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Having described the three divisions into which all the material of nature falls, I must add a few general remarks on the subject. And this must be premised, that the atmosphere belongs to the class of bodies that possess unity. What exactly this means, and why it must be laid down as an axiom, will appear if I go back a little, and entering more fully into the subject, tell you that certain bodies are continuous, and certain formed, by a union of different elements.35 Continuity may be defined as unbroken union of parts one with another. Unity is continuity without a break; it is the contact of two bodies joined to one another. There can be no shadow of doubt that of the bodies around us which we see and handle, and which are either perceived or perceive, certain are composite. They are so either through nexus or through mere accumulation; take as illustrations a rope, corn, a ship. Again, there are bodies that are not composite, as a tree, a stone. You must, therefore, grant that likewise among the objects that elude sense, and are grasped only by thought, some are possessed of unity36 [while some arise from junction of parts]. See how careful I am of your susceptibilities. If I had chosen to employ the jargon of philosophy, I might have got out of the difficulty by merely saying “united bodies.” You must, in turn, be duly grateful for this concession to your weakness! What am I driving at? This: if at any time I speak of “unity” in this connection, bear in mind that it is not used of number, but has reference to the composition of a body that coheres through no external aid, but by its own unity. To this category the atmosphere belongs.