which they give rise, either in their natural condition, or, as is usually the case, under certain artificial conditions to which the ingenuity of man has learned to subject them. As these aggregates are the lowest which can be perceived, they have been denominated elements, and are by some supposed to constitute the ultimate units of matter. But, independently of certain direct evidence against this view, it is far more consistent with what is now known of matter, and with the laws of thought, to regard them as the first or lowest stages of aggregation whose activities are capable of appealing, either directly or indirectly, to our senses. It is really no more probable that the so-called elements are the lowest subdivisions of matter than that the remotest stars visible are actually at the confines of the universe.
That these elements are capable of manifesting themselves to sense is the sole reason of our recognizing their existence; and the history of their discovery, by which their number has been so greatly increased, shows that their modes of manifestation are often so subtile as to escape all but the most thorough methods of detection. Many of these elements now universally recognized remained for a long time wholly unsuspected, and these then belonged to the great class of unknown aggregates. This interesting chapter in the history of science should suffice to teach us that below the known of to-day there lies a wide belt of the knowable unknown, and that other and still lower orders of aggregates will doubtless yet be induced to reveal their existence.
A reason for regarding these elementary substances as ultimate units has been supposed to be found in their great stability, which causes them to behave as if such were the case.
While there is one possible exception to this in the case of oxygen and the peculiar phenomena of ozone and antozone, it is indeed true, so far as known, of all the remaining elements, that they have thus far resisted all attempts to decompose them. This, however, aside from the possibility of doing so still, is really no evidence of their absolutely elementary character, but only indicates what the whole theory of evolution would admit, if not require, that all aggregates which could possess the properties requisite for the composition of such masses as are capable of affecting the senses, or of so affecting other masses as to make themselves known to the human intellect, must possess a degree of inherent stability sufficient to resist all human efforts to disintegrate them. While, therefore, it is very probable that, just as the alkalies and alkaline earths, which, at the beginning of the present century, were regarded as elementary, have yielded to the galvanic battery and proved to be composite, so a few more of those now classed as elements will at no distant day be similarly decomposed by the higher appliances yet to be devised; it is nevertheless entirely consonant with the view of the constitution of matter here maintained, that there shall remain upon the plane of human investigation a