SCIENTIFIC SUMMAEY. 
237 
of one of the unicellular plants. How, says Dr. Hicks, “ is a single cell to 
be distinguished from another stagle ceU ? What reliable characters are to 
be fixed upon as of generic value ? When we consider through what various 
forms those cells pass whose life-history has been carefully watched — as, for 
example. Protococcus pluvialis, the very species with which Mr. Archer 
illustrated his paper, — how are we to tell to what genus any single cell 
belongs ? How can we tell whether it be a fixed form, a separate entity, or 
merely a transitional form of some other growth ? When, again, we find, as 
I have shown, that cells quite similar in all respects are produced during the 
segmentation of the gonidia of the Lichens, Mosses, Lycopodia, Prasiola, &c., 
in what manner, may it be asked, are we to tell to what group the cell belongs, 
and how can we say that it is certainly a separate palmeUaceous plant?” 
From these queries it is quite evident that Dr. Hicks means to contend that 
the only clue to the true nature of such plants is a study of their develop- 
ment — a knowledge of their various transitional phases. Respecting the 
characters upon which considerable weight has been laid, the writer concludes 
that none of the following can enable the observer to identify the cell as 
belonging to a particular species : — 
1st. The position of the nucleus. 
2nd. The size of the cell. 
3rd. The disposition of the chlorophyll. 
4th. The form of the cell. 
The first is useless because in certain forms the nucleus is absent ; the second, 
because size depends upon a variety of conditions, such as temperature, the 
rate of segmentation, &c. ; the third, because in many forms the contents are 
quite homogeneous ; the fourth, because the variety of forms is almost 
unlimited. Dr. Hicks terminates his remarks upon this portion of the 
subject by asking : — “ Upon what, then, are we to fix ? No answer seems 
practicable but that which I have already indicated, namely, upon the 
gradations assumed during its whole life-history. If it be asked. How can 
this be attained ? it must be honestly answered, — with much labour and careful 
observation. Better trace one form well than endeavour to attain an 
apparently large result by that which cannot be relied upon, although it may 
have the attraction of being definite.” — Vide Quarterly Journal of Micro- 
scopical Science, October. 
CHEMISTRY. 
A New Method of estimating Carbonic Acid has recently been aimounced 
by Messrs. Tyndall and Barrett, of the “ Royal Institution.” It appears that 
the air expired by the lungs, and which contains carbonic acid in considerable 
quantity, can be more minutely and accurately analysed and examined by 
physical than by the ordinary chemical means. The new method is based 
upon the fact that carbonic acid, although in regard to the ordinary sources 
of heat a feeble absorber, becomes a very powerful one when the caloric is 
developed by carbonic oxide flame. The following is the way in which the 
experiment proving the accuracy of the new process was performed by Mr. 
Barrett. A small flame of carbonic oxide is caused to burn regularly within 
