Processes in the Useful Arts. 347 



of the screws g. In the openings of each of these caps is placed a piece k, 

 with two points, and with the end of the screw fixed in the middle of it, 

 so that the plate hhh, may, by means of the screws, be made to apply 

 to any curve, and retained in that position so as to be transferred to an- 

 other substance. Bull, des Sc Technol Jan. 1825, p. 41. 



7. Method of Consuming the Smoke of Steam- tioiler Furnaces- By Mr G. 



Chapman. 

 As this contrivance has been found to answer the purpose of consuming 

 the smoke, it has been honoured by the London Society of Arts with the 

 large silver medal. Mr Chapman heats the air which promotes the com- 

 bustion of the smoke before it is admitted into the furnace, by making 

 the grate-bars hollow from end to end, and causing the cold air to pass 

 through them into two boxes, one in front of the grate, and the other be- 

 hind it. In the ordinary method of supplying the grate with fuel by the 

 front-door, about forty or fifty cubic feet of cold air is admitted into the 

 furnace. Mr Chapman, on the other hand, uses a cast-iron hopper from 

 which the coals are introduced in a moment without letting in any per- 

 ceptible quantity of cold air. When fresb coals are put in, the smoke 

 has at first the appearance of a light-gray vapour, but in a few seconds it 

 becomes nearly invisible- A drawing and a fuller account of this fur- 

 nace will be found in the Transactions of the Society ofArtsfbr 1824, vol- 

 xlii. p. 32. 



8. Menstruum for Biting in on Steel Plates for Fine Engravings. By Mr 



Edmund Turrell. 



This useful process, for which Mr Turrel has received the large 

 gold medal of the London Society of Arts, has been the result of a great 

 number of well-directed experiments, made for the purpose of discovering 

 a menstruum, which should corrode the steel with great facility, while it 

 produced a beautiful, clear, and deep line. The following is the com- 

 pound fluid which he found the most perfect. 



Take four parts by measure of the strongest pyroligneous, (chemically 

 called acetic acid, J and one part of alcohol or highly rectified spirits of wine ; 

 mix these together, and agitate them gently for about half a minute, then 

 add one part of pure nitric acid ; and when the whole are thoroughly 

 mixed, the menstruum is fit to be poured upon the etched steel plate. 



This fluid holds the oxide of the metal in perfect solution, so that the 

 whole of the lines appear beautifully bright, and continue so till the bit- 

 ing in is completed. Very light tints are corroded in about one minute 

 and a half, and a considerable degree of colour is produced in about fifteen 

 minutes. When this mixture is poured off the plate, it should be washed 

 with one part of alcohol, and four of water, and the stopping out should be 

 effected by asphaltus dissolved in essential oil of turpentine, so as to 

 flow freely from a hair pencil. In making the above menstruum, the 

 purity of the ingredients is a matter of great importance. 



The steel plates used for engraving may be saved from rust, by warm- 



