86 NATURE 
a, 
of these samples yielded, with peptone, Bacteria; not so the 
other two. All three were prepared with the utmost caution re- 
specting atmospheric dust, &c. That, moreover, the positive 
result could not be caused by an accidental admixture of germs 
was amply proved by the often repeated control-experiments. 
It appears, therefore, that, besides the glucose and the peptone, 
a third substance is needed for generating Bacteria, a body pre- 
sent in the ordinary glucose (starch-sugar), but removed by puri- 
fication. The nature of this body I have not yet been able to 
ascertain. But however important, this matter has no direct 
bearing upon the question of abiogenesis. For that this third 
unknown body cannot be (as some will probably presume) a 
germ, my control-experiments and also the above-described ex- 
periment, wherein the sugar was boiled with acid, do sufficiently 
prove. D. Huizinca 
Groningen, May 23 
Flight of Birds 
Some time since I had occasion to ascend a mountain in the 
neighbourhood. The wind was blowing over the ridge-like 
crest of the mountain with a velocity of, I should say, ten or 
twelve miles an hour, sweeping with increased rapidity through 
certain transverse gorges cutting the ridge at right angles, In 
one of these I observed a hawk hovering in search of prey. In 
the midst of this rapid air current the bird remained apparently 
fixed in space, without fluttering a wing, for at least two 
minutes. After a time it gently changed its position a few 
feet with a slight motion of its wings, and then came to rest 
again as before, remaining apparently as motionless as the rocks 
around it. From my nearness to it a change of position of an 
inch would have been clearly visible, and yet except when it 
seemed to desire to change its point of observation no motion of 
any kind could be detected. How is this to be accounted for? 
Does a bird possess the power of giving an extremely rapid 
tremulous motion to its wings invisible even at a small distance, 
similar in its nature to the wing vibration of certain insects, 
which, as any one may have noticed, have a similar power of 
apparently fixing themselves in space over a flower, for example, 
notwithstanding a condsiderable amount of motion in the air in 
which they are suspended ? F 
If any of your correspondents would kindly take the trouble 
to throw some light on these points they would greatly oblige 
one who is unfortunately placed out of reach of the ordinary 
means of reference. J. GUTHRIE 
Graaff Reinet, Cape Colony, April 2 
THERMO-ELECTRICITY 
ibs subject I have chosen is one intimately connected 
with the names of at least two well-known members 
of this University—the late Prof. Cumming and Sir 
William Thomson. It possesses at present peculiar 
interest for the physicist ; for, though a great many general 
facts and laws connected with it are already experi- 
mentally, or otherwise, secured to science—the pioneers 
have done little more ‘than map the rough outlines of 
some of themore prominent features of a comparatively new 
and almost unexplored region. Some of its experimental 
problems are extremely simple, others seem at present to 
present all but insuperable difficulties. And it does not 
appear that any further application of mathematical 
analysis can be safely, or at least usefully, made until 
some doubtful points are cleared up experimentally. 
The grand idea of the conservation, or indestructibility, 
of energy :—pointed out by Newton ina short Scholium 
a couple of centuries ago, so far at least as the progress 
of experimental science in his time enabled him to extend 
his statements :—conclusively established for heat at the 
very end of last century by Rumford and Davy; and 
extended to all other forms of energy by the splendid 
researches of Joule :—forms the groundwork of modern 
physics. ) p 
Just as, in the eye of the chemist, every chemical 
ehange is merely a re-arrangement of indestructible and 
unalterable matter; so to the physicist, every physical 
* Abstract of the Rede Lecture delivered in the Senate House, Cambridge» 
May 23, 1873. 
| Way 29, 1873 
change is merely a transformation of indestructible 
energy ; and thus the whole aim of natural philosophy, 
so far at least as we yet know, may be described as the 
study of the possible transformations of energy, with their 
conditions and limitations ; and of the present forms and 
distribution of energy in the universe, with their past and 
future. 
It is found by experiment that some forms of energy 
are more easily or more completély transformable than 
others, and thus we speak of higher and lower forms, 
and are introduced to the enormously important con- 
sideration of the degradation, or, as it is more commonly 
called, the dissipation, of energy. The application of 
mathematical reasoning to the conservation of energy 
presented no special difficulties which had not, to some 
extent at least, been overcome in Newton’s time: but it 
was altogether otherwise with the transformations of 
energy. And itis possible that, had it not been for the 
wonderfully original processes devised by Carnot in 1824, 
we might not now have secured more than a small 
fraction of the immense advances which science has 
taken during the last thirty years. 
For a transformation of heat we must have bodies of 
different temperatures. Just as water has no “head” 
unless raised above the sea-level, so heat cannot do work 
except with the accompaniment of a transference from a 
hotter to a colder body. Carnot showed that to reason 
on this subject we must have cyc/es of operations, at the 
end of which the working substance is restored exactly to 
its initial state. And he also showed that the test of a 
perfect engine (ze. the best which is, even theoretically, 
attainable) is simply that it must be veverszble. Ty this 
term we do not mean mere backing, as in the popular 
use of the word, but something much higher—viz. that, 
whereas, when working directly, the engine does work 
during the letting down of heat from a hot to a cold 
body ; when reversed, it shall spend the same amount of 
work while pumping up the same quantity of heat from the 
cold body to the hot one. Asareversible engine may be 
constructed (theoretically at least) with any working sub- 
stance whatever, and as all reversible engines working 
under similar circumstances must be equivalent to one 
another (since each is as good as an engine can be) it is 
clear that the amount of work derivable from a given 
amount of heat under given circumstances (z.¢, the amount 
of transformation possible) can depend only upon the 
temperatures of the hot and cold bodies employed. In 
this sense we speak of Carnot’s Function of Temperature, 
which is as imperishably connected with his name as is 
the Dynamical Equivalent of Heat with that of Joule. 
Building upon this work of Carnot, Sir W. Thomson gave 
the first adso/uze definition of temperature—that is a defini- 
tion independent of the properties of any particular 
substance. Perhaps there is no term in the whole range of 
science whose meaning is correctly known to so few even of 
scientific men, as this common word temperature. It 
would not, I think, be an exaggeration to say that there 
are not six books yet published in which it is given with 
even an approach to accuracy. The form in which the 
definition ultimately came from the hands of Joule and 
Thomson enables us to state as follows the laws of trans- 
formation of energy from the heat form. “ 
1, A given quantity of heat has a definite transforma- 
tion equivalent. 
2. But only a fraction’of this heat can be transformed 
by means even of a perfect engine: and this fraction is 
DEFINED as the ratio of the range through which the heat 
actually falls to that through which it might fall—were it 
possible to obtain and employ bodies absolutely deprived 
of heat. 
This definition has two great advantages. 1st, The 
utmost amount of work to be got from heat under any 
circumstances of temperature is determined by precisely 
the same law as that assigning the work to be had from 
} 
