546 
MARE RE 
[Aprit 8, 1897 
those of the solvent required, because the effect of transpiration 
of more of the solvent into the solution is not in any way to 
alter the individual complexes. The change in available energy 
of the system, on dilution, thus solely arises from the expansion 
of the complexes into a larger volume ; and it can be traced into 
exact correlation with the change of available energy that occurs 
in the expansion of a gas. This argument meets the objection 
that a true theory should involve a knowledge of the molecular 
actions between the various molecules. It would seem that 
with just the same cogency it might be argued that a real in- 
vestigation of the connection of the alteration of the freezing- 
point of a liquid by pressure, and its change of volume on 
freezing, should involve a knowledge of the individual molecular 
actions in the liquid ; and so it would, had we not the means of 
evading molecular considerations that is afforded by Lord 
Kelvin’s great principle of availability, which is for this very 
reason at the basis of all physical theory. 
There is, however, one point to be remembered, namely, 
that the theoretical osmotic pressure is a limiting value which 
may not be reached by an actual arrangement, unless we can 
be certain that it works reversibly, and so without heating 
effects. 
The remark has been made by Lord Kelvin, that the con- 
nection between Henry’s law and the osmotic law must break 
down when the solution of the gas is accompanied by change in 
its state of molecular aggregation. It is also probable, from the 
fundamental ideas as to dissociation and aggregation, that such 
change would usually be partial, and not uniform over all the 
dissolved molecules; so that it is not to be expected that 
Henry’s law would, in such circumstances, hold good. The 
point in which the argument, as set forth in precise form by 
Lord Rayleigh (NATURE, Joc. c#t. p. 254), becomes then in- 
applicable, is that the gas expelled from solution by the osmotic 
process must be considered as emerging in the actual state of 
aggregation differing from that of its free condition, and its 
return to the latter state involves further change of available 
energy. 
If the considerations above stated, which will be most suitably 
developed in detail in another connection, are valid, it follows 
that Prof. Poynting’s recent suggestion (Phz?. A/ag., October 
1896), with a view to evading the necessity of the ionic dis- 
sociation hypothesis, cannot avail, as it would not lead to the 
desired value for the osmotic pressure; that pressure depends 
on the number of molecular complexes involving the dissolved 
substance, that exist in the dilute solution, but not on their 
individual degrees of complexity. 
THE OSTRICH: 
“THE ostrich, S¢ruthio camelus, has been observed with 
interest from very early times ; it has frequently been the 
subject of remark by African travellers; and it has been 
domesticated and farmed in the Cape Colony for some thirty 
years. Yet it is remarkable how little is known about it in 
scientific circles, and how many misconceptions still prevail as 
to its nature and habits. 
This article is founded on personal observations made during 
nine years of uninterrupted ostrich-farming in the Karroo of 
the Cape Colony, and during travels about the country 
generally. In large ostrich camps, some of which are a couple 
of miles in diameter, numbers of birds of both sexes run in 
what is practically a wild state, seldom interfered with in any 
way, except when rounded up to be plucked or to be fed in a 
drought. The habits of birds thus farmed differ in no way 
from those of native wild birds, except perhaps that monogamy 
is more difficult. 
NUMBER OF SPECIES. 
All the differences on which the arguments for classifying the 
ostrich into three species are founded, are commonly present 
among the ostriches of the Cape Colony—that is, of South 
Africa generally ; for a great many of the Cape ostriches are 
the progeny of birds brought down from ‘‘ The Interior”’”—the 
Kalahari Desert, Damaraland, and beyond. There is, I think, 
little doubt that all South African ostriches are of one species ; 
individual variations, accentuated by local differences of food 
and climate, are quite sufficient to account for all supposed 
varieties. I do not think that, on the evidence which I have 
1 Abridged from a long article, by S. C. Cronwright Schreiner, in the 
March number of The Zoologist. 
NO. 1432, VOL. 55] 
been able to gather, there is any justification for maintaining 
that there is more than one species of ostrich. 
Tue EGG OF THE OSTRICH. 
The ostrich hen lays every other day, and the egg weighs 
about three pounds ; it is a tasty and nutritious food however 
prepared, very rich, and excellent for making pastry and cakes. 
It is generally computed to be equal to two dozen fowls’ eggs ; 
but this must be on account of its superior richness, for, from 
personal experiment, the empty shell of a fairly large one 
exactly held the contents of eighteen fowls’ eggs. It takes 
about forty minutes to boil an ostrich egg hard. The period 
of incubation is about six weeks, 
LEAPING AND SWIMMING. 
The old idea that an ostrich can only leap over a very low 
fence, or across but the narrowest sluit (gully), is incorrect. 
The birds will, when startled (never deliberately), sometimes go 
over a six-strand wire fence nearly five feet high, putting one 
foot on one of the middle wires, and striding over with the 
other. They will go over a stone wall in the same manner, if 
too high for them to step upon ; and I have seen a cock take a 
standing jump on to the top of a wall five feet high, beyond 
which were his chicks. 
Even as a chick the ostrich is a powerful swimmer. I have 
known several birds swim some distance down the Great Fish 
River when it was running fairly strong, and have heard, on 
what seems trustworthy evidence, of a cock that was carried a 
long way down the same river when it was running nearly level 
with its precipitous banks in the stormy season ; he was some 
hours in the water before he could get out, but emerged 
unhurt. 
How ir FEEDS, AND WHAT IT WILL SWALLOW. 
The ostrich feeds \in a peculiar manner. It tosses the food 
into a sack in the upper part of the neck, and then swallows it. 
I have seen a bird toss fully a quart of mealies (Indian corn) 
into this sack before swallowing ; and it is no uncommon thing 
to see two ‘‘ swallows” travelling down the neck at the same 
time with a clear interval between them ; or to see one of them 
(if of large and loose food, e.g. grain) slide back into the sack 
after being swallowed, if the bird lowers its head to continue 
feeding before the food has travelled some considerable distance 
down the neck. The food travels slowly, and performs a 
complete circuit of the neck before reaching the crop. Crushed 
bones are greedily eaten; if too large a piece should stick in 
the neck, it. is a simple matter to cut it out and sew the wound 
up again. The wound, asa rule, heals quickly, and causes but 
little inconvenience. As is well known, ostriches will swallow 
almost anything small enough to pass down the neck. An 
ostrich’s crop always contains a large quantity of smooth stones, 
many of them brightly coloured. 
Ilow THE OsTRICH RUNs. 
Considerable misconception prevails as to the manner in 
which the ostrich runs. It seems to be still generally held that, 
when running, it spreads out its wings, and, aided by them, 
skims lightly over the ground. This is not correct. 
When a bird really settles itself to run it holds its head lower 
than usual, and a little forward, with a deep loop in the neck. 
The neck vibrates sinuously, but the head remains steady, thus 
enabling the bird, even at top speed, to look around with. 
unshaken glance in any direction. The wings lie along the 
sides about on a level with, ora little higher than, the back, 
and are held loosely just free of the plunging ‘‘ thigh.” There 
is no attempt to hold them extended, or to derive any assistance 
from them as organs of flight. 
When an ostrich, after a long run, is very tired, its wings 
sometimes droop ; this is due to exhaustion ; they are never, by 
a running bird exerting itself to the utmost, held out away from 
the sides to lighten its weight or to increase its pace. But the 
wings appear to be of great service in turning, enabling the bird 
to double abruptly even when going at top speed. 
THE NEsT. 
As the breeding season approaches, a cock and hen will pair, 
and, having selected a site congenial to their inclinations, 
