628 SCIENCE PROGRESS 



colourless product, the use of aluminium is remarkably advan- 

 tageous. The comparative weakness of aluminium at the high 

 temperatures which are customary in varnish-making was at 

 first considered to be a drawback to the use of the metal but in 

 practice it has been found that this is more than offset by the 

 fact that aluminium is unaffected by furnace gases. The sulphur 

 compounds of aluminium are only formed with great difficulty 

 and as it is the sulphur from the coke fires which plays havoc 

 with copper and iron varnish pots, this cause of deterioration is 

 inoperative in the case of aluminium. 



The second case to be examined under this heading embraces 

 industries such as those engaged in the manufacture of jams, 

 preserves, fruit and other essences, in which materials are used 

 which either contain acids or from which acids are easily pro- 

 duced. Jam boilers, for instance, use copper boilers. Although 

 the amount of copper dissolved by fresh fruit is insignificant, if 

 left uncleaned a jam pan will acquire a green coating of copper 

 salts in the course of an hour, owing to the attack of the copper 

 by the acids in conjunction with the oxygen of the air. This 

 difficulty obtains in a number of trades and in some cases, as, for 

 instance, the distillation of the higher fatty acids, it is necessary 

 to reject, owing to their bright green colour, the first runnings 

 from the condensers if the still has stood idle for a few hours. 

 Here again aluminium offers a definite advantage, combined of 

 course, in the case of materials destined for human consump- 

 tion, with the fact already referred to, that no poison can gain 

 an entry. 



There now remains to be considered the fact that a number 

 of chemicals which attack most metals vigorously dissolve 

 aluminium with extreme reluctance. The most striking instance 

 of this kind is indubitably that of nitric acid. This material, 

 generally considered to be the most corrosive of chemicals, 

 attacks aluminium so slowly that pipes, tanks, hoods and even 

 pumps of the metal are used on a large scale both in the manu- 

 facture of the acid and in the manufacture of explosives in which 

 nitric acid is used. In the process which is now being worked 

 on a very large scale, whereby oxides of nitrogen are produced 

 from the air by electric discharges, the cooled gases are conveyed 

 through aluminium pipes to the towers in which nitric acid 

 is produced. 



Sulphurous acid and sulphuretted hydrogen are similarly 



