306 PHILOSOPHICAL TRANSACTIONS. [aNNO 17'28. 



those which had been only wetted at different times, cleansed better than those 

 which were steeped; the rust covering all the surface of the latter without 

 raising the scale; whereas in the former, as soon as one part of the metal is 

 detached, it is attracted by the menstruum, and the surface is raised into 

 blisters of rust. 



These dissolvents, the author observes, though weak in themselves, yet 

 produce the effect as well as the stronger, which are much dearer : but among 

 the latter he prefers vinegar, which being very plentiful in France, may be 

 used with little cost. For you need only dip eacli leaf into it, and take it out 

 again immediately, leaving it afterwards in some moist place, and it will be 

 scaled in 48 hours, if care be taken to repeat this 3 or 4 times in a day. The 

 scaling will still be more expeditious, if you dissolve a little sal-ammoniac in 

 the vinegar, a pound or two to a puncheon ; for as the vinegar dissolves iron 

 well, so sal-ammoniac rusts it sooner than any other salt : but this must be 

 used very moderately, and the leaf must be left to steep in clean water, to 

 dissolve any particles of it that may stick to its surface, which may other- 

 wise make it rust after it is tinned. If you scale with vinegar, and want to do 

 it at a less expence, you need only plunge the leaves once or twice at far- 

 thest, and when the vinegar is dried on the surface, sprinkle it with water ; or 

 dip them into it, and take them out immediately. 



There are several other ways of making iron rust ; as, keeping it in a moist 

 cellar, exposing it to the dew, sprinkling it with simple water, several times in 

 a day, which will still act quicker by dissolving sal-ammoniac in it. In those 

 countries where the pyrites is common, the vitriolic waters will scale them 

 soon enough, which are almost as cheap as common water: you need only 

 heap the pyrites together, and leaving them to moulder in the air, make after- 

 wards a lixivium with them and common water, which lie will have the desired 

 effect. 



But as the leaves of iron are sensibly much easier cleansed on one side than 

 the other, the bad side rarely taking the brilliant polish in the tinning, but 

 having always some spots, which happens because in the beating one side is 

 more exyjosed to the action of the hammer, and is therefore better plained, 

 the author again advises not to steep them, but only to moisten them, in order 

 to make them rust, moistening that side only that wants it most : whereas if 

 you steep them, as the bad side will take double or triple the time of the other, 

 the acid menstruum will dissolve the surface, and occasion a loss of iron. 



He next gives two cautions necessary to be followed; the first is in the 

 management of the plates before they come to be prepared; which is in the 

 beating of them, to change the place of each in its turn, that every one may 



