114. When we pour sulphuric acid on black oxide of manganese and apply heat, why do we obtain oxygen?

115. How can we obtain oxygen from the oxide of manganese without sulphuric acid?

116. What is the derivation of the term pneumatic?

117. Describe the general principle upon which a pneumatic trough is made?

42. In a pure state, oxygen is devoid of colour, taste, or smell-it can, however, be easily distinguished from any other gas by the following properties :

Experiment 21.-Having carefully removed one of the jars (fig. 5) from the shelf of the pneumatic trough, with its mouth immersed in a tray of water, suspend in the oxygen a piece of lighted charcoal; it will immediately burst into a splendid flame. A similar effect is produced upon a coil of red hot iron wire, or a taper that has been just extinguished, the wick still remaining red.

FIG. 6.

Experiment 22.-A piece of ignited phosphorus, placed in fig. 6, burns with so intense a flame in oxygen that the eye can scarcely bear its brilliancy. This is a dangerous experiment; in fact, any experiments with phosphorus should be carefully performed by young persons, as this extraordinary element ignites by the heat of the hand, and produces a more dreadful burn than red hot iron. It cannot be sufficiently impressed that it should be cut under water, upon the memory when required, into pieces not exceeding the size of a pea, which should be removed with a pointed wire.

QUESTIONS.

118. When a piece of lighted charcoal is suspended in oxygen, what peculiar effect is produced?

119. Does red hot iron wire burn in oxygen?

120. Does ignited phosphorus burn in this gas ?

121. Is a glow-worm affected by it?

122. What two great classes of compounds are formed by oxygen uniting with other bodies?

123. When charcoal burns in oxygen, what is formed? 124. 'What does iron form under similar circumstances? 125. Does oxidation, which signifies the absorption of oxygen, take place in the atmosphere ?

126. In the combustion of coal, candles, &c., is this gas actively engaged?

127. What proportion of our atmosphere does oxygen form ?

128. Is it instrumental in supporting life?

NITROGEN.

44. This gas is so termed, because it enters into the composition of nitre. It was formerly termed azote (from a, the Greek negative, and wý, life), because it destroys life. This title was objectionable, ́as there are many other destructive agents. Nitrogen is lighter than air, and forms four-fifths of the volume of our atmosphere; the remaining fifth is oxygen. It is found in all animal substances, and seems to promote vegetation in a considerable degree, as it is invariably found to form a considerable portion of those parts of plants which perform the most impor