586 MODERN METHODS OF SCIENCE. 
Lavoisier, in the latter part of the last century, elucidated the 
phenomenon of respiration and the production of animal heat by 
one of the most beautiful of theories, based, to all appearances, 
upon well observed facts; yet at the present day more delicate 
observations, and the discovery of the want of balance between 
the inhaled oxygen and exhaled carbonic acid subverted that 
beautiful theory, and we are left entirely without one. It is true 
we have collated a number of facts in regard to respiration, molec- 
ular changes in the tissues, ete., all of which are recognized as 
having something to do with animal heat; still it is acknowledged 
that we are incapable of giving any concrete expression to the 
phenomenon of respiration and animal heat as Lavoisier did 
eighty or ninety years ago. 
Electricity is the same now as it has ever been, yet it was once 
spoken of as a fluid, then as a force, now as an energy readily 
convertible into caloric or mechanical energy ; and in what light 
it will be considered fifty years hence no one can predict. 
Now what I desire to enforce here is that, amid all these changes 
and revolutions of theories, so called, it is simply man, the inter- 
preter, that has erred, and not Nature; her laws are the same ; we 
simply have not been able to read them correctly, and perhaps 
never shall be. ; 
What, it may be asked, are we to do then? Must we cease the- 
orizing? Not at all. The lesson to be learned from this is, to be 
more modest in our generalizations; to generalize as far as our 
carefully made out facts will permit us, and no farther; to check 
e imagination and not to let it run riot and shipwreck us upon 
some metaphysical quicksand. 
The fact is, it becomes a question whether there is such a thing 
as a pure theory in science. No true scientific theory deserves 
the name that is not based on verified hypotheses ; in fact, it is 
but a concise interpretation of the deductions of scientific facts. 
Dumas has well said that theories are like crutches, the strength 
of them to be tested by attempting to walk with them. And 
might farther add that very often scientists, who are without sure- 
footed facts to carry them along, take to these crutches. 
It is common to speak of the theory of gravitation, when there 
is nothing purely hypothetical in connection with the manner in 
which. it is studied ; in it we only see a clear generalization of ob- 
served laws which govern the mutual attraction of bodies. If at 
