16 
THE VOYAGE OF H.M.S. CHALLENGER. 
chloride is first subjected to a stream of dry CO 2 , so that, when used in the estimation 
of the COj, no error may be introduced by the absorption of the gas. The calcium 
chloride tube is fitted with an india-rubber stopper, in order to exclude the outside air 
when cooling and during the process of weighing. 
The weighed sample is placed in the large bulb, a little water added, the different 
parts adjusted and finally wiped dry, and then the whole is weighed by suspending 
from the beam of the balance by a fine platinum wire. The acid is then run in little 
by little, until all eflervescence has ceased ; the contents of the bulb are heated with 
constant shaking to near the boding point, cooled, and finally weighed, and from the 
loss in weight (weight before experiment minus weight after) the percentage of calcium 
carbonate is calculated. When these analyses were first made, Ludwig’s apparatus was 
used, but this was modified to the present form by the substitution of an ordinary 
calcium chloride tube for the circular basal one, as there was a certain amount of 
inconvenience experienced in heating. 
It may be urged that the determination by carbonic acid, unaided by the oxide of 
calcium test, indicates but approximately the quantity of carbonate of calcium, for the 
deposit may contain carbonates of magnesia and iron. It is sufiicient to observe that 
these bodies are only present in very small quantities. Besides, the composition of a 
deposit at the same station and even in the same sample varies sometimes by a quantity 
greater than the error committed. It is true that small quantities of iron, alumina, 
phosphates, &c., are always dissolved in the above process, but this partial dissolu- 
tion has no marked influence on the biological or mineralogical determinations that 
require to be made. This method was then sufficiently exact for our purpose. Complete 
quantitative analyses were made of many of the deposits, but the length of time 
required to do this for all samples was not compensated by any real utility in the 
investigation, while in many cases the quantity of a deposit was insufficient for such an 
analysis. 
By thus removing the carbonate of lime with dilute hydrochloric acid from each 
de|>osit, our descriptions are conveniently divided into two parts, one dealing almost 
exclusively with materials of organic origin — the shells and skeletons of carbonate of 
lime secreting organisms, — and another referring to the part in which the mineral element 
predominates in the great majority of cases ; in some localities, however, there are 
associated with the minerals properly so called, the remains of siliceous organisms, 
together with the silicified casts of calcareous organisms, and these may together or 
8cj)arately make up the greater part of the deposit after the removal of the carbonate 
of lime. 
That part of the deposit which remains in the vessel after the removal of the carbonate 
of lime is called in the descriptions Residue. In the examination of this residue the 
heavier particles were separated by decantation and examined by reflected light in the 
