12 Dr. Thomson* s Answer [Jan. 



carbonate of chromium. Muriatic acid will readily dissolve the 

 protoxide of chromium. Filter the solution to get rid of the 

 charcoal, and evaporate it to dryness to get rid of the excess of 

 acid. If the muriate of chromium thus obtained be dissolved 

 in water and mixed with ammonia, protoxide of chromium will 

 be precipitated in abundance. If Dr. Ure will follow the simple 

 directions here given, I shall pronounce him a bungler indeed 

 if he cannot obtain protoxide of chromium in any quantity 

 whatever. 



9. If the reader will indulge me in one quotation more from 

 this most delectable morsel of criticism, I will not trouble him 

 again. But I am unwilling to withhold any thing which is 

 likely to display the uncommon candour, profound knowledge, 

 and gentlemanly feelings, of the worthy Andersonian Professor 

 of Glasgow. The passage I allude to is as follows : " His 

 number for tartrate of potash is unquestionably wrong ; and, 

 indeed, though it were right, his conclusion would be erroneous ; 

 for the mother-water (as he elegantly terms the limpid superna- 

 tant liquid) contains, under his proportions, both tartaric acid 

 and oxide of lead. Let it be tested with sulphate of soda, and 

 it will become cloudy ; with sulphuretted hydrogen, and it will 

 become very black; or with nitrate of lead, and tartrate of lead 

 will fall. Thus the principle of Richter, of whose apphcation 

 our Doctor is so vain, becomes under his management quite 

 deceptious." (Review, p. 138.) 



As I consider the method which I took to determine the 

 composition of tartrate of potash to merit the attention of prac- 

 tical chemists, and as I made no allusion to the principles on 

 which this method is founded in my late work, I shall take the 

 present opportunity to point them out. There are some acids 

 which cannot be precipitated completely from sahne solutions 

 by any reagent in our possession. Tartaric acid is in this predi- 

 cament. Tartrate of lime is the most insoluble of all the tar- 

 trates, but the salts of lime do not precipitate this acid, unless 

 we evaporate the mixture to dryness ; but tartaric acid is so 

 liable to be altered by heat, that this method is not quite safe. 

 Nitrate of lead precipitates tartaric acid immediately ; but tar- 

 trate of lead is not insoluble. Of course when nitrate of lead 

 and tartrate of potash are mixed in the atomic proportions after 

 the precipitate has subsided, a little tartrate of lead will remain 

 in solution. I take care to concentrate this liquid so much that 

 the tartrate of lead which it retains can amount only to a small 

 fraction of a grain. I then test it with nitrate of lead and tar- 

 trate of potash — neither of which salts are capable of acting 

 upon the tartrate of lead present ; but if either lead or tartaric 

 acid exist not in combination with each other, these reagents 

 will indicate its presence. By this method we have it in our 

 power to deterwi»e the quantity of tartaric acid iii a salt suffi-. 



