^54* Mr Kerr on Open Bent Tttdes, 4’^. 
The structure of the open bent tube may be somewhat variedy 
and the tube be still equally adapted to its purpose. Thus the 
descending part of the second branch may be left open, and a' 
stopcock placed in the second angle. 
The advantages of the open bent tube are nearly the same, 
but greater than those of the sealed bent tube. The formation 
of the smallest quantity of gas is detected by it, and the great 
advantage is gained of being able to ascertain the nature of the 
gas evolved at any period of an experiment, unaltered by at- 
mospheric air 
About two years ago, while endeavouring to discover the 
quantity of oxygen in the yellow and red substances produced 
by exposing phosphorus to heat, or for a long time to light, I 
thought of such tubes as I have described in my first communi- 
cation, in order to detect witk accuracy the quantity of gas 
evolved by the action of nitric acid on the substances mentioned. 
My inquiries did not prove altogether satisfactory, but I soon 
found that the tubes were exceedingly convenient for performing 
* After the account of the sealed bent tubes was corrected for the press, I ob- 
served, in Dr Ure’s Chemical Dictionary, under the article Carbonate, an account, 
which had previously escaped my notice, of an instrument invented by him, which 
bears a considerable resemblance to mine, though it is only adapted to a particular 
case. He recommends it exclusively for discovering the quantity of gas disengaged 
from a carbonate, by means of muriatic acid. It consists of a glass-tube, of the 
same strength and diameter as that usually employed for barometers, having one 
end blown out into a strong egg-shaped bulb, about 2 inches long, and If inch wide. 
The tube is recurved like a syphon, and its other end is left open. In using this 
instrument, the Doctor first introduces a small bit of the carbonate into the empty 
tube, and makes it slide along to the globular part. The whole tube is then filled 
with mercury, through which he subsequently contrives to pass a quantity of di- 
luted muriatic acid, sufficient to dissolve the carbonate. As diluted muriatic acid 
has no action on mercury, at ordinary temperatures, all the gas evolved must be 
carbonic acid ; and, as the Doctor ascertains the capacity of the bulb, and gra- 
duates the straight part of the tube between the bulb and the bend, he is able to 
determine with great accuracy the quantity of carbonic acid contained in calcareous 
spar, and some other carbonates. But as the curvature of his instrument is not 
dilated toward its shut branch, any gas evolved near that curvature cannot be col- 
lected with any degree of accuracy. In order to collect gas with his instrument, 
mercury must be employed, to confine both the acid and the substance acted on to 
the shut branch ; therefore, it can be of no use when the acid employed acts on 
mercury. The tubes recommended by me can be employed in experiments wi^i' 
almost any acid. 
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