182 

Letters to the Editor. 
[The Editor doesnot hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, nor to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of NATURE. No notice ts 
taken of anonymous communications. | 
On the New Element Hafnium. 
In a former letter to Nature (January 20, p. 79) 
we announced the discovery of a new element with 
atomic number 72, for which the name hafnium was 
proposed. Evidence was given that this element 
1s a homologue of zirconium in accordance with 
theoretical expectations (Bohr, “ Theory of Spectra 
and Atomic Constitution,” p. 114, Camb. Univ. Press, 
1922). Continued experiments enable us to complete 
the statements in the former letter. By the addition 
of a known quantity of tantalum (73) to our samples, 
and by a comparison of the intensity of the Ta-lines 
with the Hf-lines, a closer estimate of the amount of 
hafnium present has been obtained. We have in- 
vestigated a great number of zirconium minerals 
from different parts of the world. They all contained 
between 5-10 per cent. of hafnium. In samples of 
commercial zirconium oxide investigated, we have 
found the new element, amounting in one case to 
as much as 5 per cent. Starting from the latter 
substance, by means of a chemical method which is 
also adapted to separate zirconium from the other 
tetravalent elements, we have been able to obtain 
several grams of a preparation in which the presence 
of about 50 per cent. of hafnium could be established. 
Conversely, we have succeeded in preparing zirconium 
in which no hafnium lines could be observed. Further 
particulars about the method of preparation and 
provisional determination of the atomic weight will 
be published shortly in the communications of the 
Copenhagen Academy. D. CosTrEr. 
G, HEvrEsy. 
Universitetets Institut for teoretisk Fysik, 
Copenhagen, January 31. 

The Latent Period in Lubrication. 
SOMETIMES in a scientific inquiry results accrue 
which are called, in laboratory slang, “ pretty ’’; the 
pieces of the puzzle have fallen together in a fashion 
so pat as to give artistic pleasure. 
Most lubricated surfaces have the curious property 
that the friction falls after the Imbricant has been 
applied, until a steady state is reached after an 
interval which may vary from a few minutes to a few 
hours. For example, a clean surface of glass lubri- 
cated with pure heptoic acid, the slider being in 
position, the initial value of the coefficient of friction 
at 12° was #=0-51, but in 4o minutes it had fallen to 
its steady value, u=0-40. 
This latent period, as it may be called, is shortened 
by a rise in temperature and, apparently, by mechan- 
ical agitation ; and is manifested by surfaces lubri- 
cated only by a film of insensible thickness as well as 
by those flooded with liquid. 
The most striking fact, however, is the influence of 
the slider. The final steady state is never reached 
unless the slider is in position. Surfaces which have 
been freely exposed to vapour or to an excess of fluid 
resting on them always have high friction when first 
put in contact. The lowest value is given only by a 
film of lubricant which has been enclosed for some 
NO. 2780, VOL. 111] 
NATURE 
[FEBRUARY I0, 1923, 
time between two solid faces. Such is the puzzle, and 
the explanation is curiously simple. 
A molecule of an aliphatic acid or alcohol is like a 
rod loaded at one end. Putting a drop of one of these 
substances on a clean surface is like flinging a handful 
of such rods, picked up at random, at it; some hit 
and stick by the loaded ends, others by the unloaded 
ends. Condensation from the vapour is similar 
except that the rods are flung singly. _ oto 
It is practically certain that friction is lowest when 
all the rods are orientated in the same way, a condi- 
tion which will be reached only when the wron 4 
orientated molecules have had time to evaporate off 
into the fluid or vapour and have been replaced b 
molecules rightly orientated. The latent period is 
the time occupied by this readjustment. a 
So long, however, as the layer is exposed to fluid or 
vapour it is always losing or gaining molecules by — 
evaporation and condensation, and some of those oF: 
arriving will be wrongly orientated. The layer will 
reach a steady state, but it will not be that of least 
friction because at any moment a fraction of the 
molecules will be wrongly orientated. Orientation 
will be as perfect as possible and friction at its lowest 
only after a layer has been for some time shielded 
from evaporation by being enclosed between solid 
faces. ee 
If this explanation be correct there should be no ~ 
latent period when both ends of the rods are alike. — 
This is so. In normal paraffins both ends are alike, __ 
and in no circumstances do surfaces lubricated by normal 
paraffins show a latent period. SY 
The fact that a latent period exists is of importance 
to practical lubrication. The molecular process which 
causes it is, we believe, of importance in the mechanics 
of living matter. Physiologists will note how it 
recalls du Bois Reymond’s theory of muscle and nerve. 
Ipa DOUBLEDAY. 
W. B. HARDY.) 7 ee 
Scientific and Industrial Research Department, = 
16 and 18 Old Queen Street, 
Westminster, London, S.W.1. 
January 23. 









The Rule of Priority in Nomenclature. 
In Nature for February 3, p. 148, Mr. F. Chapman 
mentions three distinct proceedings that may affect 
the stability of nomenclature in zoology. Concerning 
those that arise from differences of opinion as to the 
classificatory value of certain shapes or structures, 
or those that depend on the advance of knowledge, 
on corrections of fact, or on the need for breaking 
up unwieldy groups, it is useless to argue. Nosystem 
of classification and nomenclature devised by man 
can cope with such inevitable changes. 
The third proceeding, with which alone I venture 
to deal here, is the discovery that a name in general 
use was predated by a name that hitherto has been 
left in obscurity, and the consequent enforcement of 
the law of priority. On this point Mr. Chapman’s 
letter overflows with good sense; but it has all been 
said before. His laments, however, will not have 
been entirely wasted if you will permit this consolatory 
reply—namely, that in the year 1913, at the Inter- 
national Congress of Zoologists in Monaco, an agree- 
ment was reached in the largely attended section on 
nomenclature and confirmed in plenary session, by 
which the International Commission on _ Zoological 
Nomenclature was given power, on certain conditions, 
to suspend the rules in those cases where their 
operation was contrary to the general convenience. 
The Commission has, on the request of various 
¥ 
