VOL. LXXVII.3 PHILOSOPHICAL TRANSACTIONS. I7l 



In performing the experiments on the magnetism of brass, I generally used a 

 magnetic needle suspended in a peculiar manner, as it is described in my last 

 lecture ; viz. a common sewing needle, or a piece of steel wire rendered mag- 

 netic, and suspended by means of a chain of hair ; which sort of suspension I 

 find not only from the experiments then made, but also by several subsequent 

 trials, to be the nimblest hitherto contrived ; because some substances which 

 seem to be quite destitute of magnetism, by not attracting any of the magnetic 

 needles otherwise suspended, will sensibly affect this. However, notwithstand- 

 ing the nicety of this method for discovering a very low degree of magnetic 

 attraction, it was found still inferior to that of exploring substances floating on 

 the surface of quicksilver, as used by M. Brugman. It seemed therefore neces- 

 sary to repeat some of those experiments on brass, and also on platina, by ex- 

 amining their magnetism by this means, viz. by putting the pieces of brass or 

 grains of platina on the surface of quicksilver, and then presenting a strong 

 magnet near them. The result of those experiments was, that very seldom a piece of 

 brass, or grain of platina, occurred, which was not affected by the magnet ; and 

 even when they were not affected by it, their indifference, as it may be expressed, 

 was not very clear and decisive ; and indeed there are very few substances in na- 

 ture which, when examined by this means, are not in some degree attracted by 

 the magnet, so general is the dispersion of iron, or such is the tendency which 

 most bodies have towards the magnet. 



Such brass which in the former experiments appeared to have no magnetism 

 naturally, nor to accquire any by hammering, was now found to be mostly mag- 

 netic, though in so very small a degree as to be discoverable only when floating 

 on quicksilver. The same was the case with the grains of platina before they 

 were hammered ; but after hammering, their attraction towards the magnet 

 became more evident ; whereas those pieces of brass, which naturally had not 

 any degree of magnetism sufficient to affect the needle, nor acquired any by ham- 

 mering, but yet showed some tendency towards the magnet when floating on 

 quicksilver, never, or very seldom, had that tendency increased by hammering. 



As in the course of those experiments it naturally occurred to observe several 

 particulars, which may be of use to those persons who wish to repeat these expe- 

 riments, I shall now subjoin the principal of them. It is necessary first of all 

 to observe, that the vessel in which the quicksilver is put, for the purpose of 

 examining the magnetism of divers bodies, must be at least 6 inches in dia- 

 meter ; otherwise the substances that are set to float on the mercury, will be 

 continually running towards the sides of the vessel, on account of the curvature 

 of the surface of that metal, which in narrow vessels begins at a greater distance 

 from the edge, than in vessels of a larger diameter. 



It is also necessary to keep the quicksilver very clean and very pure ; the want 



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