and nitrogen, with traces of other gases, must consist in ultimate analysis of myriads of gaseous molecules. There are molecules of oxygen, and molecules of nitrogen, as well as molecules of the other gases. No doubt the molecule may consist of parts; indeed we know, as a matter of fact, that it does consist of parts; it is therefore conceivable that the molecule could be subdivided, but these parts, whatever other properties they might have, would certainly not possess the characteristics of the original gas to which the molecule belonged.
It is something to have learned so much at all events with regard to the composition of the air we breathe. But we have been able to find out a great deal more, and to discover that the behaviour of these little molecules is of the most extraordinary and vivacious description. That air is eminently mobile is sufficiently obvious; but the air which fans our faces, or the wind which turns our windmills, or even the hurricane which devastates whole towns, gives only a very imperfect idea of the real mobility of air. The hurricane may move, perhaps, at a pace of a hundred miles an hour, but that is a velocity which might almost be described as sluggish in comparison with the molecular movements of the ultimate particles of gases. For our present purpose we may think of the air, not as it blows over the mountain tops, but as it lies in some secluded cavern where all seems absolutely still. No doubt to our ordinary methods of investigation the air and every particle of it may seem in such a case to be absolutely quiescent, but we have ascertained that the apparent quietness is really the result of the want of sufficient acumen in our organs of sense. Had our perceptive powers been endowed with the necessary subtlety, we
