606 
NATURE 
[APRIL 25, 1907 
LETTERS TO THE EDITOR. 
{The Editor does not hold himself responsible for opinions 
expressed by his correspondemts. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice is taken of anonymous communications.] 
Electrical Method of Extracting Soot from Air in 
Flues. 
AsBouT two years ago I observed that a body positively 
electrified to 100 volts became covered with soot in a day, 
while a negatively charged body remained comparatively 
clean. 
I have reason to believe that the observation was first 
made by Lord Kelvin many years ago, but it does not 
seem to be well known. 
Our laboratory mechanic, Mr. Black, has_ recently 
applied this to cleaning air by inserting in an air-flue a sheet 
of wire gauze connected to the positive 250-volt supply. 
The electrified wire gauze is very efficient in extracting 
the soot from the air, and the method provides a simple 
means of cleaning the air supplied to large buildings in 
towns where the air is laden with soot. 
GEORGE W. WALKER. 
Physical Laboratory, The University, Glasgow, April 16. 
Paradoxes and Principles. 
Your critic has written his notice of my 
of Nature and Science’ (Nature, February 7, p. 341) 
without giving reasonable care to the examination of the 
book, and has in consequence made a damaging state- 
ment as to fact which is so extravagantly untrue that 
it goes far beyond the limits of fair comment. 
He says that I neglect general principles in the ex- 
planation of paradoxes, and tells me how they ought to 
be explained, “‘. . . by showing that the abnormal pheno- 
mena are determined by precisely the same laws as the 
normal phenomena; to ‘ explain” why a balloon rises it is 
necessary to propound the general principles of gravita- 
tional mechanics and to show that it rises for the same 
reason as a stone falls. But Dr. Hampson eschews general 
principles.” 
This is grossly untrue. My book teems with statements 
of and references to general principles ‘‘ propounded”? in 
explanation of paradoxes, exactly on the system recom- 
mended to me. I have not undertaken to explain the 
rising of balloons—a thing not regarded by me or my 
acquaintances as paradoxical—but I have explained why 
water stands in an inverted tumbler, and have done it 
exactly on the lines prescribed by your critic; propound- 
ing, p. 85, the general principle of fluid pressure, ‘* gases, 
like liquids, are fluids, and transmit pressure equally in 
all directions. The air, then, transmits in all directions 
the pressure due to its own weight, and it thus presses 
upwards beneath ...,’’ and, pp. 92, 197, the general 
Principle of gravitational attraction:—‘‘ The ordinary 
meaning of the word ‘ weight’ is an earthward-tending 
force which can be used as a measure of the quantity of 
material. It depends upon the mutual attraction between 
the material and the earth.”” ‘‘. .. the force . . . must 
vary inversely as the square of the distance in correct 
agreement with the law of gravitation.”’ 
A few of the many other references to general principles 
invoked in explanation of paradoxes are as follows :— 
P. 19, “‘ Heat makes things expand.’’ 
P. 29, “It is a general law of nature that a moving 
body tends to keep moving straight on at the same speed.” 
_ P. 32, “*... resists . . . by virtue of the great law of 
inertia, the strong tendency possessed by all moving things 
to resist interference with their motion.’ y 
f P. 33, “Like all other things, it tries hard to keep to 
its original direction of movement.’’ 
P. 78, “A fluid, when pressed upon, transmits the full 
pressure equally in all directions.”’ 
P. 93, ‘‘. . . its tendency at any moment is, in accord- 
ance with this law of inertia, to go straight on.” 
eB Gyre, 1S . the centrifugal force increases in propor- 
tion to the square of the velocity.’’ 
P. 116, ‘The general principle that weights, in 
descending, cannot produce more power than they con- 
sume in being raised the same height.’ 
NO. 1956, VOL. 75] 
“ Paradoxes ~ 
P. 118, “* All things with which we are acquainted have 
some heat.” 
P. 149, “‘ The principle that it requires heat energy to 
convert water into vapour .. .”’ 
P. 154, ‘* Gases and vapours are very poor conductors 
of heat.” 
P. 170, ‘‘ The sudden expansion and conversion into 
vapour require much heat.” 
P. 179, ‘*The great law of the conservation of 
” 
energy - . 
P. 212, ‘‘ The doctrine that no element could by any 
means be changed into anything else.”’ 
P. 211, The persistence of matter, 
lines. 
If your critic does not intend to maintain the nonsensical 
proposition that a book for popular reading by the un- 
initiated should bristle with quantitative formula, how can 
he say that in writing the above and many dozens of 
similar passages I eschew general principles? Is this his 
‘idea of scientific method ’’? 
His criticism, as a guess, was, of course, not unlikely to 
be true of a book for such readers as I had in view. But 
when a critic has not time to read the book entrusted to 
him for judgment, would it not be fairer to the journal 
and to the author if he excused himself from the task of 
preparing a notice? 
Of the value of his criticisms as to style and method, 
which it would take pages to discuss, I leave your readers 
to judge by the circumstance that the one statement 
capable of being definitely tested by a few quotations 
shows such complete carelessness about facts as to render 
the critic, if not malicious, certainly incompetent. 
February 11. W. Hampson. 
stated in sixteen 
A scientiFic ‘‘ principle’? is a proposition assumed to 
be true universally, which is made the basis of deductions. 
I said that Dr. Hampson ‘‘ eschewed general principles ”’ 
in the sense that he does not expound these propositions 
or make them the basis of his ‘‘ explanations.” 
In refutation of this statement Dr. Hampson quotes 
one paragraph and fourteen short sentences. (The wn- 
quoted reference to p. 211 is not evidence.) Presumably 
he considers these passages as convincing as any that he 
can find; certainly none could be more conclusive of the 
justice of my criticism. For of the fourteen sentences 
nine do not deal with principles at all; some of them 
state non-universal experimental generalisations, others 
particular cases of general theorems, of which no proof 
is offered. In the remaining five, two “ principles’’ are 
mentioned, of which one, the conservation of energy, is 
merely named, but is nowhere propounded; the other 
““ principle ’? is Newton’s first law of motion. 
In order to justify my contention decisively, it is really 
only necessary to point out (1) that for the last twenty 
years Newton’s laws of motion have not been accepted 
as adequate principles of mechanics, and (2) that none of 
the attempts at stating the first of those laws is successful. 
However, I will make every possible concession and admit, 
for the sake of argument, that Newton’s laws are “ prin- 
ciples,’’ and that Dr. Hampson has stated one of them. 
But then, where are the others? It is impossible to found 
mechanics on the first law alone. It is just because Dr. 
Hampson has neglected the second law, which introduces 
the conception of “force ’’ and all its consequents, that 
his writings abound in confusion. Thus, in his longer 
quotation, which he holds up as a model of exposition, 
he has left the imagination of the reader to divine the 
nature and effects of ‘‘ pressure’’ and ‘‘ force’’; it so 
happens that in this case the ambiguity is not serious, 
but elsewhere it is extremely serious. It is appalling to 
think in what mazes he would have entangled himself if 
he had not been so discreet (but inconsistent) as to omit 
Pascal’s famous hydrostatic paradox from his list. 
I repeat, then, that Dr. Hampson has attempted to 
explain the results of science without enunciating its 
principles. With fifteen quotations at his disposal he can 
produce no better evidence against that judgment than 
four inaccurate statements of a single antiquated principle 
which was never regarded as a sufficient foundation for 
even one of the many branches of physics with which he 
deals ! Toe REVIEWER. 
