinations is that which it forms with hydrogen gas, the result of which is a peculiar gaseous acid. CAROLINE. Just as chlorine and hydrogen gas form muriatic acid? In this respect chlorine and iodine seem to bear a strong analogy to each other. MRS. B. That is indeed the case; so that if the theory of the constitution of either of these two bodies be true, it must be true also in regard to the other; if erroneous in the one, the theory must fall in both. But it is now time to conclude; we have examined such of the acids and salts as I conceived would appear to you most interesting. --I shall not enter into any particulars respecting the metallic acids, as they offer nothing sufficiently striking for our present purpose. CONVERSATION XX. ON THE NATURE AND COMPOSITION OF VEGETABLES. MRS. B. We have hitherto treated only of the simplest combinations of elements, such as alkalies, earths, acids, compound salts, stones, &c.; all of which belong to the mineral kingdom. It is time now to turn our attention to a more complicated class of compounds, that of ORGANISED BODIES, which will furnish us with a new source of instruction and amusement. EMILY. By organised bodies, I suppose, you mean the vegetable and animal creation? I have, however, but a very vague idea of the word _organisation_, and I have often wished to know more precisely what it means. MRS. B. Organised bodies are such as are endowed by nature with various parts, peculiarly constructed and adapted to perform certain functions connected with life. Thus you may observe, that mineral compounds are formed by the simple effect of mechanical or chemical attraction, and may appear to some to be in a great measure the productions of chance; whilst organised bodies bear the most striking and impressive marks of design, and are eminently distinguished by that unknown principle, called _life_, from which the various organs derive the power of exercising their respective functions. CAROLINE. But in what manner does life enable these organs to perform their several functions? MRS. B. That is a mystery which, I fear, is enveloped in such profound darkness that there is very little hope of our ever being able to unfold it. We must content ourselves with examining the effects of this principle; as for the cause, we have been able only to give it a name, without attaching any other meaning to it than the vague and unsatisfactory idea of au unknown agent. CAROLINE. And yet I think I can form a very clear idea of life. MRS. B. Pray let me hear how you would define it? CAROLINE.