78 
NATURE 
[Sune 2, 1870 
with that of stones of which the falls were witnessed : 
the proportion being only about 4 or 5 per cent. -Thus 
in the collection at the British Museum we have 106 
specimens of iron (siderites and siderolites together) 
indubitably meteoric, of which four only were seen to 
fall, while there are 179 of meteoric stones, of which 
only five have zo¢ been seen to fall. Surely, however, 
we have a sufficient explanation of this in the nature 
of the bodies themselves. The one, a mass of solid 
iron, besides possessing far greater permanence than a 
soft and porous rock permeated by small particles of 
readily rusting metal, would, if lying on the ground, be 
at once recognised by any one familiar with the metal, 
and would be preserved for use or as a curiosity ; while 
the meteoric stone so found would equally naturally be 
neglected, unless the finder knew a meteorite as well 
as M. Meunier. Hence the iron meteorite is both 
better preserved and more surely recognised; and 
hence though its fall is a far rarer event in nature than 
that of the meteoric stone, our collections are com- 
paratively rich in iron meteorites. That the American 
Continent has furnished so many meteoric irons to our 
collections is, no doubt, due to the ignorance of the uses 
of iron on the part of the ancient inhabitants of that con- 
tinent, and to the comparatively unpeopled nature of the 
country. It is in the United States, and scattered over 
the plains and valleys of Mexico, or lying unrusted under 
the clear dry air of the Cordilleras of the Andes, that 
most of these iron masses have been found. They afford 
an ocular proof that, though after a longer or shorter time 
such irons must sink into a shapeless mass of oxides, 
yet under favourable conditions they can and do last 
through long generations before this destructive process 
is consummated. Indeed, if they do not, what becomes 
of M. Meunier’s main argument? And if they do, the 
anomaly of their coming into our collections, while the 
stones, the fall of which has not been witnessed, are 
absent, does not seem so inexplicable. 
M. Meunier advances another argument in support of 
his theory founded on the similarity of composition of 
certain meteorites that he has examined, and in which 
he recognises what he terms a Stratification of different 
recurring varieties. 
Assuming the correctness of this statement, we fail to 
see its logical connection with his theory. It is no new 
fact in the mineralogy of meteorites, whether of stone or 
iron, that the same minerals and combinations recur in 
them, and that certain of them look like chips from the 
same block. It is on this very account that a com- 
munity of origin for those belonging to each group of 
them, if not indeed for the whole of even these groups 
themselves, has been so long suspected. But is not this a 
community of origin that links them not only one with 
another, but probably also to other bodies in space, and 
that by a much further reaching chain than one which 
would bind them down only to our tiny orb and its 
satellite? Indeed, the very remarkable parallelism 
between their constituent elements and those which have 
been revealed by prismatic analysis as existing in activity 
on the surface of the sun, gives to this question of the 
origin of meteorites an interest of that expectant kind 
which holds us, as it were, listening for the announcement 
of what may be the next new discovery in solar physics— 
some fact that may illuminate as by electric light the 
whole solar system, and, clearing up the mystery that 
surrounds the comets, the zodiacal light, the solar corona, 
and even our aurora, may tell us why the “ fiery tears of 
St. Lawrence” and other meteor showers do not descend 
on us as wbddes mérpwv, and even explain the source 
whence the meteorites do really come. We shall then be 
able better to decipher the characters in which the history 
of the meteorites is written, a history which assuredly is 
engraved, though in hieroglyphic language, on these 
messengers from space to our world. Expectation of this 
kind should surely invest our reasoning with the kind 
of caution which befits men who, feeling in the twilight 
after a “quest” of this kind, are conscious that they 
cannot be very far off from touching it in very truth, 
and, as it were, with their hands. We venture to think 
that M. Meunier has not, on this oceasion, succeeded 
in attaining to the object of that quest. 
N. S. MASKELYNE 
WHAT IS ENERGY? 
Lys 
N our first article it was shown that energy, or the 
power of doing work, is of two kinds, namely, energy 
due to actual motion, and that due to position. We ended 
by supposing that a stone shot vertically upwards had 
been caught at the summit of its flight and lodged on the 
top of a house ; and this gave rise to the question, What 
has become of the energy of the stone? To answer this 
we must learn to regard energy, not as a gualily, but 
rather as a ¢hing. 
The chemist has always taught us to regard quantity 
or mass of matter as unchangeable, so that amid the 
many bewildering transformations of form and quality 
which take place in the chemical world, we can always 
consult our balance with a certainty that ¢¢ will not play 
us false. But now the physical philosopher steps in and 
tells us that energy is quite as unchangeable as mass, 
and that the conservation of both is equally complete 
There is, however, this difference between the two things 
—the same particle of matter will always retain the same 
mass, but it will not always retain the same energy. As 
a whole, energy is invariable, but it is always shifting 
about from particle to particle, and it is hence more 
difficult to grasp the conception of an invariability of 
energy than of an invariability of mass. For instance, 
the mass of our luminary always remains the same, but 
its energy is always getting less. 
And now to return to our question,-— What has 
become of the energy of the stone? Has this dis- 
appeared? Far from it; the energy with which the stone 
began its flight has no more disappeared from the universe 
of energy, than the coal, when we have burned it in our 
fire, disappears from the universe of matter. But this 
has taken place :—the energy has changed its form 
and become spent or has disappeared as energy of actual 
motion, in gaining for the stone a position of advantage 
with regard to the force of gravity. 
If we study this particular instance more minutely, we 
shall see that during the upward flight of the stone its 
energy of actual motion becomes gradually changed 
into energy of position, while the reverse will take place 
Ye ee ee 
