Lifluence of Copper, SfC. on Magnetic Needles. 317 



■ The hard crystals when separated are found to be efflores- 

 cent, like those of the usual kind ; and they ultimately give off 

 all their water, leaving only dry sulphate of soda. When a 

 given weight was heated in a platina crucible, one half their 

 M'eight passed off as water, the rest being dry salt. They, 

 consequently, contain eight proportionals of water, or 72 sul- 

 phate of soda, and 8 x 9 = 72 water. The usual crystals of 

 sulphate of soda contain 10 proportionals of water. 



When crystallized sulphate of soda is heated in a flask, a 

 part of it dissolves in the water present, whilst the rest is thrown 

 down in an anhydrous state. The solution at 180° appears to 

 contain one proportional of salt 72, and 18 proportionals of 

 water 162 ; from which, if correct, it would result, that when 

 the crystals are heated to 180°, 4 of the salt take all the water, 

 whilst I separate in the dry state.— Q,uarterlij Journal. 



INFLUENCE OF COPPER, ETC., ON MAGNETIC NEEDLES. 



M. Arago communicated to the Academy of Sciences his 

 experinents relative to the oscillations of a magnetic needle 

 surrounded by different substances. He had ascertained that 

 the copper rings with which dipping-needles are generally 

 surrounded exerted on the needles a very singular action, the 

 effect of which was rapidly to diminish the amplitude of the os- 

 cillations without sensibly altering their duration. Thus when 

 a horizontal needle suspended in a ring of wood by a thread 

 without tension was moved 45° from its natural position, and 

 left to itself, it made 145 oscillations before the amplitude was 

 reduced to 10°. In a ring of copper, the amplitude diminished 

 so rapidly that the same needle, removed 45° from its natural 

 position, only oscillated 33 times before the arc was reduced to 

 10°. In another ring of copper, of less weight, the number of 

 oscillations between the arcs of 45° and 10° were (56. The time 

 of the oscillations appeared to be the same in all the rings. 

 In the ring of wood 145 oscillations from 45° to 10° 



copper 33 45 10 



In a lighter copper ring 66 'I'S 10* 



. Rev. Ency. 



MEDICINAL LEECHES. 



A report has lately been laid before the French Academy 

 of Sciences, by MM. Uumcril and Latreille, on a memoir by 

 MM. Pelletier and Huzard, jun., containing researches upon 

 leeches. . . 



« The authors of this memoir had been commissioned to 

 obtain information for the civil authorities relative to the 

 means of putting an end to llie cumphiints which are often 



made 



