96 Mr. M. Carey Lea on the 



stant stirring 20 cub. centim. of a ten per cent. ^ solution of 

 iodine. Turn off the heat, cover the beaker with a glass 

 plate, and allow it to cool slowly with the bath. Next day 

 throw the crystals on a filter and wash with 70 per pent, 

 alcohol. Press between blotting-paper and dry at ordinary 

 temperature. 



Herapathite appears to be a very permanent substance. 

 Some that I prepared over thirty years ago has been kept in 

 a stoppered, but not sealed, white glass bottle and freely 

 exposed to light during this time. It appears to be quite 

 unchanged, and was used in part of the work just described. 



In preparing the test solution time is gained by using the 

 barium carbonate in considerable excess and by frequent 

 shaking. The herapathite should be in fine powder. After 

 the green particles have entirely disappeared, which will 

 usually take a day or two, the solution must be filtered 

 off and be placed in contact with a fresh portion of barium 

 carbonate, with frequent shaking for several days. The 

 complete removal of the last traces of sulphuric acid cannot 

 be judged of, as might be supposed, by allowing a portion 

 to evaporate and examining for the crystals of herapathite. 

 A specimen may dry up to a yellow varnish and yet may 

 contain sulphuric acid, as may be shown by evaporating to 

 dryness with a little ammonia, exhausting the residue with 

 very dilute hydrochloric acid, and adding a drop of barium- 

 chloride solution. But there is a very much simpler and at 

 the same time more effective means of testing. A solution is 

 allowed to evaporate spontaneously after adding a drop or 

 two of solution of oxalic acid. Then, if there is the slightest 

 trace of herapathite present, it will show itself in extremely 

 characteristic crystals, either small rosettes of black prisms or 

 in thin, light brown, transparent plates, which are black when 

 crossed. A great many acids have this property of forcing 

 out a crystallization ; for example acetic, citric, tartaric, 

 malic, and even arsenic. On the other hand, hydrochloric, 

 hydrobromic, and nitric acids have not this power. 



The explanation appears to be this. Iodoquinia is a col- 

 loid, and when present in large excess is able to prevent the 

 crystallization of the relatively small proportion of herapathite 

 present. The addition of either of the first-named series of 

 acids appears to convert the colloid base into a crystalline 

 salt, which of course no longer interferes with the crystal- 

 lization of the herapathite. 



Method of applying the test. — After having ascertained in 

 the above-mentioned manner that the test solution is abso- 

 lutely free from herapathite, a few drops of the liquid to ba 



