49 
which required it to move rapidly. Again, when it was dis- 
covered that an acid and an alkali were produced at the poles, 
together with oxygen and hydrogen, when water was decom- 
posed by electricity, it was supposed by some that electricity 
had the power of generating acids and alkalies; but Davy sup- 
posed that this might be the result of the circumstances in 
which the experiments had been performed ; he therefore 
varied those circumstances, until he performed the experiment 
without any acid or alkali having been detected. 
Having now, by the methods indicated, obtained a certain 
body of tolerably trustworthy facts or materials for science, the 
next step is to give them cohesion, or convert them into 
science — to bind them into as few unities as possible. We 
have now to pass from facts to inferences, from the senses to 
the intellect ; to bring into play that unifying power of the 
mind by which we detect the one in the many, and discover 
the special law, of which various facts are illustrations. This 
is done by what I may perhaps be permitted to call an inductive 
guess. 
The mind that is trained to close and cautious inference, 
and at the same time possesses a special aptitude for general- 
ization, will almost instinctively see the hypothesis that supplies 
the needed explanation. As in the case of Pascal, who, 
rejecting the previous idea of nature having an abhorrence of 
a vacuum, conceived that air had weight ; or in that of Roger 
Bacon, explaining by refraction the bending power of a convex 
lens towards the perpendicular, while his predecessors thought 
it to be the result of the material of the substance through which 
the light passed, the form having been supposed to be of no 
importance. All persons, however, have not been of this accu- 
rate character. Most discoverers have tried many suppositions 
before they have hit upon the right one ; numbers have passed 
iD review before their judgment has selected any as probable; 
and even of those so selected, not one may have survived the 
test of experiment. The weakest analogies, the most whimsical 
notions, the most apparently absurd theories, may pass through 
the teeming brain, and no recoi’d may remain of more than the 
hundredth part Kepler, for example, imagined and discarded 
no fewer than nineteen hypotheses before he established the 
actual fact regarding the motion of Mars, and then applied to 
it the correct term “ elliptic.” 
But although a guess or hypothesis may be erroneous, it does 
not follow that it is useless; it may be a means of collecting 
and binding together evidence for a certain purpose, which, 
although eventually useless in the proving of that for which it 
VOL. x. E 
