OF ARTS AND SCIENCES. 31 



and should heat the tube with its oxidizing flame only. An iron cone 

 may be used for increased draught, but not an iron or glass cylinder 

 for the greater radius of heat given by the latter tends to throw the 

 mirror farther along the tube, and to deposit it irregularly. A con- 

 venient rest for the reduction tube is made by soldering three stout 

 copper wires to an old binding screw or post, curving the ends to fit 

 the tube and branching them out, so that the tube lies flat in the 

 curved ends. The binding screw travels vertically on a brass rod 

 melted into a flat, heavy piece of lead, or screwed into tlie base of an 

 old Bunsen burner. 



When the apparatus is found free from arsenic, the extract is added. 

 Previous to tliis it has been measured or weighed, preferably the 

 latter. The test tube having its weight marked on it, the weight 

 of the extract is quickly determined. Weighings can be made on a 

 balance sensitive to fifty milligrams, which is enough for all practical 

 purposes. 



A few drops of the extract are at first added. If no mirror appears 

 in three or four minutes, one eighth to one quarter of the rest may be 

 added, and if in five minutes more there is no mirror, the whole of the 

 extract may be introduced. This cautious addition is necessary in 

 order not to obtain too large a mirror, else a difficulty in estimation 

 might arise, or a new determination might have to be made. A 

 twenty-five minute run is sufficient for the deposition of all the arsenic 

 when the size of the mirror formed in the first fifteen minutes is not 

 larger than that corresponding to 0.05 mgr. of arsenious oxide. If 

 the mirror forming is likely to be larger than this, it is better, after 

 weighing, to start another mirror with another portion of the extract, 

 than to wait for the complete deposition of a mirror which may be 

 too large for comparison with the standards. 



The set of standard mirrors is made as follows. One gram of 

 arsenious oxide, purified by repeated sublimation, is dissolved with 

 the aid of a little sodic bicarbonate (free from arsenic), and, after 

 acidification with dilute sulphuric acid, is made up to a litre. Of 

 this standard solution (I.), containing 1 mgr. AS2O3 to 1 c. c, ten 

 c. c. are taken and made up to a litre, giving the standard solution 

 (II.) containing 0.01 mgr. to 1 c. c. Of this solution, 1 c. c, 2 c. c, 

 3 c. c, etc., are carefully measured from a burette and introduced into 

 the reduction flask of the apparatus, giving the mirrors corresponding 

 to the same number of hundredth-milligrams. It is necessary to 

 make two or more mirrors of the lower amounts, as, even with care- 

 ful drawing, the cross sections of the deposition tubes differ, so that 



