120 RESEARCHES UPON ATOMIC WEIGHTS. 



rium. It is economical, however, to wait until crystallization has ceased, since 

 the solubility of the acid at room temperature is so high, a saturated solution 

 at 20° containing over 75 per cent of acid. The first crop of crystals was 

 drained as completely as possible, usually with centrifugal drainage in a plat- 

 inum centrifugal machine,^ and the product was twice recrystallized. On ac- 

 count of the high solubihty of the acid and the low temperature coefficient of 

 solubility between 0° and 100° the yields of acid were very small, hence the 

 mother-liquors were worked up for several successive yields of crystals. The 

 thrice recrystallized acid was preserved in a desiccator containing solid caustic 

 potash. It was designated Sample I. 



The solutions of iodic acid, as well as the solid substance and even iodine pent- 

 oxide made from the acid, possessed to a slight degree a peculiar odor which 

 may be described as "aromatic." This odor persisted through the crystaUiza- 

 tion although with continual diminution, but was not entirely absent from acid 

 which had been crystallized three times. It was later proved that the odor is 

 not due to the acid itself because after a sufficient number of crystallizations 

 the odor disappears. Furthermore, it was experimentally proved that no ap- 

 preciable amount of iodine was contained in this vapor, for when a concen- 

 trated solution of the acid is boiled no iodine can be detected in the distillate, 

 and when a current of air is conducted over the solid at ordinary temperatures 

 into a solution of sulphur dioxide no iodine can be found in the sulphurous acid 

 solution. Even at high temperatures the pentoxide loses in weight very slowly, 

 10 gm. losing only i mg. at 240° in 4 hours. By far the greater part of this loss 

 is due to decomposition of the pentoxide, for free iodine can be easily detected in 

 the air which has passed over the substance at high temperatures. 



The possibility of the presence of metallic impurity in the iodic acid, arising 

 from the action of iodine on the glass of the flask in which it was condensed, 

 was tested by evaporating to dryness the mother-liquor of the last crystalliza- 

 tion of the original solution and heating the residue in a weighed platinum boat 

 in a current of pure dry air until the pentoxide was completely decomposed. 

 The boat gained in weight only 0.0006 gm., part of this gain being due to the 

 formation of platinous iodide. Faint tests for sodium and calcium only could 

 be obtained with the residue in the spectroscope. 



SILVER. 



Silver was purified by methods already found in this laboratory to be effective 

 for the purpose (page 22). A dilute solution of silver nitrate was first precipi- 

 tated with hydrochloric acid in excess, and the precipitate was thoroughly 

 washed with pure water. The chloride was next reduced by means of invert 

 sugar and an excess of sodium hydroxide, and the metal, after long-continued 

 washing by decantation, was fused upon charcoal. After the buttons had been 



* Baxter: Jour. Amer. Chem. Soc, 30, 286 (1908). 



