202 COLOR. 



vision ; while the third, in some mysterious way, act upon 

 the principles of chemical affinity. 



And here it is proper to say that, in reading works of 

 science and philosophy, we must keep clearly in mind the 

 distinction between the facts which are brought to light by 

 the observations and experiments of scientific men, and the 

 theories by which they attempt to explain them. Facts, 

 once established by proper evidence, remain uncontrovert- 

 ed from age to age. We can rely with confidence upon 

 them. But the theories are continually changing. They 

 are only suppositions which may be imagined to account 

 for the facts, and ought to be received with great caution. 

 There is direct and positive proof that sound is produced 

 by vibrations of some material substance, but there is no 

 such direct proof in respect to luminous radiations. It is 

 only a matter of inference and reasoning. The reasoning 

 is, that we can only conceive of two modes by which a 

 force can be transmitted through space, namely, by the 

 progressive motion of material particles, and by an imdu- 

 latory movement of an intervening medium / and as it has 

 been abundantly proved that it is not and can not be the 

 former, we may safely infer that it must be the latter. It 

 may be so ; but then, on the other hand, it may be sup- 

 posed possible that modes of the transmission of force can 

 exist of which, having no experience of them, we can have, 

 in our present state of knowledge, no conception. It is not 

 very safe for minds as limited in their attainments and 

 powers as ours to conclude that a phenomenon that we 

 witness must necessarily be accomplished in one of two 

 ways simply from the fact that we do not know of any 

 third. 



However this may be, the scientific world at the present 

 day are almost universally convinced that the phenomena 

 of light, heat, electricity, and the like, are all the results of 



