of making oxygen is interesting since it was the first means known of preparing the gas; it is never used nowadays to prepare large quantities of oxygen, since other substances are known which contain a large proportion of oxygen, and give it off, when they are heated, more easily than mercuric oxide does-substances which are also preferable on account of their greater cheapness. Potassium chlorate is most frequently employed :

EXP. 2. Place in a clean dry test-tube a little potassium chlorate, and heat it as in Exp. 1. The white salt, after decrepitating (crackling), fuses (or melts) and when further heated, appears to boil; the small bubbles which are given off consist of oxygen gas, as may readily be proved by holding in the mouth of the test-tube a burning or glowing splinter of wood as described in Exp. 1.

Potassium chlorate gives off oxygen gas much more readily than does mercuric oxide; but if it is mixed with small quantities of certain other substances, which themselves appear to undergo no change, its oxygen is driven off by heat with extreme facility; of these substances manganic oxide (black oxide of manganese) is the one usually chosen.

EXP. 3.-Powder some potassium chlorate (about as much as would fill a watch-glass) finely in a mortar, mix with it, by rubbing them together in the mortar, about one-fifth as much powdered manganic oxide, and heat a small quantity of the mixture in a test-tube; the oxygen will begin to come off as soon as the mixture is heated, and a comparatively gentle heat will cause the gas to be rapidly evolved.

In the preceding experiments the oxygen was detected in the tube in which it was prepared, and was allowed to pass away freely into the air. When the gas has to be collected in a vessel unmixed with air, it is made to pass through a bent glass-tube (the delivery tube), which is fitted by means of a cork air-tight into the mouth of the test-tube; the end of this tube dips into some water, and the bubbles of gas are allowed to rise into a vessel full of water and inverted over

the end of the delivery tube. This process of "collecting" oxygen is fully described in the following experiment: in the performance of which two students may advantageously work together, one attending to the regulation of heat to the mixture, the other to filling the gas-jars.

EXP. 4.-Select a sound cork, of such a size that, after having been softened by being squeezed or by being rolled with gentle pressure on the floor under the foot, it fits tightly into the mouth of the test-tube to be employed. Then bend (6) a piece of hard glass tubing about fourteen inches in length, into the form shown in the figure; so adapting the bends by trial that when the apparatus is fitted together the FIG. 2.

bottom of the test-tube may be at a convenient height in the flame, the end of the delivery tube at the same time dipping about an inch under water. Make a hole through the centre of the cork (8), of such a size that the glass tube fits tightly into it. Then test whether the apparatus is air-tight by fitting the glass tube into the cork, and the cork into the test-tube, and blowing down the open end of the delivery tube; no air must be heard to escape, or must be seen to bubble out on moistening the cork: if air does escape a fresh cork must be taken. Now pour into the perfectly dry testtube the oxygen mixture (see Exp. 3) off a piece of paper folded into a trough, or scoop up the mixture from the mortar with the mouth of the test-tube, until the tube is about onethird full, and fit in the cork and delivery tube.

Before heating the tube fill the jar in which the oxygen is

to be collected with water, close it with a stopper or groundglass plate (or with the hand), invert its mouth into water three or four inches in depth, contained in an earthenware pan or bowl, and carefully remove the stopper or plate. If this operation has been performed with proper precaution the jar will be entirely filled with water, and no air-bubble will remain. Next proceed to heat the upper part of the oxygen mixture, holding the tube in the right hand; keep the lamp slowly moving with the left, in order to prevent any part of the glass from being suddenly and strongly heated, which would be liable to crack it. Oxygen gas will soon be evolved, but will not at once appear at the end of the delivery-tube, since it has first to drive out the air which filled the apparatus; as soon as a slip of wood glowing at its end is kindled, when held at the mouth of the delivery-tube, the oxygen has driven out the air, and is beginning to escape; the end of the delivery-tube is then at once dipped under water beneath the mouth of the jar, and the stream of bubbles rising into it will rapidly displace the water. As soon as the jar is full of gas, close its mouth under water with the stopper or glass plate, and remove it for experiment. The jar may also be removed by slipping under its mouth a small dish or saucer, the water taken out in the saucer then closes the mouth of the jar air-tight.

Precautions. The water must be removed from the pan, when it rises inconveniently high, by means of a small porcelain dish; if at any time the gas should be given off too rapidly, the flame should be removed until the current slackens; the lower portions of the mixture should be heated only after the upper parts refuse to yield any more gas: when the process is to be stopped, the end of the delivery-tube must be removed from the water before the gas has ceased to bubble out, and the test-tube must not be allowed to touch cold or wet objects, which would cause the hot glass to crack.

Several bottles filled with oxygen will be required for the following experiments, or the same bottle may if necessary be refilled with the gas according to the above directions, after the completion of each experiment.

Note. The use of ground-glass plates, which must close the mouth of the jar perfectly air-tight, is much easier than that of stoppers. An earthenware "bee-hive shelf" which may be used in a common earthen pan, or a "pneumatic trough," is also convenient, since it supports the jar during the process of eollection.

Oxygen gas is remarkable for the energy with which it combines with or burns many substances: three examples are given of this property in Expts. 5, 6, and 7.

EXP. 5.-Select a splinter of wood charcoal or a small piece about the size of a nut; the experiment is more brilliant if the surface of the charcoal formed originally part of the bark of the tree. Bind this upon a "deflagrating spoon" with a little fine iron or copper wire; then adjust the wire handle of the spoon in the brass cap, so that when held beside the bottle of oxygen with the cap on a level with the mouth of the jar, the little metal cup is about an inch from the bottom of the bottle. Now heat the charcoal in the Bunsen flame, or better the blowpipe flame (4), until a part of its surface glows when held in the air, and quickly place it into the bottle of oxygen, with the brass plate covering the mouth of the bottle. (See fig. 7, p. 17.) The charcoal will burn much more brilliantly than in air, throwing off sparks if its surface was "barky:"

C + O2 = CO2.

When it ceases to burn pour into the bottle a little clear lime-water from a small beaker or test-tube, quickly close the bottle and shake the liquid round inside it; the clear liquid becomes milky, indicating the presence of carbon dioxide gas, as will be hereafter explained.

EXP. 6. Remove the charcoal from the deflagrating spoon and replace it by a piece of sulphur as large as a pea; heat the spoon in the flame until the sulphur melts and begins to burn with a pale blue almost invisible flame. Then place the spoon into a fresh jar of oxygen, the sulphur will at once burn with a much larger flame, which emits a beautiful violet light:

S+ O2 =SO2.

Sulphur dioxide (sulphurous anhydride) gas remains in the

bottle, its presence is proved by its suffocating smell, also by pouring a little water into the bottle and shaking it round. Sulphurous acid is thus formed, and is recognised by dropping into the water a piece of blue litmus-paper, which is immediately reddened, and by pouring in a few drops of red potassium dichromate solution, the colour of which changes to green.

EXP. 7.-Cleanse the deflagrating spoon from any remaining sulphur, and place into it a small piece of phosphorus no larger than a pea. The phosphorus may be cut with a knife, but it must be touched only with wet fingers, and should be handled as little as possible, since it is liable to catch fire by the heat of the hand; it is always kept under water, being dried only immediately before being used by pressing it between filter-paper or blotting-paper, or with a dry cloth. Set fire to the phosphorus by holding the spoon in the flame, and notice how it burns in the air; then place the spoon in a jar of oxygen the phosphorus will burn most brilliantly, producing a white substance called phosphorus pentoxide (or phosphoric anhydride) :—

P2+05 = P205.

When the phosphorus has ceased to burn pour in a little water and shake it round in the jar, the white substance dissolves, yielding phosphoric acid :

P205 + 3H20=2H,PO4,

and the water may now be proved to be acid by dropping into it a piece of blue litmus paper, which will be immediately reddened.

Test for Oxygen.-A convenient test for oxygen is to introduce into the gas a slip of wood with a spark at the end, which is caused to burst into flame. Only one other gas possesses this property, and it is readily distinguished from oxygen by other means. This test only detects oxygen when it is in a pretty pure condition.