August 31,1872.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
165 
changing characters of human nature and-human society 
How is it that experience of this sort, vast as it is, can | 
guide us in a matter so different from itself F How does 
scientific thought, applied to the development of a kan¬ 
garoo fcetus or the movement of the sapinexogen, make 
prediction possible for the first time in that most important 
of all sciences, tho relations of man with man ? In the 
dark or unscientific ages men had another way of apply¬ 
ing experience to altered circumstances. They believed 
for example that the plant called Jew’s-ear, which does 
bear a certain resemblance to the human ear, was a 
useful cure for diseases of that organ. This doctrine of 
signatures, as it was called, exercised an enormous in¬ 
fluence on the medicine of. the time. I need hardly tell 
you that it is hopelessly unscientific ; yet it agrees with 
those other examples that we have been considering in 
this particular; that it applies experience about the shape 
of a plant, which is one circumstance connected with it, 
to dealings with its medicinal properties, which are other 
and different circumstances. Again, suppose that you 
had been frightened by a thunderstorm on land, or your 
heart had failed you in a storm at sea ; if any one then 
told you that in consequence of this yrnu should always 
cultivate an unpleasant sensation in the pit of your 
stomach, till y r ou took a delight in it, that you should 
regulate your sane and sober life by the sensations of a 
moment of unreasoning terror; this advice would not be 
an example of scientific thought. Yet it would be an 
application of past experience to new and different cir¬ 
cumstances. But you will already have observed what 
is the additional clause that we must add to our definition 
in order to describe scientific thought and that only. 
The step between experience about animals and dealings 
with changing humanity is the law of evolution. The 
step from calculated places of Uranus to the existence of 
Neptune is the law of gravitation. The step from ob¬ 
served behaviour of crystals to conical refraction is made 
up of laws of light and geometry. The step from old 
bridges to new ones is the laws of elasticity and the 
strength of materials. The step then from past expe¬ 
rience to new circumstances must be made in accordance 
with an observed uniformity' in the order of events. 
This uniformity has held good in the past in certain 
places; if it should also hold good in the future and 
in other places, then, being combined with our expe¬ 
rience of the past, it enables us to predict the future, j 
and to know what is going on elsewhere; so that we 
are able to regulate our conduct in accordance with this 
knowledge. I want to make a little clearer the fact 
that what you call the evidence for a thing depends upon 
the assumption that this uniformity is valid at places 1 
and times at which it has not been observed. The aim 
of scientific thought, then, is to apply past experience to 
new circumstances ; the instrument is an observed uni¬ 
formity' in the course of events. By r the use of this in¬ 
strument it gives us information transcending our ex¬ 
perience, it enables us to infer things that we have not 
seen from things that we have seen ; and the evidence 
for the truth of that information depends, as we have 
seen, on our supposing that the uniformity holds good 
beymnd our experience. 
I now want to consider this uniformity a little more 
closely'; to show how the character of scientific thought 
and the force of its inferences depend upon the character 
ol the uniformity of nature. I cannot of course tell y'ou 
all that is known of this character without writing an 
encyclopaediabut I shall confine my'self to two points 
of it about which it seems to me that just now there is 
something to be said. I want to find out what we mean 
when we say that the uniformity of nature is exact; 
and what we mean when we say that it is reasonable. When 
a student is first introduced to those sciences which have 
come under the dominion of mathematics, a new and 
wonderful aspect of nature bursts upon his view. He 
has been accustomed to regard things as essentially ( 
more or less vague. All the facts that he has hitherto 
known have been expressed qualitatively', with a little 
allowance for error on either side. Things which are 
let go fall to the ground. A very observant man may 
know also that they fall faster as they go along. But 
our student is shown that, after falling for one second in 
a vacuum, a body is going at the rate of 32 feet per 
second; that after falling for two seconds, it is going twice 
as fast; after going two and a half seconds, two and a 
half times as fast. If he makes the experiment, and 
finds a single inch per second too much or too little in 
the rate, one of two things must have happened : either 
the law of falling bodies has been wrongly stated, or the 
experiment is not accurate ; there is some mistake. He 
finds reason to think that the latter is always the case ; 
the more carefully he goes to work, the more the error 
turns out to belong to the experiment. Again, he may 
know that water consists of two gases, oxygen and 
hydrogen combined together; but he now learns that 
two pints of steam at a temperature 150 deg. centigrade 
will alway's make two pints of hy'drogen and one pint of 
oxy'gen at the same temperature; all of them being 
pressed as much as the atmosphere is pressed. If he 
makes the experiment and gets rather more or less than 
a pint of oxy'gen, is the law disproved? No; tho 
steam was impure, or there was some mistake. Myriad j 
of analyses attest the law of combining volumes ; the 
more carefully they are made, the more nearly' they coin¬ 
cide with it. The aspects of the faces of a crystal are 
connected together by a geometrical law, by which, four 
of them being given, the rest can be found. The place 
of a planet at a given time is calculated by' the law of 
gravitation; if it is half a second wrong, the fault is in 
the instrument, the observer, the clock or the law; now, 
the more observations are made the more of this fault is 
brought home to the instrument, the observer and the 
clock. It is no wonder then that our student, contem¬ 
plating these and many like instances, should be led to 
say, “ I have been shortsighted; but I have now put on 
the spectacles of science, which nature has prepared for 
my eyes; I now see that things have definite outlines, 
that the world is ruled by exact and rigid mathematical 
laws ; “ Ivai su, theos, geometreis.” It is our business 
to consider whether he is right in so concluding. Is the 
uniformity of nature absolutely exact, or only more 
exact than our experiments ? At this point we have to 
make a very' important distinction. There are- two 
ways in which a law may' be inaccurate. The first way 
is exemplified by that law of Galileo which I mentioned 
just now; that a body falling in vacuo acquires equal 
increase of velocity in equal time3. No matter how 
many feet per second it is going, after an interval of a 
second it will be going thirty-two more feet per second. 
We now know that this rate of increase is not exactly 
the same at different heights, that it depends upon the 
distance of the body from the centre of the earth; so 
that the law is only approximate ; instead of the.increase 
of velocity being exactly equal in equal times, it itself in¬ 
creases very slowly' as the body' falls. We know also that 
this variation of the law from the truth is too small to be 
perceived by' direct observation on the change of velocity. 
But suppose we have invented means for observing this, 
and have verified that the increase of velocity is inversely 
as tho squared distance from the earth’s centre. Still 
the law is not accurate; for the earth does not attract 
accurately towards her centre, and the direction of attrac¬ 
tion is continually varying with the motion Oi tne sea ; 
the body will not even tall in a straight line. -The sun 
and the planets, too, especially the moon, will produce 
deviations ; yet tho sum of all these errors will escape out 
new process of observation, by being, a great deal smaUer 
than the necessary errors of that observation. . But 
when these again have been allowed for, there i3 still 
the influence of the stars. In this case, however, we 
only' give up one exact law for another. It may still be 
held that if the effect of every particle of matter in the 
universe on tho falling body' were calculated according 
