2l8 



NATURE 



[August iS, 1910 



in the hollow of Indio Viejo, leads Mr. Marsters to urge a 

 thorough examination of the pampas between the valleys 

 of the Caraveli and the Chaparra. 



Bulletins 71 and 73, on water-supply for agricultural 

 districts, show the wide view taken of their responsibilities 

 by this energetic body of engineers. G. A. J. C. 



iV£TF PROCESS FOR PRODUCING PROTEC- 

 TIVE METALLIC COATINGS. 

 'T'HERE are various means of coating metals with other 

 metals, generally of a protective nature ; thus, for 

 example, electroplating, galvanising, and drawing the metal. 

 A new process of considerable interest, invented by M. U. 

 Schoop, of Zurich, was described in a paper read before 

 the Engineers and Architects Society of Zurich on .April 13, 

 and reproduced in Metallurgical and Chemical Engineer- 

 ing [vol. viii., p. 404). 



The chief point in the new process is that the metal in 

 the liquid condition is sprayed upon the surface to be 

 coated. The atomising of the metal may be brought about 

 in a variety of ways. The metal is melted in a closed 

 crucible, and is forced out of a nozzle by the pressure of 

 gases or vapours ; or the subdivision of the metal may be 

 brought about by causing two jets of metal to pass out of 

 separate nozzles, so thai they meet at a suitable angle. 

 These, being under pressure, break up into a fine spray, 

 which is carried forward and coats any object placed near 

 to the spray. 



Schoop Process for Prod 



of Metallic Coatings, 



The most satisfactory method, however, appears to be 

 to cause the subdivision of the metal by the use of com- 

 pressed gases. The metal which is to be employed for 

 coating purposes is melted in a closed crucible which has 

 a pipe at the bottom, ending in a nozzle with a 

 capillary opening. Another pipe enters the upper part of 

 the crucible, which is connected with a source of com- 

 pressed gas, the gas being pre-heated by passing through a 

 coil, d. Another tube is connected externally with the gas- 

 pressure tube and ends in a nozzle, which is situated below 

 the metal outlet. The compressed gas impinges with great 

 pressure on the fine stream of metal, which is also being 

 forced out under great pressure. The metal is thus very 

 finely atomised, and is blown against the article to be 

 ^.-oated. h. Various gases may be used to press the metal 

 out of the nozzle, for e.xample, an inert gas such as 

 nitrogen, and this may also be used for atomising ; but, on 

 the other hand, for atomising agent an oxidising gas may 

 be used. It has, for instance, been found possible to 

 produce coatings of lead peroxide of a firmly adhering 

 character. 



Different metals naturally behave differently, depending 

 upon their fluidity, melting point, oxidisability, and so 

 •on. The material of the crucible also depends upon the 

 metal which is to be melted in it. Thus for melting 

 aluminium and its alloys iron or steel crucibles cannot be 

 used. It appears, indeed, that under high pressures metals 

 alloy much more readily and quickly than under ordinary 

 atmospheric pressure. Obviously alloys may be melted and 

 sprayed, but the metals require to be mixed in their 

 eutectic proportions. 



NO. 2129, VOL. 84] 



What probably happens in the spray process is that 

 when the metal forced out under pressure leaves the 

 nozzle, it is met by the gas stream, also under pressure. 

 The stream of metal is thus torn up and converted into 

 a mist. The individual particles of the metal when sprayed 

 on to the surface to be coated lose their surface tension 

 and their globular form, and are pressed out on the 

 surface in the form of a circular film. These circular 

 films overlapping become automatically welded into a con- 

 tinuous homogeneous coating. Calculation shows that 

 with a pressure of 25 atmospheres the metallic particles 

 have a speed of 20 km. per second — i.e. twenty-five times 

 the speed of the bullet from the gun used in the German 

 Army. Owing to this enormous speed of the metal mist, 

 metallic surfaces of a surprisingly high density are pro- 

 duced. For example, the density of rolled tin was found 

 to be 7-47, and that of the sprayed tin 7-42. 



-A point of great interest is that the metallic mist is at 

 a comparatively low temperature, at about 40° C. to 60° C., 

 in spite of the fact that the molten metal as it is ejected 

 has a temperature of several hundred degrees, and also 

 the atomising gas. This rather surprising fact is prob- 

 ably due to the sudden reduction of tension of the gas, 

 whereby the metal mist, which presents a relatively very 

 great surface, is also cooled. This is, however, a very 

 great advantage, because it is possible to give a metal 

 coating to substances of low melting point or of high 

 inflammability, such as paper, wood, celluloid, wax, &c. 

 The hardness of the metal film is greater than the hard- 

 ness of metallic films produced by other 

 methods, e.g. casting or rolling. This 

 great hardness should be of advantage 

 in the manufacture of electrotypes and 

 cuts, in which hardness is an import- 

 ant feature. Microscopic examination 

 also shows that the surface is abso- 

 lutely homogeneous. 



In carrying: out the operation, the 

 metallic surface to be coated is first 

 freed from oxide or adhering dirt. In 

 many cases, especially when metals are 

 to be coated, it is found advantageous 

 to pre-heat the surface before spray- 

 ing on the metal. It is not, how- 

 ever, absolutely necessary to pre-heat ; 

 consequently, with a portable ap- 

 paratus, metallic structures such as 

 bridges, machinery, and so on, could 

 easily be coated, and it is claimed 

 that it would be no inore expensive 

 than painting. At to the applications, they are almost 

 numberless. It may be used for all cases in which 

 plaiting is at present employed. Paper may be coated 

 with metal so that it can be used for packing such 

 things as chocolate, tea, and so on, in place of tin foil. 

 The cases of balloons can be coated — an aluminium coating 

 I /30th mm. thick is sufficient to render it quite gas-tight. 



The wooden propellers of aeroplanes coated in this 

 manner are protected against the action of the weather, 

 and the air friction is considerably reduced. Wall-paper, 

 stage scenery, and textiles can all be coated with metals. 



Certainly the process is one of great interest : and one 

 point of particular note is, it is now possible to coat 

 metals with aluminium, which will be perhaps one of the 

 most useful applications of this new process. In this case 

 it will be necessary to employ a crucible the material, of 

 which will not alloy with the aluminium. 



THE GALITZIN SEISMOGRAPH. 

 ■P)R. SCHUSTER has recently presented to Eskdale- 

 -^ muir Observatory a pair of Galitzin seismographs, 

 and the instruments have been set up under the personal 

 direction of Prince Galitzin, member of the Imperial 

 .Academy, St. Petersburg. Similar instruments have 

 been in use in Russia for some time, with very remarkable 

 results, and they are rapidly being adopted by the principal 

 Continental observatories. 



The object secured by these instruments is that they 

 record, not merely the occurrence of an earthquake on a 

 large scale of magnification, but also give an exact repro- 



