3o6 DEVELOPMENT OF HEAT. chap.
point, which may assume the form of a brush (aigret) if there be a suflScient outflow of positive electricity. Hie diseharge is discontinuous, as can easily be proved by making use of a rotating mirror ; the hissing noise also indicates that the discharge is discontinuous. When the discharge takes place in the air a smell of ozone becomes perceptible ; many other chemical actions are also brought about by this action of points. For instance, in the air some oxidation products of nitrogen are formed as well as ozone ; in acetylene, benzene is formed ; in an atmosphere of carbon monoxide and water vapour combination takes place, and formic acid is produced, if carbon dioxide is used oxygen is evolved (this reaction corresponds with the process of vegetation); nitrogen and hydrogen give ammonia, which is again partially decom- posed ; sulphur dioxide and oxygen give sulphur trioxide ; cyanogen and hydrogen give hydrocyanic acid ; and nitrogen and oxygen, in presence of water, give ammonium nitrate, a compound whose presence has also been detected after lightning.
The same reactions can also be brought about by a spark discharge, which only differs from the " silent " or " dark " discharge in its greater intensity. A gas may be brought to the glowing point when it is enclosed between two condenser plates separated by an insulator (e.g, glass), when these are connected with the poles of a high tension alternat- ing current machine. In this case there is formed a com- paratively large quantity of ozone, as in the discharge from the poles of a Tesla alternating current machine.
The most remarkable method of bringing about chemical actions by the silent discharge is that found by Berthelot (18), The apparatus devised by him is shown in Fig. 57. Two thin-walled glass tubes, a and b, are arranged concentrically one within the other. The outer tube h is furnished at its upper end with side tubes, c and d, and immediately above these it is sealed on to a. The tube a is flUed with sulphuric acid, and b is immersed in a cylinder filled with the same liquid. When solid substances are to
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