386 
NATURE 
[March 16, 1871 

LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications.) 
Eozoén Canadense 
I CANNOT understand Mr. T. Mellard Reade’s right to fling 
the taunt at those who maintain the foraminiferal nature of 
Eozoon, that ‘‘each disputant takes up a different position, and 
shifts it as occasion requires.” 
I have never taken up any other position than this: that the 
best-presrved specimens of the Canadian Eozodn exhibit an un- 
questionably foraminiferal structure. J am supported in this by 
every British naturalist with whom I am acquainted, as specially 
conversant with foraminiferal organisation, viz.. by Messrs. H. 
B. Brady, T. Rupert Jones, W. K. Parker, and Prof. W. C. 
Williamson ; whilst the most eminent authorities in micro- 
mineralogy and pseudomorphic structure, viz., Messrs. David 
Forbes, T. Sterry Hunt, and H. Sorby, altogether disown 
Eozo6n as a mineral. 
I have further asserted, and I do not in the least ‘‘ shift” my 
position, that the character of the Canadian Eosoon zs altogether 
independent of that of later ophites. The occurrence of true 
Eozoic structure in the newest Tertiaries would only show that 
Eozoon, like Lingula, has maintained its continuity through a 
long succession of geological epochs. On the other hand, the 
occurrence of minerals presenting superficial resemblances to 
true Eozoonal structure, can be of no account to such as really 
understand the latter. 
If the Skye ophite, for example, possesses a true “‘ nummuline 
layer” in combination with other characteristic Eozoinal features, 
its presence ina formation of later ‘‘ geological time than the 
Laurentian,” furnishes no argument whatever against its organic 
character. 
If, on the other hand, the supposed ‘‘ nummuline layer”’ in 
the Skye ophiteis nothing but a lamella of chrysotile, the exis- 
tence of such a pseudomorph can only affect the opinions of 
such as are incompetent to distinguish the two by those micro- 
scopic tests on which experienced observers feel perfect reliance. 
Since I do not feel called upon to expend valuable time in 
giving to Mr. T. Mellard Reade the instruction which he requires 
to qualify him for discussing this question, I now leave him to 
the enjoyment of his own opinion. Whenever he shall have 
shown, by work of his own, his competence to criticise the obser- 
vations of others who have made a special study of the subject 
he discussed, I shall be most happy to afford him the same 
opportunity of forming his judgment as to the organic nature of 
Eozoén, by an examination of my preparations, that I have given 
to the many eminent naturalists, who have thus fully satisfied 
themselves of the justice of my conclusions. ~ 
W. B. CARPENTER 

Dr. John Hopkinson on ‘‘ The Overthrow of the Science 
of Electro-Dynamics” 
As I see you have reprinted at length Dr. Hopkinson’s paper 
with the above title, in which he criticises severely, not to say 
ungenerously, some papers of mine published in the Quarterly 
Journal of Science and Chemical News, you will think it only 
fair to publish my reply, in which I think I shall show that, in 
the course of his short paper, Dr. Hopkinson has committed 
mistakes at least as grave and important as any he imputes to 
me. Let us see if this is not the case. 
Dr. Hopkinson quotes one of my articles as follows: ‘‘ They 
(that is Joule and Scoresby) calculate the maximum theoretical 
power of a grain of zinc to be 158 foot-pounds, and yet using 
permanent magnets, which, by their own statement, were so badly 
constructed as to have only a quarter the power they ought to 
have had, with the poles of the electromagnets never approach- 
ing the permanent magnets nearer than } of an inch (and what 
an enormous loss is incurred here !); with an engine constructed 
almost at haphazard, and with scarcely a consideration of the 
best principles or of the most advantageous construction of such 
engines, they actually obtained a result of 102°9 foot-pounds out 
of a calculated theoretical maximum of 158. With a little care 
and consideration, I do not hesitate to say the duty per grain of zinc 
might easily have been increased tenfold.” On which he observes, 
“‘Tt is hardly credible, but the above looks very like a confu- 
sion between Force and Work ! The author seems to assume that 
if the forces in operation in an engine are greater, that the engine 

will necessarily produce more work from the same quantity of 
fuel. In these experiments the quantity of zinc (a—4) used to 
produce work W is observed ; if the engine was made more 
powerful, if the permanent magnets were four times as strong, 
and the electromagnets passed } of an inch from them, doubt- 
less W would be greater, but so also would (a—4), and it does 

not follow that ( ¥ , with which we are concerned, would be 
a-6) 
at all changed. What becomes, then, of the dogmatic assertion 
that the duty or a grain of zinc would be increased tenfold ?” 
Why he should say, ‘‘It is hardly credible, but the above 
looks very like a confusion between Force and Work,” I know 
not. I cannot plead guilty to having made the slightest con- 
fusion between the two. Ido think the total of the force used 
is a measure of the work produced. But Dr. Hopkinson tries to 
persuade us that a well-constructed engine would do no more 
duty than an ill-constructed one, and consequently, I presume, 
that the magnets might possibly be weakened ad infinitum, and 
removed to ever so great a distance, without necessarily affecting 
the efficiency of the engine. And then he ventures to criticise 
my papers as full of fallacies !_ I retort that it is hardly credible, 
but that the above looks very like a confusion between (a — 6) and 
6! Inthese experiments of Joule and Scoresby’s the quantity of 
zinc used to produce work W, is represented by the authors not as 
(a— 4) but as 4, and therefore the duty per grain of zinc is not 
Wt Vine 
a-s Te 
and the electromagnets are passed nearer to them, not only 
does W increase but 4 also diminishes. So that was I not justi- 
fied in saying that the duty of a grain of zinc could in a better- 
constructed engine be probably increased tenfold? And if it be 
increased only twofold, or even half as much again, then, allowing 
for waste, I have proved my point, and disproved Joule’s mechanical 
equivalent of heat. Might I not retort fairly on Dr. Hopkinson 
that the Manchester Literary and Philosophical Society ‘* never 
ought to have permitted this paper to appear in their Pro- 
ceedings ?” 
Next let us take Dr. Hopkinson’s next criticism. My argu- 
ment is this :—That if the doctrine of the mechanical equivalence 
of heat be, that production of energy absorbs, and destruction of 
energy produces, a definite amount of heat, and if we find cases, 
as those of elastic wires, and water below its maximum density, 
in which destruction of energy produces cold, not heat, the doc- 
trine of the mechanical equivalent of heat cannot be universally 
true. To this argument Dr. Hopkinson replies that the facts I 
quote as paradoxes are simple deductions from the two laws of 
thermo-dynamics. Quite true ; but this only shows that one of 
the laws of thermo-dynamics is inconsistent with the doctrine of 
the mechanical equivalence of heat. Might I not retort again on 
Dr. Hopkinson that ‘‘a hostile critic should at least understand 
the meaning of what he criticises” ? 
If I said anything which seemed to imply that a minimum of 
work in an engine was inconsistent with a maximum of duty, I 
freely retract the expression ; and I also acknowledge that the 
argument drawn from the fire-syringe had better have been omit- 
ted. But my point was proved abundantly without it. 
But still, as the maximum of work done by a battery before it 
is worn out is only a multiple of the maximum duty of a grain of 
zinc, I do think it is a start/ing thing, though not mathematically 
impossible, that this maximum of work should prove to be no 
work at all. 
Perhaps you will allow me to add that I have read Sir W. 
Thomson’s paper read before the British Association in 1852, to 
which your own reviewer referred me. No doubt you will think 
it presumptuous in me to say so, but I think that in that paper 
he has mixed up two totally distinct questions, namely, the cold 
produced by the decomposition of water into its elements at two 
electrodes, and the heat produced by the resistance of the film of 
hydrogen or oxygen or oxide, to the passage of the current. 
The first is a fixed determinate quantity ; the second an acci- 
dental one depending on the character of the surface of the 
electrode, and the ease with which it throws off the film of 
hydrogen or oxygen. These two points affect the question, as 
well as the polarisation, and the specific power ot retaining or 
transmitting heatexercised by various electro-motive combinations. 
M. Favre suggests the formation, sometimes, of peroxide ot 
hydiezen; but this supposition is unnecessary, and, moreover, 
would not remove the difficulty, for peroxide of hydrogen is so 
unstable a compound, that it would soon be resolved into oxygen 
and when the permanent magnets are stronger, 
