392 SCIENCE, 
porary ladders and passages, or which must 
be supported till we can see how to fill in 
the understructure. To give the hypothe- 
ses equal validity with facts is to confuse 
the temporary scaffolding with the building 
itself. 
But even if we take this view of the tem- 
porary nature of our molecular and ethereal 
imaginings, it does not lessen their value, 
their necessity to us. 
It is merely a true description of our- 
selves to say that we must believe in the 
continuity of physical processes, and that 
we must attempt to form mental pictures of 
those processes, the details of which elude 
our observation. For such pictures we 
must frame hypotheses, and we have to use 
the best material at command in framing 
them. At present there is only one funda- 
mental hypothesis—the molecular and ethe- 
real hypothesis—in some such form as is 
generally accepted. 
Even if we take the position that the form 
of the hypothesis may change as our knowl- 
edge extends, that we may be able to devise 
new machinery—nay, even that we may be 
able to design some quite new type to bring 
about the same ends—that does not appear 
to me to lessen the present value of the hy- 
pothesis. We can recognize to the full how 
well it enables us to group together large 
masses of facts which, without it, would be 
scattered apart, how it serves to give work- 
ing explanations, and continually enables 
investigators to think out new questions for 
research. We can recognize that it is the 
symbolical form in which much actual 
knowledge is cast. We might almost as 
well quarrel with the use of the letters of 
the alphabet, inasmuch as they are not the 
sounds themselves, but mere arbitrary sym- 
bols of the sounds. 
In this country there is no need for any 
defence of the use of the molecular hypoth- 
esis. But abroad the movement from the 
position in which hypothesis is confounded 
[N. S. Von. X. No. 247. 
with observed truth has carried many 
through the position of equilibrium equally 
far on the other side, and a party has been 
formed which totally abstains from mole- 
cules as a protest against immoderate in- 
dulgence in their use. Time will show 
whether these protesters can do without 
any hypothesis, whether they can build 
without scaffolding or ladders. I fear that 
it is only an attempt to build from balloons. 
But the protest will have value if it will 
put us on our guard against using molecules. 
and the ether everywhere and everywhen. 
There is, I think, some danger that we may 
get so accustomed to picturing everything 
in terms of these hypotheses that we may 
come to suppose that we may have no firm 
basis for the facts of observation until we 
have given a molecular account of them, 
that a molecular basis is a firmer founda- 
tion than direct experience. 
Let me illustrate this kind of danger. 
The phenomena of capillarity can, for the 
most part, be explained on the assumption 
of.a liquid surface tension. But if the sub- 
ject is treated merely from this point of 
view it stands alone—it is a portion of the 
building of science hanging in the air. The 
molecular hypothesis then comes in to give 
some explanation of the surface tension, 
gives, as it were, a supporting understruc- 
ture connecting capillarity with other 
classes of phenomena. But here, I think, 
the hypothesis should stop, and such phe- 
nomena as can be explained by the surface 
tension should be so explained without ref- 
erence to molecules. They should not be 
brought in again till the surface-tension ex- 
planation fails. It is necessary to bear in 
mind what part is scaffolding, and what is 
the building itself, already firm and com- 
plete. 
Or, as another illustration, take the Sec- 
ond Law of Thermodynamics. I suspect 
that it is sometimes supposed that a molec- 
ular theory from which the Second Law 
