e 
Sept. 2, 1885 | NATURE 
It seems also worthy of suggestion that our data for the average 
giadient of temperature may be somewhat fallacious. Recent 
observations (Cad/enger Expedition) show that the lower stratum 
of the ocean is occupied by water at near the freezing tempera- 
ture, whilst the mean annual temperature of the earth’s surface, 
where the borings have been made, must be at least 30° higher. 
It does not then seem impossible that the mean temperature 
gradient for the whole earth should differ sensibly from the mean 
gradient in the borings already made. 
The foregoing remarks have not been made with a view of 
showing Sir William Thomson’s argument from the cooling of 
the earth to be erroneous, but rather to maintain the scientific 
justice of assigning limits of uncertainty at the very least as 
wide as those given by him. Prof. Tait (“ Recent Advances in 
Physical Science,” 1885) cuts the limit down to 10,000,000 
years ; he may be right, but the uncertainties of the case are 
far too great to justify us in accepting such a narrowing of the 
conclusion. 
The third line of argument by which a superior limit is sought 
for the age of the solar system appears by far the strongest. This 
argument depends on the amount of radiant energy which can 
have been given out by the sun. 
The amount of work done in the concentration of the sun 
from a condition of infinite dispersion may be computed with 
some accuracy, and we have at least a rough idea of the rate of 
the sun's radiation. From these data Sir William Thomson 
concludes (Thomson and Tait, ““ Natural Philosophy,’ Appen- 
dix E) :— 
“Tt seems, therefore, on the whole most probable that the sun 
has not illuminated the earth for 100,000,000 years, and almost 
certain that he has not done so for 500,000,000 years. As for the 
future, we may say, with equal certainty, that inhabitants of the 
earth cannot continue to enjoy the light and heat essential to 
their life for many million years longer unless sources now 
unknown to us are prepared in the great storehouse of 
creation.” 
This result is based on the value assigned by Pouillet and 
Herschel to the sun’s radiation. Langley has recently made a 
fresh determination, which exceeds Pouillet’s in the proportion of 
eight to five.! With Langley’s value Thomson’s estimate of time 
would have to be reduced by the factor five-eighths. 
It has been suggested by Croll that the primitive solar nebula 
may have been hot. ‘his heat must have arisen from the col- 
lision of two or more masses ; if their relative velocity before 
collision was that due simply to their mutual attraction, the heat 
so generated is already counted in the heat generated by the 
concentration of the sun froma state of infinite dispersion. But 
if the relative velocity existed otherwise than from their mutual 
attraction, then the total heat in the sun might exceed that due 
simply to concentration. Sir William Thomson considers the 
hypothesis very improbable. The term improbability seems, 
however, almost to lose its meaning in these speculations, and at 
least we know by the spectroscope that actual nebulz do consist 
of incandescent gases. 
In considering these three arguments I have adduced some 
reasons against the validity of the first argument, and have en- 
deavyoured to show that there are elements of uncertainty sur- 
rounding the other two ; nevertheless they undoubtedly constitute 
a contribution of the first importance to physical geology. 
Whilst then we may protest against the precision with which Prof. 
Tait seeks to deduce results from them, we are fully justified in 
following Sir William Thomson, who says that “‘the existing 
state of things on the earth, life on the earth, all geological 
history showing continuity of life, must be limited within some 
such period of past time as 100,000,000 years.” 
If I have carried you with me in this survey of theories bear- 
ing on geological time, you will agree that something has been 
acquired to our knowledge of the past, but that much more 
remains still to be determined. 
Although speculations as to the future course of science are 
usually of little avail, yet it seems as likely that meteorology and 
geology will pass the word of command to cosmical physics as 
the converse. 
At present our knowledge of a definite limit to geological 
time has so little precision that we should do wrong to summarily 
* Langley (Anan. Res. R. A. S. 1885) estimates that 3 calories per minute are 
received by a square centimetre at distance unity. ‘[his gives for the total 
annual radiation of the sun 4°38 X 1033 calories. Thomson gives as Pouillet’s 
eslimate 6 X 103’ times the heat required to raise 1 lb. of water 1° Cels., or 
2°72 X 1033 calor.es. 
473 
reject any theories which appear to demand longer periods of 
time than those which now appear allowable. 
In each branch of science hypothesis forms the nucleus for the 
aggregation of observation, and as long as facts are assimilated 
and co-ordinated we ought to follow our theory. Thus even if 
there be some inconsistencies with a neighbouring science we 
may be justified in still holding to a theory, in the hope that 
further knowledge may enable us to remove the difficulties. 
There is no criterion as to what degree of inconsistency should 
compel us to give up a theory, and it should be borne in mind 
that many views have been utterly condemned, when later know- 
ledge has only shown us that we were in them only seeing the 
truth from another side. 
SECTION B 
CHEMICAL SCIENCE 
OPENING ADDRESS BY WILLIAM CROOKES, F.R.S., V.P.C.S., 
PRESIDENT OF THE SECTION 
A GLANCE over the Presidential addresses delivered before 
this Section on former occasions will show that the occupiers of 
this chair have ranged over a fairly wide field. Some of my 
predecessors have given a general survey of the progress of 
chemical science during the past year; some, taking up a tech- 
nological aspect of the subject, have discussed the bearings of 
chemistry upon our national industries; others, again, have 
passed in review the various institutions in this country for teach- 
ing chemistry ; and in yet other cases the speaker has had the 
opportunity of bringing before the scientific world, for the 
first time, an account of some important original researches. 
On this occasion I venture to ask your attention to a few 
though s on the very foundations of chemistry as a sclence—on 
the nature and the probable, or at least possible, origin of the 
so-called elements. If the views to which I have been led may 
at first glance appear heretical, I must remind you that in some 
respects they are shared more or less, as I shall subsequently 
show, by not a few of the most eminent authorities, and notably 
by one of my predecessors in this chair, Dr. J. H. Gladstone, 
F.R.S., to whose brilliant address, delivered in 1883, I must beg 
to refer you. 
Should it not sometimes strike us, chemists of the present day, 
that after all we are in a position unpleasantly akin to that of our 
forerunners, the alchemists of the Middle Ages? These necro- 
mancers of a time long past did not, indeed, draw so sharp a 
line as do we between bodies simple and compound ; yet their 
life-task was devoted to the formation of new combinations, and 
to the attempt to transmute bodies which we commonly consider 
as simple and ultimate—that is, the metals. In the department 
of synthesis they achieved very considerable successes; in 
the transmutation of metals their failure is a matter of 
history. 
But what are we of this so-called nineteenth century doing in 
our laboratories and our libraries? Too many of us are content 
to acquire simply what others have already observed and dis- 
covered, with an eye directed mainly to medals, certificates, 
diplomas, and other honours recognised as the fruits of * pass- 
ing.” Others are seeking to turn the determined facts of 
chemistry to useful purposes ; whilst a third class, sometimes 
not easily distinguished from the second, are daily educing novel 
organic compounds, or are racking their ingenuity to prepare 
artificially some product which Nature has hitherto furnished us 
through the instrumentality of plants and animals. The prac- 
tical importance of such investigations, and their bearing on the 
industrial arts and on the purposes and needs of daily life, have 
been signally manifested during the last half-century. 
Still a fourth class of inquirers working at the very confines 
of our knowledge find themselves, occasionally at least, face to- 
face with a barrier which has hitherto proved impassable, but 
which must be overthrown, surmounted, or turned, if chemical 
science is ever to develop into a definite, an organised, unity. 
This barrier is nothing less than the chemical elements commonly 
so called, the bodies as yet undecomposed into anything simpler 
than themselves. There they extend before us, as stretched the 
wide Atlantic before the gaze of Columbus, mocking, taunting, 
and murmuring strange riddles which no man yet has been able 
to solve. 
The first riddle, then, which we encounter in chemistry is,. 
‘‘ What are the elements?” Of the attempts hitherto made to- 
