On Scientific Method. 
[October, 
49 ° 
of which would be impossible were the hypothesis correct. 
To take an instance : — The upholders of the Phlogistic 
theory affirmed that when a metal is burned it parts with 
phlogiston ; that the product of combustion is metal minus 
phlogiston ; and that the re-transformation of the product 
of combustion into metal is brought about by the absorption 
of phlogiston. The upholders of what might be called the 
Oxygen theory affirmed that when a metal is burned it com- 
bines with oxygen ; that the produdf of combustion is metal 
plus oxygen ; and that the re-transformation of the product 
of combustion into metal is brought about by the removal 
of oxygen. Each hypothesis had facets in its favour; each 
explained many fadts. But the fadt discovered by Davy, in 
1807, that the metals potassium and sodium are adtually 
produced by the removal of oxygen from those substances 
which are themselves formed when these metals are burned, 
could not be explained in terms of the phlogistic theory. 
Either the fadt or the theory must give way. The fadt was 
established beyond a doubt ; therefore the theory — in its 
then accepted form at any rate — had to succumb. 
A good scientific hypothesis must, then, be in keeping 
with fadts ; but it does not follow that it must be simple, or 
that it must make no claims upon our belief. The hypo- 
thesis which well explains the fadts concerning light is, we 
might almost say, absurd in the demands which it makes 
upon our credulity. “We are asked by physical philoso- 
phers to give up all our ordinary prepossessions, and believe 
that the interstellar space which seemed so empty is not 
empty at all, but filled with something more solid and elastic 
than steel. As Dr. Young remarked, ‘ the luminiferous 
ether pervading all space, and penetrating almost all sub- 
stances, is not only highly elastic, but absolutely solid.’ Sir 
John Herschel has calculated the amount of force which 
may be supposed, according to the undulatory theory of 
light, to be exerted at each point in space, and finds it to be 
1,148,000,000,000 times the elastic force of ordinary air at 
the earth’s surface, so that the pressure of the ether upon a 
square inch of surface must be about 17 billions of pounds. 
Yet we live, and move without appreciable resistance through 
this medium, indefinitely harder and more elastic than 
adamant. All our ordinary notions must be laid aside in 
contemplating such an hypothesis ; yet it is no more than 
the observed phenomena of light and heat force us to 
accept.”* 
* Principles of Science, vol. ii., p. 145. 
