seek to find out how they are related; that is, to discover the laws which connect them. The process of reasoning by which we discover such laws is called induction. As we can seldom be sure that we have apprehended all the related facts, it is clear that our inductions must generally be incomplete. Hence it follows that oonclusions reached in this way are at best only probable; yet their probability becomes very great when we can discover no outstanding fact, and especially so when, regarded provisionally as true, they enable us to predict phenomena before unknown.
In conducting our experiments, and in reasoning upon them, we are often guided by suppositions suggested by previous experience. If the course of our experiment be in accordance with our supposition, there is, so far, a presumption in its favor. So, too, in reference to our reasonings: if all our facts are seen to be consistent with some supposition not unlikely in itself, we say it thereby becomes probable. The term hypothesis is usually employed instead of supposition.
Concerning the ultimate modes of existence or action, we know nothing whatever; hence, a law of nature cannot be demonstrated in the sense that a mathematical truth is demonstrated. Yet so great is the constancy of uniform sequence with which phenomena occur in accordance with the laws which we discover, that we have no doubt respecting their validity.
When we would refer a series of ascertained laws to some common agency, we employ the term theory. Thus we find in the "wave theory " of light, based on the hypothesis of a universal ether of extreme elasticity, satisfactory explanations of the laws of reflection, refraction, difEraction, polarization, etc.
3. Measurements.—All the phenomena of Nature occur in matter, and are presented to us in time and space. Time and space are fundamental conceptions: they do not admit of definition. Matter is equally indefinable: its distinctive characteristic is its persistence in whatever state of rest or motion it may happen to have, and the resistance which it offers to any
