1878.] 
On Residual Phenomena . 
23 
into the discrepancy, Laplace suggested that the heat libe- 
rated by the compression of the air resulting from sound- 
vibrations might be the missing faCtor. Its influence was 
accordingly calculated, and was found to supply the complete 
solution of the difficulty. Nor was this all ; the law of the 
evolution of heat on the compression of bodies received at 
the same time a most signal verification. 
These few instances will suggest certain reflections. 
Residual phenomena, though possibly of very frequent oc- 
currence, will be overlooked save by the patient and accurate 
investigator who is content with nothing less than certainty. 
Had Arfwedson quietly assumed that the saline body formed 
during his operations was sulphate of magnesia, without 
subjecting it to any quantitative examination, lithia would 
probably have remained undiscovered for some time, and the 
honour would doubtless have fallen to the share of some 
other chemist. Had the comparison between the experi- 
mental and the theoretical velocity of sound been made in a 
careless manner, the discrepancy would have escaped atten- 
tion, and the question ultimately solved by Laplace might 
never have been raised. But there is another danger to 
which Liebig has drawn attention, and which he has happily 
illustrated by an incident in his own early career. The 
experimentalist may perceive a difference between theory and 
practice, between results or properties actually observed and 
those which in his opinion were to have been expeCted, but 
may content himself with framing some hypothesis to 
account for the difficulty without resorting to any aCtual 
investigation. Such a line of conduCt prevented Liebig from 
anticipating Balard in the discovery of bromine. He had 
actually obtained the new element in a state of approximate 
purity, but had assumed it to be chloride of iodine, had 
imagined an hypothesis to explain its peculiarities, and had 
set it aside unexamined. This event, as he tells us, served 
him as a caution for the rest of his career. How many 
residual phenomena, which have been really noticed, escape 
investigation in virtue of some superficial and baseless expla- 
nation we can scarcely even conjecture. 
But another question arises here : — How is a residual phe- 
nomenon to be distinguished, on the one hand, from those 
errors of observation and experiment which cannot be abso- 
lutely avoided, even by the most careful operators, and, on 
the other, from those discrepancies which prove that a law 
or a theory is essentially erroneous, and must be rejected. 
It is scarcely possible, we fear, to lay down any absolute rule 
which shall in all cases direCt the enquirer how to overcome 
