DISCOVERY 



9 



vapours for concealing military operations are produced 

 by the combustion of mLxtures of chlorides of metals 

 or non-metals, or chlorinated hydrocarbon derivatives, 

 with a metal, non-metal, or a sulphide, or with a 

 mixture of an oxide and reducing agent, or with 

 a mixture of an oxide, an excess of a reducing agent, 

 and an oxidizing agent. For example, the mixture 

 may contain' tetrachlorethane 40, zinc oxide 20, 

 zinc dust 15, calcium silicide 15, and sodium chlorate 

 10 per cent." 



If carbon tetrachloride is used as the chlorinated 

 hydrocarbon, the production of smoke really depends 

 on its interaction with the zinc, zinc chloride thus being 

 formed. In order to prevent the deposition of carbon 

 at the same time, oxides are added, and the metals of 

 these oxides, after furnishing oxygen to the carbon, 

 are also transformed into chlorides and add to the 

 smoke. Kieselguhr is often added, and this serves 

 the purpose of absorbing the carbon tetrachloride, thus 

 furnishing a pasty mixture which can readily be 

 handled, and of which the constituents do not separate 

 out. In order to start the reaction it is necessary to 

 heat a portion of the mixture to a high temperature 

 by means of a special thermite igniter. Once the 

 reaction has started it proceeds with great vigour, and 

 a dense white smoke is emitted in great volume. 

 The particles of this smoke consist essentially of the 

 hygroscopic chlorides of zinc and other metals, with 

 the result that tlie cloud becomes denser a short time 

 after it has been formed. 



The Berger mixture could not be made in this 

 country, since some of the necessary ingredients were 

 not available in quantity. Extended series of ex- 

 periments and trials were therefore carried out at the 

 Royal Naval Experimental Station, Stratford, E., with 

 the result that several different mixtures, containing 

 ingredients which were available in quantity, were 

 devised. These mixtures were manufactured on a 

 large scale and filled into floats for use at sea. These 

 floats consisted essentially of a cyUndrical metal 

 container, approximately 8 inches by 28 inches, sur- 

 rounded by an air chamber of sufficient size and shape 

 to make the whole thing float upright in the sea with 

 a free-board of a few inches. The mixture, as soon as 

 it was made, was put into the float and a lid soldered on. 

 The lid contained a central, screwed hole, for the in- 

 sertion of an igniter when occasion demanded ; until 

 required this hole was sealed by a screw plug. Around 

 the central hole was a number of smaller holes or vents, 

 closed by means of soldered-on tin discs. The method 

 of using a float was to insert the igniter, strike the 

 friction tab on the top, and then throw the float over- 

 board. The delay La the fuse in the igniter was 

 suf&cient to allow the float to come to the surface of 

 the water before the thermite really caught fire. As 



soon as the smoke mixture started burning the heat 

 developed was sufficient to melt the solder fastening 

 the tin discs over the vents, with the result that the 

 latter opened, and smoke issued in volume from the 

 vents and the central hole where the igniter had been. 

 A single float would burn for 15-20 minutes, emitting 

 a cloud which would extend as an efficient screen for 

 1,000 yards or more. With several floats a most 

 satisfactory screen could be obtained. 



Instead of throwing the float overboard, it could 

 be burnt on deck if the wind conditions were satis- 

 factory. 



In order further to simplify the working of a float, 

 an igniter was devised which was actuated by coming 

 into contact with water. AU that was necessary was 

 to tear the cover off the igniter after it had been 

 inserted in the float, and then throw the float overboard. 

 The igniter contained a special chemical mixture which 

 ignited as soon as it came in contact with the water ; 

 this fired the thermite, which then started the combus- 

 tion of the smoke mixture. 



There is no doubt that this mixture was also the 

 most satisfactory for certain purposes on land, but 

 it was not used to any great extent, as the supply of 

 material was sufiicient only for naval purposes. 

 Successful efforts were made at Stratford to adapt 

 it for use in Stokes bombs, and had the war continued 

 it might have found extended appUcation in that 

 and other directions. 



The last smoke-producing substance to which 

 reference will be made is chlorosulphonic acid. It is 

 well known that this substance fumes violently when 

 exposed to moist air, and the Germans made use of this 

 property in a stationary smoke apparatus for land 

 purposes. Crude chlorosulphonic acid, or a mLxture 

 of chlorosulphonic acid and oleum, was contained in 

 a drum suspended over a vessel containing quicklime. 

 When the drum was turned over, the chlorosulphonic 

 acid ran out through a hole on to the lime. The heat 

 of reaction between the lime and the chlorosulphonic 

 acid was sufficient to vaporize the excess of the latter, 

 which was thus driven into the air and gave a dense 

 smoke when the air was sufficiently moist. On a dry 

 day, when the air contained very little moisture, the 

 smoke was apt to be somewhat thin. 



When the production of smoke from chlorosulphonic 

 acid for naval purposes was discussed at Stratford, 

 it was pointed out that the German method involved 

 a considerable waste of smoke-producing material, 

 which was used up in combining with the lime. Since 

 there are numerous sources of heat on board ship, it 

 was suggested that they should be utilized for vapo- 

 rizing the chlorosulphonic acid and so forming smoke. 

 This suggestion was accepted by Commander Brock, 

 and after an extended series of experiments satisfactory 



