iS79-3 
of Physical Law. 
279 
particles of light shooting off from a distant luminous body 
with this immense velocity, and, falling upon a mirror, their 
motion would not merely be checked, but the elasticity of 
these light particles must be assumed to be so perfect that 
they rebound with an equal velocity. 
According to the undulatory theory, the light consists of 
vibrations of a medium which fills all space. Since the 
velocity of transmission of these vibrations is so great it 
follows that the elasticity of this medium must be 
10,000,000,000 times as great as that of the hardest steel. 
Space is not now regarded as a void, but is filled with a 
medium which, as Thomas Young remarked, “ is not only 
highly elastic, but absolutely solid.” And yet, as we walk 
through space, the solid atoms which compose our bodies 
experience not the slightest resistance. Such ideas, although 
they can be conceived, cannot be realised. We have had 
no previous experience with materials possessing such pro- 
perties, and such ideas must necessarily appear strange to 
us ; but they are no more strange than the phenomena of 
light which we directly observe, and which force us to this 
or to some other theory equally marvellous. Only those 
who have carefully examined the subject can realise how 
weighty is the evidence in favour of the undulatory theory 
of light ; but where such stupendous conceptions are in- 
volved, a slavish acceptance of any theory, even by them, 
would be in the highest degree objectionable. We are not 
the friends of theories, but of truth. 
It is not surprising, then, that in the progress of our 
sciences many errors of reasoning and in the interpretation 
of faCts have been committed. You are all familiar with 
the ideas of Newton in regard to the nature of light, ideas 
which were not in themselves absurd, which were firmly 
believed in by this man of such transcendent power, but 
which were clearly negatived by results of subsequent expe- 
riment. 
Lavoisier’s idea that all acids were compounds of oxygen 
received a complete refutation when the constitution of 
prussic and muriatic acids became known. In faCt the 
errors of scientific men are well-nigh innumerable, not be- 
cause they are men of science, but because they are men , 
and we are probably justified in saying, quite in general, 
that if the man who never committed a mental blunder be 
found, we shall also find a man who never conceived a 
vigorous thought. The faCl that the results of scientific 
men can usually be checked by observation and experiment 
perhaps diminishes their liability to err, and enables them 
