49* Mr. Hume on the Detection of Arsenic. 



i. month or five weeks with once winding up ; but, to pr?- 

 vent the weight's having any effect upon the pendulum, it 

 is wound up every fortnight. 



This effect, however, is not produced by the cause that 

 has been supposed ; for the attraction between two bodies 

 of a few pounds weight is too minute to produce anv sensi- 

 ble effect upon each other, when one of them is in motion. 

 But the true cause will be easily understood if we reflect, 

 that when the weight has descended as low as the bob, the 

 space in which the pendulum then vibrates is very narrow, 

 and consequently it meets with greater resistance from the 

 air, than when the weight is either above or below that 

 place. 



The truth of this hypothesis may be easily proved thus ; 

 When the weight has descended as low as the centre of 

 oscillation of the pendulum, let the clock-case door stand 

 open, and in less than half an hour the pendulum will vi- 

 brate in its usual arc, if not in a greater. 



The cause of this increase of the arc is evident; for, thq 

 air being less confined, the pendulum meets with less re- 

 sistance. 



I am, sir. 



Your obedient servant, 



Lynn, Oct. 9, 1812. , E. WalKER. 



L. Mr. Hume on the Detection of Arsenic. 

 To Mr. Tilloch. 



Sir, In the three papers, which I quoted in my last letter to 

 you, 1 gave no specific process for detecting arsenic by means 

 of the less soluble alkalies, the earths, — at least by those 

 generally known bv the term, alkaline earths ; I shall there- 

 tore beg you will indulge me by inserting the following 

 short experiments, in the first number of your excellent 

 Miscellany. 



Experiment 1. — One grain of white oxide of arsenic 

 was boiled in ten ounces by measure of distilled water, to- 

 gether with a few grains of carbonate of magnesia. This 

 mixture, being filtered and allowed to cool, proved to be a 

 solution oi arsenite of magnesia; and was obedient to the 

 silver-test, either when applied in the form of simple nitrate 

 or of ammoniaco-niirate of silver. 



Experiment 2. —The same plan was practised with car- 

 bonate of lime in lieu of that of magnesia, and the result 

 was a pure arsenite of lime. 



Experiment 



