402 On the present Mode of finding 



precipitate from a given quantity of the arsenic ; the result in 

 all cases must, therefore, be nearer the truth, being more 

 evident to the senses. 



One experiment will sufficiently elucidate the plan I pur- 

 sue. Let one grain of white oxide of arsenic and the same 

 quantity of carbonate of soda be dissolved by boiling, in 10 

 or 12 ounces of distilled water, which ought to be done in 

 a glass vessel; to this let a small quantity of the nitrate of 

 silver be added, and a bright yellow precipitate will instantly 

 appear. This is a more decisive test than sulphate of cop- 

 per, which forms Scheele's green (arseniate of copper); and 

 though my process answers very well with potass or even 

 lime-water, yet I am inclined to prefer the common sub- 

 carbonate of soda. I remain, sir, your obedient servant, 



Jos. Hume. 



Long-Acre, 

 May 19, 1809. 



LXXI. On the present Mode of finding the Rates of 

 Timekeepers. By a Correspondent. 



vJf all the requisites by which a seaman is enabled to con- 

 duct his ship from one distant climate to another, no one 

 appears to be of greater importance than an accurate know- 

 ledge of the time. The parliament of Great Britain, aware 

 of its necessity, have, for the last century, offered a large 

 reward to any person who may contrive a machine, that will 

 keep time within certain limits of error during a long voyage. 

 So anxiously has this been desired by that part of the nation 

 who have any interest in its commercial or maritime con- 

 cerns, and so great has been the honour awaiting the per- 

 son who shall produce this desideratum, joined to the in- 

 citement naturally arising from the hope of obtaining a large 

 reward, that many ingenious watchmakers and mechanics, 

 both at home and abroad, have exhausted their utmost skill 

 in the endeavour to bring it to perfection. Although they 

 have hitherto found this impracticable, yet by the repeated 

 attempts, and successive improvements of various hands, 

 such an approximation to the truth has been attained, as 



reflects 



