Intelligence and Miscellaneous Articles. 317 



test of the value of commercial iodide of potassium, assuming the 

 salt of which we are now speaking as a standard of comparison. 

 How far it may be depended upon as a test, will appear from what 

 follows. 



If tartaric acid in solution be added to a solution of pure iodide 

 of potassium, the commixed solutions are at first colourless, but 

 quickly become slightly yellow, owing to the action of atmospheric 

 oxygen on the hydriodic acid which is thus generated. 



On making this experiment with the salt in question, I found, 

 to my great astonishment, that free iodine, in quantity, was in- 

 stantly developed. I was at first at a loss to account for so great a 

 difference in the behaviour of this salt to that which I had prepared 

 myself, and knew to be pure iodide of potassium ; but from the ap- 

 pearance of the crystals of this salt, and from the circumstance of 

 its not being soluble in water to the extent that it should be, I 

 suspected the existence of iodate of potash in it, and I have since 

 convinced myself of the fact of its presence. 



I find, if we add tartaric acid solution to a solution of iodate of 

 potash, no change of colour takes place, but that bitartrate of potash 

 is deposited in abundance, and, as a matter of course, iodic acid set 

 at liberty : this solution instantly decomposes iodide of potassium in 

 solution, giving rise to free iodine in great abundance ; or, if we 

 add a drop of solution of tartaric acid to a solution of pure iodide 

 of potassium, to which even a minute quantity of iodate of potash 

 has been added, free iodine is instantly developed. 



Tartaric acid appears, then, from the experiments I have made, 

 to be a very delicate test of the presence of iodate of potash in 

 iodide of potassium, and will be found a very ready and useful one 

 for this purpose in the hands of the dispensing chemist, showing 

 him that any specimen of this salt in which free iodine is thus de- 

 veloped, is actually of less value than one in which no trace of 

 iodine appears on the instant of its application ; inasmuch as iodide 

 of potassium, in a given weight, includes more iodine than iodate 

 of potash does ; as is seen at once by the atomic composition of 

 these two salts. 



It is well known to every chemist, that one of the methods very 

 commonly resorted to for the production of iodide of potassium is 

 that of acting upon iodine with potash water. In this way we form 

 iodate of potash at the same time ; six atoms of potash and six 

 atoms of iodine giving birth to five atoms of iodide of potassium, 

 and one atom of iodate of potash ; which latter, if suffered to re- 

 main mixed with the iodide, would increase the produce of the 

 manufactured salt nearly five per cent., at the expense of its purity 

 and crystalline beauty. Lancet, Aug. 29, 1840. 



ON PEPSIN THE PRINCIPLE OF DIGESTION. 



M. Wasmann has succeeded in isolating pepsin, the peculiar prin- 

 ciple of the gastric juice, described by M. Schwann, in the follow- 

 ing manner : 



The glandular membrane of the stomach is to be separated with- 

 out cutting it ; it is to be washed and digested in distilled water at 



