376 Account of some remarkable Facts observed in the 



tion, the increase of the forementioned velocity is perfectly ac- 

 counted for. There is, therefore, no reason to suppose that the 

 duration of the day is not sensibly constant. 



Let us for a moment admit, with M. de Laplace, that this du- 

 ration surpasses at present that of the time of Hipparchus, by 

 the hundredth of a decimal second. The duration of a century 

 now, or of 36,525 solar days, would" be longer than the duration 

 of a century 2,000 years aii^o, (Hipparchus lived about 1 20 years 

 before our aera,) by 365/'25. In this interval of time, the moon 

 describes an arc of 534'''.6 ; this quantity, therefore, expresses 

 the difference between two arcs traversed by the moon in a cen- 

 tury now, and in one of the time of Hipparchus; but as these 

 arcs, determined by observation and corrected by the secular 

 equation, do not differ by a quantity so large, we may conclude 

 that in this long interval the duration of the day has not varied 

 one hundredth of a second. 



LXIV. Accoujit of some remarkable Facts observed in the 

 Deoxidation of Metals, particularly Silver and Copper. Btj 

 Samuel Lucas, Esq.* 



W Sheffield, May 31, 1815. 



HEN I had the pleasure of seeing you in Man- 

 chester, I mentioned having observed that pure silver, when 

 melted, and while in a fluid state, had the property of uniting 

 •with a small proportion of oxygen, not only from the atmosphere, 

 but also from other bodies which gave it out at a suitable degree 

 of heat, as some of the nitrates for instance ; and that the oxy- 

 gen thus absorbed remains united with the silver only so long as 

 it continues in a fluid state, or, while fluid, until some substance 

 be applied having a more powerful attraction for the oxygen. 

 In proof of this, I now send, for your inspection, a few specimens 

 of silver that has been in the different states, and which carry the 

 external marks ; and also a bottle of the gas collected from silver, 

 which had been exposed to the influence of the atmosphere by 

 cupellation. 



If silver in large quantities, after having been exposed in a 

 melted state to a current of oxygen gas or atmospheric air, be 

 allowed gradually lo cool, the surface first becomes fixed or so- 

 lid ; this soon bursts, ebullition ensues, and an elastic vapour in 

 considerable quantity escapes, driving before it a portion of the 

 internal fluid metal, which, becoming solid as it is brought to 

 the surface, produces the protuberances as shown by the accom- 

 panying specimen No. 1. This ebullition continues from ;J to | 



• From Manchester Memoirs, vol. iii. 



an 



