Oct. 26, 1871 | 
This communication has run on to so great a length that Iam 
unable to touch uron other points in which I find myself totally 
disagreeing with Prof. Wiliam on. I cannot, however refrain 
from expressing my astonishment at the persistence of the histo- 
logical views implied by the description of the *‘cambium,”’ or 
growing cellular tissues of plants, as *‘some protoplasmic ele- 
ment,” or again as ‘‘some protoplasmic lsyer.” Similar ex- 
pressions were used by Nehemiah Gr-w about 200 years ago, 
and employed for some time by writers subsequent to him. At 
the present I imagined their interest was wholly historical. 
W. T. THISELTON DYER 

THE points at issue between Prof. Williamson and myself re- 
main in the same position as at first. He has not yet answered 
one of my objections. He still holds that in Lepidodendron we 
have a vascular medulla, outsite which is a series of fibro- 
vascular bundles which are not closed, but go on forming new 
tissues by means of a cambium layer like a dicotyledonous stem. 
From my own observations, and from the study of recent Con- 
tinental authorities, I have no hesitation in stating that the 
central ‘‘medulla” of Prof. Williamson consists of the united 
closed fibro-yascular bundles, while the investing cylinder is the 
modified primitive tissue which increases in diameter by means 
of the mercstem layer of Nageli. If Prof. Williamson will refer 
to Sachs’ Lehrbuch, Ed. 2, p. 397, he will find good reasons 
given for the statement there made, that /svéfes contains 20 cam- 
dium in the stem ; but that the stem increases in the same way 
as Draceéna, t.e. by a meristem layer in the primitive tissue. As 
long as Prof. Williamson believes in a central vascular medulla 
in these Lycopodiaceous stems, all his other conclusions must 
likewise be false. 
W. R. M‘Nas 
Royal Agricultural College, Cirencester, Oct. 21 
{*,* We would suggest that this controversy be now closed, 
until the publication of Prof, Williamson’s new material.—ED.] 

Blood-Spectrum 
IN the account of the Progress of Science in Italy in NATURE 
for October 12, Mr. W. Mattieu Williams says that Prof. C. 
Campani has shown that the spectrum of an ammoniacal solu- 
tion of carmine is undistinguishable from that of blood, and 
that perhaps I should be able to tell whether any difference can 
be distinguished by more minute examination. In my first 
paper on this subject, so long ago as 1865*, I alluded to this simi- 
larity, and in subsequent papers + I have shown how the colour- 
ing matter of blood can be distinguished from that of cochineal, 
and even a small quantity recognised when mixed witha rela- 
tively considerable quantity of taat dye. I have always argued 
that in such inquiries we must not rely on the spectrum, but 
compare the action of various reagents. On adding a little 
boric acid to an aqueous solution of blood, no change takes 
place in its spectrum, whereas that of cochineal is com- 
pletely altered. This effect is not produced in the case of 
carmine suspended in water, but the absorption-bands of blood 
are at once removed by deoxidising the solution with a ferrous 
salt, which, on the contrary, has no effect inthe case of carmine 
orcochineal. Weak acids decompose hcemoglobin into hzematin, 
which vives entirely different spectra, but they do not cause any 
permanent change in the colouring matter of cochineal or car- 
mine. In my opinion there is no more probability of an expe- 
rienced observer mistaking these substances for blood, because 
the ammoniacal solutions give nearly the same spectrum, than 
of achemist confounding aluminium bronze with gold, because 
they are of nearly the same colour. H. C. Soxsy 
Broomfield, Sheffield, Oct. 2 

Are Auroras Periodical ? 
THE following note on auroras is transcribed from the /owa 
Instructor and School Fournal for April, 1866. As it suggests 
a hypothesis similar to that proposed by Mr. Wilson, in your 
journal for September 7, it may not be destitute of interest. 
DaniEL KIRKWOOD 
Bloomington, Ind., Oct. 4 
“© The Aurora Borealis of February 20, 1866 
‘¢ Those who witnessed the grand auroral display of the 20th 
* Quat. Fourn. of Science, vol. ii. p. 208. nd 
+ Medical Press and Circular, New Series, vol. xii, p. 67; Monthly 
Micros. Fourn., vol, vi. p. 15. 
NAPORE 

595 

inst., and especially those who have kept a record of similar ex- 
hibirions, may have remarked the frequency with which the 
phenomena have occurred about the same epoch, viz, from 
February 15 to February 23. Some of the most brilliant that 
have occurred at this period during the last century are the fol- 
lowing :— 
1773 February 17 1848 February 20 
1784 a 2 1851 95 18 
1794 ” 15 1852 ” 18 
1838 An 21 1866 20 20 
Besides the February epoch, any extended list of auroras will 
indicate two or three others, the most remarkable of which is that 
of November 13—18 (See Olmsted’s paper in the ‘ Smithsonian 
Contributions,’ vol, viii.) Fifty-three brilliant auroras have been 
observed since 1770. Of these, an accidental distribution would 
assign but ove to the interval between the 13th and 18th of 
November ; whereas e7g/¢ of the number have actually occurred 
at that epoch. Are such coincidences accidental, or do they war- 
rant the conjecture that, as in the case of shooting stars, there are 
particular periods at which the grand displays of the phenomenon 
most frequently occur ?” 

Forms of Cloud 
THE form of cloud represented by Prof. Poéy in his figure a, in 
this week’s NATURE, is very similar to that described by the Rev. 
C. Clouston, LL.D., in his ‘‘ Explanation of the Popular Wea- 
ther Prognostics of Scotland,” published by A. and C. Black in 
1867, and also in Dr. Mitchell’s paper ‘‘ On the Popular Weather 
Prognostics of Scouland,” Edin. New Phil. Journal, Oct. 1863. 
Dr. Clouston says that, ‘‘when properly developed it was 
always followed by a storm or gale within twenty-four hours. 
It is called ‘ pocky cloud’ by our sailors.” 
He gives a sketch from which, as he says, “it will be seen that 
this is a series of dark, cumulus-looking clouds, like festoons of 
dark drapery, over a considerable portion of the sky, with the 
lower edge well defined, as if each festoon or ‘pock’ was filled 
with something heavy, and generally one series of festoons lies 
over another, so that the light spaces between resemble an Alpine 
chain of white-peaked mountains. It is essential that the lower 
edge be well defined, for a somewhat similar cloud, with the 
lower edge of the fe-toons fringed, or shaded away, is sometimes 
seen, and followed by rain only.” 
Dr. Clouston concluded his notice by saying, ‘‘this cloud is 
well known, and much dreaded by Orkney sailors,” 
Rosert H. Scotr 
Meteorological Office, London, Oct. 20 

Elementary Geometry 
IT is scarcely worth while for an anonymous writer to defend 
his opinions ; but since a sentence in my letter of September 21 
still continues to elicit remarks, I may be allowel to add an ex- 
planation of my meaning. I stated that ‘‘no child is capable of 
taking in a subject, especially if it involves logical thought, ex- 
cept by very slow degrees ; and must at the beginning commit 
much to memory which he does not comprehend.” And I called 
this ‘‘a fact.” Mr. Wormell says in reply, that the purpose 
which geometry serves is not the exercise of the memory, and 
that it is useless if not understcod. I entirely agree with him, 
and my words, if fairly interpreted, do not convey the contrary 
opinion. 
In your last issue Mr. Cooley writes, that my principle, that 
“4 child must of necessity commit much to memory which he 
does not comprehend,” appears to him totally erroneous, and not 
entit'ed to be cilled a fact. But surely the order of Nature with 
children is to possess themselves of empyrical knowledge by the 
exercise of memory, and subsequently to get to comp:ehend 
what they have thus acquired. Would Mr. Cooley wait until 
he had made a child comprehend the principles of the decimal 
scale, before he taught him to add up two rows of figures, and to 
say, “ five and seven are twelve ; put down two, and carry one ” ? 
If he condescends to the usual course of a “hearer of lessons ” in 
this one instance, he acts upon the admission of my principle. 
To apply this to geometry (and perhaps I may be borne with 
if I use Euclid in illustration): I fancy that many a boy a¢ the 
beginning understands the three first propositions, but not the 
whole of the fourth. My plan would be, not to keep him at it 
till he did, but to let him learn it fairly well by rote, and go on, 
applying the results of the fourth by an act of faith. The second 
time he went through the book, if he had been decently taught, 
