the contained air, by pressing the sides together, while the operator attaches it to the gas bottle, (fig. 8,) or the jar for the pneumatic shelf, (vide fig. 7.) From this statement the pupil will easily

understand why a bent tube, terminating in a hollow glass ball, is placed over gas lamps in shops; the watery vapour produced by the union of the hydrogen of the coal gas with the oxygen of the air being cooled in the bent tube, is collected in the ball. Familiar facts like these should always be explained to youth by their preceptors.

Experiment 33. While hydrogen unites with oxygen to form water, a violent and sudden action

takes place, which can be proved by mixing one measure of oxygen with two of hydrogen in a bladder; any other vessel should be avoided. After a slight agitation, pierce the bladder with a pointed instrument, and apply a lighted taper; a very loud explosion will ensue, with the destruction of the bladder. If it be attached to a wall, no accident can possibly occur, otherwise it would not be noticed in these pages. From this fact it will be understood why it is dangerous to apply a lighted taper to the bottle in which hydrogen is generated: a portion of air may be mixed with it, and the explosion follows; the glass vessel will generally be broken. It is hoped that this caution will never be forgotten. The firedamp of coal mines results from a similar cause.

Experiment 34.-Attach a smoking-pipe to the gas bottle, having placed some soap lather in the bowl; bubbles, called soap balloons, will be formed, which, by a little dexterity, can be detached; these will rapidly ascend; or procure at the toy-shop a French balloon, made of animal membrane, which will ascend when inflated.

Experiment 35.-The following is a beautiful illustration of the affinity or attraction which hydrogen has for oxygen :-dissolve a few grains of nitrate of silver, (lunar caustic,) composed of oxygen, nitric acid and silver, in distilled water, moisten silk or paper with the solution, and hold it closely over the bottle from which hydrogen is escaping, in a few moments it will be seen that a metallic coat of silver is exposed, resulting from the hydrogen having removed

147. Can you explain why it is dangerous to apply a lighted taper to the gas-bottle?

148. When steam passes through a red hot iron tube, why does hydrogen make its appearance at the other extremity ? 149. When you apply a lighted taper to hydrogen, with the mouth of the vessel downwards, why does it burn so slowly?

150. Would it burn without the oxygen of the surrounding air?

151. Why does it burn so rapidly when the vessel is not inverted?

152. Does a lighted taper burn in hydrogen?

153. Would an animal live in the same situation?

154. What alone supports life?

155. Describe a marked distinction that exists between oxygen and hydrogen with respect to inflammability?

156. While hydrogen is burning what is formed?

157. If we ignite the gases mixed in a bladder, does any peculiar effect take place at the moment the water is formed? 158. Explain why silver is liberated when a current of hydrogen acts upon nitrate of silver.

CHLORINE.

46. This important element was discovered by the celebrated chemist, Sir Humphrey Davy. From its greenish colour he proposed to call it chlorine, from the Greek xwpós, green. It is heavier than the atmosphere, possesses a most suffocating and pungent odour, is destructive to life, acts powerfully on metals, and partially supports combustion. Its most important properties are, that it destroys vegetable and animal colours, and neutralizes the effects of the

effluvia which escapes from decomposing animal and vegetable bodies; it is therefore extensively employed as a bleaching and disinfecting agent.

Experiment 36.-By the following process, we disengage chlorine in sufficient quantity for experiments :-heat cautiously in a retort, a mixture of one ounce of black oxide of manganese, and three ounces of muriatic acid, (a combination of hydrogen and chlorine,) the liberated chlorine is collected in the pneumatic trough like the other gases, with this exception, that warm water must be used in the trough, as the chlorine is absorbed by cold: the operator is carefully to avoid breathing the gas, as it exerts, even in small quantities, a more injurious effect upon the lungs than other noxious gases, hydrogen, &c.: a free current of air should pass through the room in which the operation is performed.

47. The explanation, termed in chemistry rationale, of the preceding process, can easily be understood, even by a young student, (fig. 11.) On mix

ing muriatic acid, which is composed of chlorine and hydrogen, with black oxide of manganese, composed of