CHEMISTRY. 13 
space it occupies. When the receiver, B, is filled, it is to be removed to 
another shelf, and replaced by a new receiver, prepared like the last. 
The philosophy of the operation by which the oxygen is liberated is as 
follows: Oxygen has a very great tendency to maintain the gaseous 
condition, and we have seen that the ordinary agents of cold and pressure, 
as usually applied, have been unable to effect its condensation into a liquid. 
In this case chemical affinity has done what the other agents failed to 
accomplish, namely. changed it from a gas into a solid. The accompanying 
mercury, also, is in a solid state, instead of its ordinary hquid condition. 
Heat, however, overcomes this combination by which the two elements are 
solidified, and the elements, expanding, resume their more natural state. 
The mercury is liquified; the oxygen converted into gas, which then passes 
over. The heat applied must of course not be sufficient to vaporize the 
mercury: this requires a temperature of 660°F. | 
A more economical way of procuring oxygen is to employ black oxyde 
or deutoxyde of manganese in the apparatus given in pl. 31, fig. 60. The 
iron retort, C, is filled with manganese, and placed in the furnace, B. In its 
mouth is inserted the bent iron gun-barrel or leaden tube, leading to the 
pheumatic trough, D. On heating the retort to redness, one portion 
of oxygen is liberated from its combination with the manganese, and driven 
over to the trough, where it is to be collected with the precautions described 
in the preceding paragraph. 
Hydrogen is likewise a gaseous elementary body. While oxygen occurs 
abundantly in the atmosphere combined mechanically, and in the solid and 
liquid constituents of the earth combined chemically, hydrogen is never met 
with but in the latter state of combination. We may indeed make a slight 
exception for the minute quantity discernible at times in the atmosphere, 
the result probably of volcanic action. It is most abundant in a combi- 
nation with oxygen forming water. The proportions of these two gases, in 
water, are one part of hydrogen and eight of oxygen by weight, and two of 
hydrogen and one of oxygen by volume. The readiest way of obtaining 
hydrogen consists in the decomposition of water. Pieces of zinc are intro- 
duced into the gas generator, A ( pl. 30, fig. 55), the vessel filled about two 
thirds full of water, and sulphuric acid poured in by degrees through the 
funnel tube, D. The water standing a little higher than the lower end of 
the funnel tube D, closes this tube, so that the gas, when generated, can 
only escape through the tube, CC’, which is inserted into the cork stopper 
of the vessel A. In the case of this generating vessel, and, indeed, in all 
apparatus used for developing gases, some arrangement is employed to 
prevent any danger of bursting by the stoppage of the conducting tube. 
This purpose is accomplished in our illustration by means of the funnel D, 
out of which the liquid will be forced by any accumulation of gas in A. 
Another contrivance, called Walter’s Safety Tube (figs. 44 and 45), is 
intended for the same purpose. Water, or any more appropriate liquid, is 
poured into the leg a ( fig. 45), so as to fill the bulb except about one third. 
A safety tube of this character is inserted into the corked stopper of the 
generating apparatus instead of the funnel D (fig. 55). Should any 
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