Manchester Memoirs, Vol. xlviii. (1904), No. 9. 3 



hydrate which will spoil the delicacy of the reaction. The 

 presence of small quantities of hydrochloric and nitric 

 acids does not mar the reaction, neither is it affected by 

 the presence of the sulphates of metals other than those 

 of the noble metals and of bismuth. Its use as a reagent 

 in group II. is thus precluded. 



Apart from the fact that the reaction which I have 

 referred to is the only one (as far as I have been able to 

 ascertain)* in which metallic copper is precipitated from 

 the aqueous solution of one of its salts by a soluble 

 reducing agent, I think I may claim that owing to its 

 great delicacy it will be found of some practical use, 

 especially as a " demonstratio ad oculos " of the presence 

 of copper. 



The limit at which the reaction is visible is, as far as 

 dilution is concerned, one part of copper in one million of 

 water, using a cylinder of 3in. diameter. As regards the limit 

 in actual weight of metal, this will, of course, depend upon 

 the smallest volume of liquid that it may be convenient to 

 employ, but taking 5 cc. as a basis, the weight of copper 

 which is still clearly discernible is 00000 1 gramme (equal 

 to O'OOi per cent, on the gramme standard). In using 

 the reaction for the detection of copper in dyed textile 

 fabrics, from one to five grammes of the material are 

 incinerated in a small porcelain dish, the ash taken up by 

 dilute nitric acid, and evaporated down on the waterbath 

 with the addition of a few drops of dilute sulphuric acid. 

 The residue is then taken up in a minimum of water and 



*Since writing this paper, I find that J. Meyer, in describing some of the 

 properties of hydrosulphurous ^ci^(Zeitschrift fu7- anorg. Chem. 1903, p. 43) 

 mentions that this substance will precipitate metallic copper from dilute acid 

 solutions of its salts. I have repeated this experiment with the commercial 

 " Hydrosulphite N F" of Meister, Lucius & Briining (a double compound of 

 hydrosulphite and formaldehyde) and find that a precipitate is obtained 

 which resembles that produced by titanous sulphate, but the reaction is not 

 nearly so delicate. 



