August 19, 1886] 
circular space on the surface of the tin plate almost clear 
of resin, showing how very much the surface-tension of 
hot resin is less than that of cold resin. 
Note of Fanuary 30, 1836.—The equations (8) and (9) 
on p. 59 of Clerk-Maxwell’s article on “ Capillary Attrac- 
tion” in the ninth edition of the “ Encyclopedia Brit- 
tanica” do not contain terms depending on the mutual 
action between the two liquids, and the concluding ex- 
pression (10), and the Jast small print paragraph of the 
page are wholly vitiated by this omission. The paragraph 
immediately following equation (10) is as follows :— 
“Tf this quantity is positive, the surface of contact will 
tend to contract, and the liquids will remain distinct. If, 
however, it were negative, the displacement of the liquids 
which tends to enlarge the surface of contact would be 
aided by the molecular forces, so that the liquids, if not 
kept separate by gravity, would become thoroughly mixed. 
No instance, however, of a phenomenon of this kind has 
been discovered, for those liquids which mix of them- 
selves do so by the process of diffusion, which is a mole- 
cular motion, and not by the spontaneous puckering and 
replication of the boundary surface as would be the case if 
T were negative.” 
It seems to me that this view is not correct ; but that 
on the contrary there is this “puckering” as the very 
beginning of diffusion. What I have given in the lecture 
as reported in the text above seems to me the right view 
of the case as regards diffusion in relation to interfacial 
tension. 
It may also be remarked that Clerk-Maxwell, in the 
large print paragraph of p. 59, preceding equation (1), 
and in his application of the term potential energy to E in 
the small print, designated by evzergy what is in reality 
exhaustion of energy or negative energy; and the same 
inadvertence renders the small print paragraphon p. 60 
very obscure. The curious and interesting statement 
at the top of the second column of p. 63, regarding a 
drop of carbon disulphide in contact with a drop of 
water in a capillary tube would constitute a perpetual 
motion if it were true fora tube not first wetted with 
water through part of its bore—‘“. . . if a drop of water 
and a drop of bisulphide of carbon be placed in contact 
in a horizontal capillary tube, the bisulphide of carbon 
will chase the water along the tube.” 
Additional Note of Fune 5, 1886.—I have caretully tried 
the experiment referred to in the preceding sentence, and 
have not found the alleged motion. 
WILLIAM THOMSON 
OUR FOSSIL PSEUDO-ALGA& 
DM EE the last half-century many paleontologists 
have described anomalous objects, some of which 
have been regarded as fossil marine Algze, and others as 
tracks of various marine invertebrate animals ; and since 
the publication of Darwin’s theory of evolution various 
attempts have been made to utilise some of these in 
formulating a pedigree for the living types of vegetation. 
Amongst those who have tried to accomplish this object 
my distinguished friend the Marquis of Saporta, and his 
colleague, M. Marion, occupy the most prominent posi- 
tion. They have in several publications described and 
figured many objects which they believe to have been 
true marine Algze, and out of which they have constructed 
the lower roots of their genealogical tree. But meanwhile 
there has grown upan enlarging school of palzeontologists 
who look with strong suspicion upon these genealogies ; 
who refuse to recognise the vegetable character of these 
objects ; who believe most of them to be casts of various 
ridges and furrows, most of which have been tracks pro- 
duced by creeping invertebrate animals or by the still more 
mechanical agencies of wind and water. At the head of 
this school Prof. Nathorst, of Stockholm, stands pre- 
NATURE 
369 
eminent. An animated controversy sprang up some time 
ago between M. Nathorst and M. Saporta relative to this 
subject. Blast and counterblast have succeeded one 
another, and the latest discharge of palzo-botanica! 
artillery has just been fired off by M. Nathorst in the 
form of a memoir entitled “ Nouvelles Observations sur 
des Traces d’Animaux et autres Phénoménes d’Origine 
purement méchanique décrits comme ‘ Algues Fossiles.’ as 
Enjoying the privilege of an intimate and valued friend- 
Ship with both these distinguished paleontologists, I am 
anxious to do full justice to both. But I must admit that 
my judgment and experiences bring me into closer agree- 
ment with the northern naturalist than with his French 
antagonist. The interesting subject discussed by them 
has long occupied my attention, and my conclusions 
respecting it have not been hastily arrived at. 
The question in debate is not whether or not marine 
Algze existed in Palaeozoic and later geological epochs : 
on this point Nathorst and Saporta are agreed. The 
abundance of phytophagous marine mollusks found even 
in the Cambrian, as in most of the other fossiliferous 
strata, clearly demonstrates that there must have been 
submarine pastures upon which they could feed. The 
question is, are numerous objects, found in strata of 
marine origin, and believed by some to be fossilised 
marine Algz, really such? To this query Saporta answers 
7es ; Nathorst’s reply is an emphatic Wo. Hence the 
elaborate controversial literature of which these two 
savants are the authors. To condense their several 
articles into an abstract is not easy, but such an abstract 
of M. Nathorst’s latest publication may be attempted, 
illustrating the general features of the discussion. 
Throughout his memoir M. Nathorst rests prominently 
upon two general propositions which appear to me to be 
unanswerable. The first is that all or nearly all these 
debatable pseudo-Algze stand out in bold demi-relief 
from the z7feriorv surfaces of the rocky slabs to which they 
are attached, and that beyond their sculptured surfaces 
they as constantly consist of a mere extension of the rock 
of which they form a part. Hence Nathorst insists that 
they are merely convex casts of what were primarily 
concave grooves or channels on the surface of the sub- 
jacent stratum. 
In reply to this opinion M. Saporta publishes figures of 
casts of vegetable fragments in demi-relief, the positions 
of which on the inferior surfaces of slabs are identical 
with those of the pseudo-Algz under discussion. One 
of these is a fragment of what appears to be a twig of a 
Conifer, of which the lower side alone is preserved in 
demi-relief. Nathorst freely admits the possible existence 
of such specimens, but he regrets Saporta’s explanation 
of them. Jfrimzs, he affirms with inexorable logic, 
that such examples are so rare and exceptional that they 
only prove the opposite of the rule which they are alleged 
by Saporta to sustain. Whenever fragments like these 
are found embedded in the rocks, they almost invariably 
display traces of both their upper and lower surfaces ; 
whereas this is scarcely ever the case with the disputed 
Fucoids, and in the very few instances where such are 
supposed to have been met with, their entirely excep- 
tional character suggests a very different explanation of 
them from that proposed by M. Saporta. 
It is difficult to understand how a cylindrical object 
sufficiently dense to produce a deep concave impression 
upon hardening mud could do so without leaving some 
trace of its upper surface upon the opposite surface of the 
sand by which that mud became overlain. Saporta’s theory 
explaining why it does not do so is surely untenable. 
That theory supposes that an organism half embedded in 
mud and overlain by sand began to decay at its upper 
surface, which decay ultimately reached the lower surface 
which rested on the mud ; that, as the decay proceeded 
downwards, the superimposed sand would finally reach 
| the concave mould in the mud which it would fill, and 
