May 4, 187% | 
each element has a definite combining power or equiva- 
lence, and that the arrangement of atoms in compounds 
is of as much importance as their kind or number. This 
work is remarkable for the conciseness of its definitions ; 
one of the first is on chemical and physical changes, in 
which it is said that “ physical changes in matter are those 
which take place outside the molecule ; they do not affect 
the molecule itself, and therefore do not alter the identity 
of the matter operated on. Chemical changes take place 
within the molecule, and hence cause a change in the 
matter itself.” Some of the definitions would not, how- 
ever, find general acceptance ; thus, an acid molecule is 
said to be “one which consists of one or more negative 
atoms united by oxygen to hydrogen ;”—a definition which 
excludes hydrochloric acid and its analogues. And a saline 
molecule is defined to be one containing a “ positive atom 
or group of atoms, united by oxygen to a negative atom 
or group of atoms,” which removes sodic chloride from 
the list of salts. The term base is confined to the hydrates 
of positive elements or groups of elements, and the 
hydrates of the metals calcium, zinc, and iron are some- 
times called calcic base, zincic base, ferrous base, 
and ferric base. The nomenclature of the acids 
is systematised, but peculiar names are the result: an 
ortho-acid is one containing as many atoms of oxygen and 
hydrogen as is equal to the equivalence of the negative 
atom or group; and a meta-acid is derived from an ortho- 
acid by the subtraction of molecules of water, thus ortho- 
phosphoric acid would be P (OH)., metaphosphoric acid 
(PO)” (OH),, and dimetaphosphoric acid (P O,)’(OH). 
These names and those of most other acids are liable to 
some misunderstanding, as the compounds they represent 
have long been known by other designations. The theo- 
retical part of the book contains chapters on elemental 
molecules and atoms, compound molecules, volume rela- 
tions of molecules, and stoichiometry. The part on inor- 
ganic chemistry is divided into eleven chapters, on hydro- 
gen, the negative monads, dyads, triads, boron, negative 
tetrads, the iron group, positive tetrads, triads, dyads and 
monads, thus treating of the elements according to their 
electro-chemical characters, commencing with the most 
negative. Each chapter is divided into sections containing 
the history, occurrence, preparation, and properties of the 
elements, and is followed by aseries of questions intended 
as exercises for the students, a method now much adopted, 
and found to be of great assistance to teachers. This 
book is another of the evidences of the rapid progress of 
pure science in America. 
Czermak’s Electric Double Lever. (Der Electrische 
Doppelhebel, von F. N. Czermak.) (Leipzig: Engel- 
mann. 1871. London: Williams and Norgate.) 
A DESCRIPTION of a most ingenious little contrivance for 
marking the exact moment in which a movement begins 
or changes its direction. The old arrangement, by which 
a lever, forming part of a circuit, comes, when set in mo- 
tion, in contact with a fixed point connected with the other 
part of the same circuit, and so closes the circuit and 
makes a signal, is modified by Prof. Czermak as follows. 
The fixed contact point is replaced by a secondary lever, 
whose axis of revolution is the same as that of the primary 
lever. This secondary lever bears at one end a contact 
point. The primary lever touches in its swing this con- 
tact point, and so closes the circuit ; it then pushes the 
secondary lever before it, but having reached the limit of 
its oscillation, leaves the secondary lever at rest ina 
position marking the farthest point of the excursion. A 
counter contact-point, however, on the other arm of the 
primary lever (where the lever is a double-arm one ; with 
single arm levers, a special arrangement is introduced), as 
the primary lever is returning into position gives to the 
secondary lever a movement in the same direction. Thus 
the two levers are continually following each other, making 
and breaking contact. The instrument is in this way 
NATURE 5 



made capable of being used for signalling all manner of 
movements. It is impossible fully to explain its construc- 
tion ina few lines, and we therefore refer the reader to 
the pamphlet itself, which, we should say, is published in 
celebration of the Jubilee of the great Leipzig Professor, 
Ernst Heinrich Weber. By the invention of his delightful 
“ Rabbit Holder,” Czermak has endeared himself to every 
physiologist, and we may well share his hope that this new 
double lever will be found no less useful. M. FOSTER 


LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Pangenesis 
Ir appears from Mr. Darwin’s letter to you in last week’s 
Narturg, * that the views contradicted by my experiments, pub- 
lished in the recent number of the ‘‘ Proceedings of the Royal 
Society,” differ from those he entertained. Nevertheless, I 
think they are what his published account of Pangenesis 
(Animals, &c., under Domestication, ii. 374, 379) are most 
likely to convey to the mind of a reader. The ambiguity is due 
to an inappropriate use of three separate words in the only two 
sentences which imply (for there are none which tell us anything 
definite about) the Aaéitat of the Pangenetic gemmules ; the words 
are ‘‘circulate,” “freely,” and “diffused.” The proper meaning 
of circulation is evident enough—it is a re-entering movement. 
Nothing can justly be said to circulate which does not return, 
after a while, to a former position. In a circulating library, 
books return and are re-issued. Coin is said to circulate, because 
it comes back into the same hands in the interchange of business. 
A story circulates, when a person hears it repeated over and over 
again in society. Blood has an undoubted claim to be called a 
circulating fluid, and when that phrase is used, blood is always 
meant. I understood Mr. Darwin to speak of blood when he 
used the phrases ‘‘circulating freely,” and “the steady circula- 
tion of fluids,” especially as the other words “freely” 
and ‘‘diffusion” encouraged the idea. But it now seems 
that by circulation he meant ‘‘ dispersion,” which is a totally 
different conception. Probably he used the word with some allu- 
sion to the fact of the dispersion having been carried on by 
eddying, not necessarily circulating, currents. Next, as to the 
word ‘‘freely.’’ Mr. Darwin says in his letter that he supposes 
the gemmules to pass through the solid walls of the tissues and 
cells; this is incompatible with the phrase “ circulate freely.” 
Freely means ‘‘ without retardation ;’ as we might say that 
small fish can swim freely through the larger meshes of a net; 
now, it is impossible to suppose gemmules to pass through solid 
tissue without avy retardation. ‘* Freely” would be strictly 
applicable to gemmules drifting along with the stream of the 
lood, and it was in that sense I interpreted it. Lastly, I find 
fault with the use of the word “diffused,” which applies to 
movement in or with fluids, and is inappropriate to the action I 
have just described of solid boring its way through solid. If 
Mr. Darwin had given in his work an additional paragraph or 
two to a description of the whereabouts of the gemmules which, 
I must remark, is a cardinal point of his theory, my misappre- 
hension of his meaning could hardly have occurred without more 
hesitancy than I experienced, but I certainly felt and endeavoured 
to express in my memoir some shade of doubt ; as in the phrase, 
p- 404, ‘‘that the doctrine of Pangenesis, pure and simple, as I 
have interpreted it, is incorrect.” 
As I now understand Mr. Darwin’s meaning, the first passage 
(ii. 374), which misled me, and which stands: “ . minute 
granules . . which circulate freely throughout the system ” 
should be understood as “minute granules . . which are 
dispersed thoroughly and are in continual movement throughout 
the system ;” and the second passage (ii. 379), which now stands: 
“The gemmules in each organism must be thoroughly diffused ; 
nor does this seem improbable, considering . the steady 
circulation of fluids throughout the body,” should be understood 
as follows: ‘* The gemmules in each organism must be dispersed 
all over it, in thorough intermixture ; nor does this seem impro: 
bable, considering . . . the steady circulation of the blood, 
the continuous movement, and the ready diffusion of other fluids, 
* Narur:, vol. iii. p. 502. 
