580 Proceedings of the Royal IrisJt Academy. 



that too great a dilution of the test solution "with the liquid under 

 examination should be avoided, as the blue colouration will not be 

 developed if water be in excess ; and even after it has been produced, 

 the addition of a certain proportion of that substance quickly causes 

 its disappearance. Such being the case, the best way of employing 

 the test, according to my experience, is to place three or four drops 

 of the molybdic solution in a small white porcelain capsule, and having 

 heated them slightly, allow one or two drops of the liquid to be 

 examined to glide or fall gently on the acid solution, when there will 

 be developed, either immediately or after a few moments, the blue 

 colouration. And where the alcohol is very largely diluted with 

 water, it is better to continue the gentle heating of the test solution 

 for some time, to concentrate it or expel as much water from it as pos- 

 sible, before adding the liquid to be tested, for in this way I have 

 succeeded in detecting the spirit in mixtures so dilute, as to give no 

 blue reaction when added immediately to the test solution on its being 

 simply wai'med. As regards the application of heat, I must observe 

 that the temperature of the acid solution must not be raised too high, 

 for if it be heated till the acid evolves its dense vapours, or begins to- 

 boil, the solution will of itself alone, from its partial decomposition, 

 develops a more or less blue colouration, which will become more per- 

 ceptible on its cooling. But such an occurrence can be easily avoided 

 by employing a water-bath as the heating agent ; for I have found 

 that a temperature of 212° F. is incapable of so acting on the test 

 solution — at least an exposure of several hours' duration to that heat 

 failed to produce the sliii,htest blue colouration, and a much lower 

 temperature than that suffices for the application of the test. 



I should here state that the molybdic or test solution which I have 

 generally employed was made by dissolving at a gentle heat one part 

 by weight of molybdic acid in ten parts of strong and pure sulphuric 

 acid, but the exact strength of this solution as regards the amount of 

 molybdic acid it contains seems to be immaterial. 



I may observe that the colouration produced in the reaction 

 stated disappears after a variable interval of exposure to the air— a 

 circumstance which is due, as I have ascertained, to the absorption of 

 moisture from the atmosphere, and not to the reoxidation of the 

 molybdenum compound, as might have been supposed; for amongst 

 other facts in proof of this, I may state that after it has thus disap- 

 peared, it may be readily restored either by expelling the water so 

 absorbed by a gentle heat ; or, more slowly, by placing the mixture 

 under a desiccator, and thus removing it by spontaneous evaporation 

 at the ordinary temperature. Such being the case, it is evident that, 

 where the test solution has been too much diluted for the immediate 

 development of the colouration described, expelling the excess of 

 water by heating the mixture on a water-bath, it may be made ta 

 exhibit itself. 



But the necessity for such evaporation should, if possible, be 

 avoided, which in most cases will be so, by using only a drop or two 



