W. De la Rue on the Navicula Spencerii. 23 
composition is not such, for the quantity of hydrogen it contains 
is much greater than that found by any investigator in the sub- 
stance acknowledged as pectic acid. The mean results of Chod- 
new’s analyses of this acid, to which those of Prof. Sacc most 
nearly approach, are as follows—for couvenience of comparison, 
Prof. Sace’s results are given in the second column, my own in 
the third: 
C : 4222 = ave 9 43°39 
H 521 593 5:86 
O 52:55 52:14 50°75 
Chadnew’s formula is C,, H,, O,,=C,, H,,0,;. 
Prof. Sacc’s “ « Oca ses Wil aes 
The results of my own analyses of the substance from wood 
differ from those expressed by the latter formula, principally in 
the larger amount of carbon found; they differ also as widely 
from those obtained in any analysis of pectic acid. My further 
reasons for believing that the substance is not such, are, first, its 
different behavior on washing with alcohol ; second, its insolubil- 
ity in boiling water; third, the form of the precipitate obtained 
from a solution in excess of alkali; and finally, the fact that while 
pectic acid is partially transformed into mucic acid on being 
boiled with nitric acid, this is not the case with the substance 
under consideration. 2 geo 
My further conclusion from the investigation is that the real 
formula of the new acid is J Rhy Dias 
Arr. V.—On the Navicula Spencerii ; by Warren De va Roe. 
(In a letter to the Editors, dated London, August 28, 1849.) 
Some time since, my attention was called to an article, in your 
Journal, for March, 1849, from the pen of Prof. J. W. Bailey, enti- 
tled, “Some remarks on the Navicula Spencerii, and on a still 
more difficult test object ;” as this article contains some strictures 
on the description® of the markings on this Navicula observed by 
Mr. Marshall and myself, I trust, you will allow me an opportu- 
nity of replying to Prof, Bailey, in your valuable Journal. 
I avail myself of this opportunity to correct Mr. Quekett, with 
respect to the nature of the markings observed by us ;—he says, 
“Mr. De la Rue has further made out that the dots are not pro- 
jections from the surface, but are either perforations or depres- 
sions ;”—now this is precisely the reverse of what I wished, but 
apparently failed, to convey to Mr. Quekett, in a conversation 
with him, respecting this and some others of the Navicula- 
* See Quekett on the use of the Microscope, page 440 and plate ix. 
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