Aucust 31, 1899] 
would sometimes be an advantage to have one presenting a 
broader view and making no reference to any specific temper- 
ature, just as the ordinary definition of critical temperature 
makes no reference to any specific pressure. 
Now, if in a fv diagram we draw the curve formed by the 
liquid and vapour lines, the indicator points corresponding to 
the ‘‘mixed state” (ze. part vapour and part liquid, each more 
or less distinctly discernible) lie wholly within the region 
bounded by this curve and the axis of volume ; also the ordinate 
of the highest point of this curve—where, of course, the tangent 
is horizontal—corresponds to the critical pressure, and the 
“critopiestic” or critical pressure line is the said horizontal 
tangent. 
All horizontal lines below the critopiestic intersect the 
region corresponding to the ‘‘ mixed state,’’ while those above 
do not, thus showing that at pressures below the critical the 
substance changes from gas to liquid, or wzce-versa, by the 
ordinary process of condensation or evaporation, z.e. by passage 
through the mixed state, while above that pressure this process 
does not take place, but the change occurs by continuous and 
imperceptible transition. 
Of course all this accords with experiment, as is pointed out 
in several, though by no means all, the standard text-books. 
Thus on p. 123 of the new edition of Clerk Maxwell’s ‘* Theory 
of Heat,” revised by Lord Rayleigh, we read :—“If we begin 
with carbonic acid gas at 50° F. we may first heat it till its 
temperature is above the critical, 88° F. We then gradually 
increase the pressure to, say, 100 atmospheres. During this 
process no sign of liquefaction occurs. nally we cool the 
substance still under a pressure of 100 atmospheres ta 50° F. 
During this process no sudden change of state can be observe, 
but carbonic acid at 50° F. and under a pressure of 100 atmto- 
spheres has all the properties of a liquid... by this process 
we have caused the substance to pass from an undoubtedly 
gaseous to an undoubtedly liquid state without at any time 
undergoing an abrupt change stmilar to ordinary liquefaction.” 
Again, on p. 206 of the ‘‘ Text-Book of Physics,” by Mr. 
Alfred Daniell, we find :—‘‘If CO, gas be exposed to a tem- 
perature above 30 ‘92 C. and be subjected to any pressure above 
73 atmospheres, it will still be a gas: a//ow zt to cool, the pres- 
sure being kept up, and it will be a liquid after tt passes 30°92 C., 
and yet the transition ts unobservable.” 
I therefore propose to define the critical pressure of a sub- 
stance as ‘* that pressure above which it is impossible to make 
the substance undergo the ovdinary process of condensation (or 
evaporation) ”—or if greater amplification is needed as ‘‘ that 
pressure above which an appropriate alteration of temperature 
causes the substance to pass from the gaseous to the liquid state 
or vice-versa, by a process of continuous and imperceptible tran- 
sition, and not, as happens below that pressure, by fassage 
through the mixed state.” 
This definition I have given in my recently published book, 
“Physics : Experimental and Theoretical,’”’ but the Z¢mes re- 
viewer, in a paragraph in that paper of July 29, characterises it 
as ‘* mere nonsense.” 
I shall be greatly obliged if you will publish this letter, to- 
gether with your opinion on the validity of my definition. 
Perhaps also some of your readers may favour me with an expres- 
sion ot their views. R. H. JupE. 
Newcastle-upon-Tyne, August 2. 
Maternal Devotion of Spiders. 
ON removing some virgin cork from the wall of a consery- 
atory a short time ago, I was much struck with the way in 
which a small black female spider clung to her two egg-bags, 
despite the fact that the piece of cork to which she was cling- 
ing had been thrown roughly to the ground. When the cork 
was about to be replaced on the wall, it became necessary to 
turn the spider adrift, in order to prevent her being crushed. 
But although the cork was shaken, she declined to budge, and 
retained a tight hold upon her precious bags. Knowing how 
fully alive to danger the spider race is in general, I thought 
that this remarkable instance of devotion to maternal prompt- 
ings on the part of a naturally sensitive creature ought not to 
be disregarded. I accordingly removed the mother very care- 
fully, and placed her on some rockwork, where I noticed she 
seemed to be very uneasy, moving restlessly about as if search- 
ing for something. I then took the egg-bags and placed them 
beside her. As I expected, she seemingly failed to recognise 
NO. 1557, VOL. 60] 
NATURE 
413 
them, or at least manifested a repugnance to them, and ran 
away for a little distance. Subsequently, however, she re- 
turned, and proceeded to examine the bags with scrupulous 
care by means of her palpi; and evidently satisfied with this 
scrutiny that they were really her own cherished property, she 
commenced to spin a web about them to secure them in their 
place. 
Rennie has described experiments with the females of certain 
spiders which carry about their egg-bags attached to their 
bodies. | When one of these spiders was molested, and its 
bag dragged with a stick, the mother seemed to lose all sense 
of personal danger in her anxiety for her unhatched offspring, 
and fought vigorously to retain her precious egg-bag. When 
forcibly deprived of the bag, she manifested great distress, and 
commenced a search for it, and, not finding it, she refused 
to leave the spot, seeming to be quite indifferent as to her 
fate. The curious part of the story is that when the egg-bag 
was finally restored to her, she refused to touch it, being 
apparently quite unable to recognise her property. In another 
case the spider regained possession of the bag as it was being 
withdrawn, and immediately refixed it in its former position. 
My spider apparently recognised her egg-bags without much 
difficulty, and, furthermore, seemed to be alive to the danger 
to which they were exposed in their new situation by her act 
of spinning a protecting web without delay. When evening 
arrived, I observed that she had drawn the bags close up under 
a sheltering leaf, and was guarding them closely, having placed 
herself between them. FrANcIs J. RowsBorHaM. 
August 23. 
THE CAMBRIDGE ANTHROPOLOGICAL EX- 
PEDITION TO TORRES STRAITS AND 
SARAWAK. 
Ape main object of the expedition was to verify and 
supplement the anthropological observations that I 
made in Torres Straits in 1888-89, with the view of 
the publication of a monograph dealing with the anthro- 
pology of the islanders using that term in its widest 
sense. A few months before leaving I received sucha 
pressing and enthusiastic invitation from Mr. Charles 
Hose for the expedition to visit the Baram district of 
Sarawak, that I felt constrained to extend the scope of 
our work by accepting his tempting offer. The party 
consisted of Dr. W. H. R. Rivers, Messrs. C. S. Myers, 
W. McDougall, S. H. Ray, A. Wilkin, C. G. Seligmann, 
and myself. 
The Torres Straits islanders are Papuans, and as they 
inhabit the remains of the old land communication 
between Australia and New Guinea it was important that 
they should be thoroughly studied before it was too late. 
The islanders have been more or less under mission 
instruction since 1872, and some time before then the 
pearl-shelling industry had commenced. Owing to the 
varied influences of the white man, modification was 
bound to take place rapidly, and unfortunately in most 
islands more or less extensive depopulation has also 
occurred. There are two distinct tribes in the archipelago 
—the eastern tribe inhabits the Murray Islands, Erub 
(Darnley Island) and Uga, and the western tribe the re- 
maining islands. The latter people have been most under 
the influence of white men, scarcely a pure-blooded native 
exists in Erub, but the Murray Islands, on account of their 
remoteness and the difficulties in reaching them owing 
to numerous coral reefs, have been less visited. As Mer, 
the chief island of this group is very fertile, and has a 
population of some 450 people, it appeared to be the best 
centre for our work. 
We reached Mer on May 6, 1898, and took possession 
of the disused mission residence, which we speedily con- 
verted into anthropological, psychological and photo- 
graphic laboratories. Here we measured 63 men, 5 
women, 30 boys, and 22 girls. The average height of the 
men is 1°653 m. (5 ft. 5 in.) ; their cephalic index is 77°5. 
Although reference is made here only to the cephalic 
index and the height, I may state that we usually made 
