136 



CHEMISTRY. 



Blitia TarikesvillicB, and other indigo-yielding 

 plants, to see if they contained indigo-blue 

 ready formed, or if, like the common woad, 

 the coloring matter exists in the vegetable cells 

 in the form of indican or some other glucoside. 

 The experiments with the Polygonum tinctori- 

 um led to the conclusion that the leaves of 

 this plant contain a substance not to be distin- 

 guished from the indican of Isatis tinctoria, 

 which by decomposition with acids yields in- 

 digo-blue and glucose, accompanied by some 

 by-products ; and that there is no proof of 

 the existence of ready-formed coloring matter 

 in the plant while the latter is living and in a 

 healthy state ; and, further, that in all indigo- 

 yielding plants hitherto examined the coloring 

 matter is derived from a glucoside which splits 

 up with great ease into indigo-blue and glucose, 

 and that this glucoside is probably in all cases 

 the same, and identical with the indican of 

 Isatis tinctoria. 



Solubility of Solids in Gases. In an inves- 

 tigation undertaken for the purpose of study- 

 ing the conditions of liquid matter as it ap- 

 proaches and passes the "critical point" that 

 is, the point at which it changes from the liquid 

 to the gaseous state Messrs. J. B. Hannay and 

 James Hauston have obtained some very curi- 

 ous and interesting results, which appear to 

 show that solids may be dissolved in gases. 

 The experiments were conducted with such 

 solvents as alcohol, ether, carbon disulphide 

 and tetrachloride, paraffines, and olefines, 

 and such solids as sulphur, chlorides, bro- 

 mides, and iodides of the metals ; and such or- 

 ganic substances as chlorophyl and the ani- 

 line dyes were employed. In a preliminary 

 report, the authors explain that, in order to 

 gain some insight into the condition of matter 

 just beyond this critical point, they dissolved 

 in the liquid a solid substance fusing much 

 above the critical point of the liquid, and ob- 

 served whether, on the latter passing its criti- 

 cal point and assuming the gaseous condition, 

 the solid was precipitated or remained in solu- 

 tion. It was not precipitated, but continued 

 in solution, or rather in diffusion, through the 

 atmosphere of vapor, even when the temper- 

 ature was raised 130 above the critical point, 

 and the gas was considerably expanded. When 

 the side of a tube containing a strong gaseous 

 solution of a solid is approached by a red-hot 

 iron, the part next the source of heat becomes 

 coated with a crystalline deposit, which slow- 

 ly redissolves on allowing the local disturb- 

 ance of temperature to disappear. Earefaction 

 seems to be the cause of this deposition, be- 

 cause, if the temperature be raised equally, 

 and the volume retained at its original value, 

 no deposition takes place. 



An attempt to examine the spectroscopic 

 appearances of solutions of solids, when their 

 liquid menstrua were passing to the gaseous 

 state, was unsuccessful in most cases, owing to 

 the fact that the substances capable of assuming 

 both states gave banded spectra with nebulous 



edges. At the suggestion of Professor Stokes, 

 the substance obtained by the decomposition 

 of the green coloring matter of leaves by acids 

 was tried, and this, which yields a very fine 

 absorption spectrum, was found to exhibit the 

 phenomenon in a marked manner whether dis- 

 solved in alcohol or ether. This compound is 

 easily decomposed by heat under ordinary cir- 

 cumstances, and yet it can be dissolved in gas- 

 eous menstrua and raised to a temperature of 

 850 without suffering any decomposition, 

 showing the same absorption spectrum at that 

 heat as at 15. 



When the solid is precipitated by suddenly 

 reducing the pressure, it is crystalline, and may 

 be brought down as a " snow " in the gas, or 

 as a " frost " on the glass, but it is always easily 

 redissolved by the gas on increasing the pres- 

 sure. These phenomena are shown to the best 

 advantage in a solution of potassic iodide in 

 absolute alcohol. "We have then," say the 

 authors, " the phenomenon of a solid with no 

 measurable gaseous pressure dissolving in a 

 gas, and not being affected by the passage of 

 its menstruum through the critical point to the 

 liquid state, showing it to be a true case of 

 gaseous solution of a solid." 



Determination of the Organic Purity of 

 Potable Waters. In a long and valuable pa- 

 per on this subject, read before the London 

 Chemical Society, Mr. 0. M. Lidy, after an ex- 

 tended discussion of the various methods which 

 have been proposed for estimating the organic 

 matter in potable waters, gives the preference 

 to a special modification of the permanganate 

 process, to which he has applied the name of 

 " the oxygen process." This consists in add- 

 ing to a known volume of the water measured 

 quantities first of dilute sulphuric acid, and 

 then of permanganate solution. The author 

 takes two equal portions of the same water, 

 adds equal volumes of permanganate and sul- 

 phuric acid to each, and allows the one to 

 stand one and the other three hours. The 

 amount of permanganate remaining is then 

 measured by means of potassic iodide and so- 

 dic hyposulphite. The results are expressed 

 in the quantities of oxygen required to oxidize 

 the organic matters in one gallon of water. 

 The relation of the result obtained in one 

 hour's to that in three hours' time permits a 

 conclusion as to the nature of the organic 

 matter, i. e., its susceptibility to oxidation, or 

 of the relation between the readily oxidizable 

 and putrescent, and the less easily oxidizable 

 or non-putrescent, matters. The paper closes 

 with certain important conclusions, which are 

 summed up, as follows : 



The ammonia process furnishes results which are 

 marked by singular inconstancy, and are not delicate 

 enough to allow the recognition and classification of 

 the finer grades of purity or impurity. The errors in- 

 cidental to the process form an array of difficulties 

 which become infinitely serious, seeing that the range 

 (from O05 to O 1 ! part per million) between pure and 

 dirty waters is comparatively so small. The combus- 

 tion process has all the evils of evaporation to encoun- 



