Mr. J. Dalton on the Nature and Properties of Indigo. 127 



precision to show the real quantity of oxymuriatic acid in so- 

 lutions. 



For example : I took a graduated tube of capacity equal to 

 300 grains of water ; I filled it with pure nitrous gas, and 

 then transferred it to a cup of the liquid oxymuriate, valued 

 above by the sulphate of iron. After repeated agitation, co- 

 vering the end of the tube carefully with my finger, I soon 

 had 100 measures of liquid in the tube : then withdrawing it to 

 a cup of water I agitated repeatedly, letting in water each time, 

 instead of oxymuriate of lime, because I was aware that the 

 100 measures already in the tube were not saturated. Soon 

 after, the process was at an end, no more nitrous gas being 

 absorbed. The 100 measures of the oxymuriate took 168 

 measures of nitrous gas to saturate them. Now deducting one- 

 sixteenth of this for the nitrous gas impregnating the liquid, 

 and for loss occasioned by the free oxygen gas in the water 

 which the nitrous gas had to combine with, there will remain 

 157 nitrous gas =*2 grain weight, which was converted into 

 nitric acid ; but if we deduct one-eighth part from the weight 

 of nitrous gas, we shall have the weight of oxygen requisite 

 to convert it into nitric acid ="175 parts of a grain; only 

 differing y^^y from the other valuation by sulphate of iron. 



To find the value of any sample of indigo, I take one grain 

 carefully weighed from a mass finely pulverized. I put this 

 into a small glass, a wine-glass for example ; then by a drop- 

 ping tube I put two or three grains of concentrated sulphuric 

 acid upon it. The two principles are next well mixed to- 

 gether by trituration with the end of a small glass rod. Water 

 is then poured in, and the colouring matter fully diffused 

 through it. The liquid is now transferred into a tall cylin- 

 drical jar, of about one inch internal diameter; more water is 

 poured in till the mixture becomes sufficiently dilute to show 

 the figure of the flame of a candle through it. Then the li- 

 quid oxymuriate is mixed with the Tujuid gradually and by 

 measure, agitating duly each time, and never putting any more 

 in till the smell of the preceding has vanished. The liquid 

 soon becomes transparent and of a beautiful greenish-yellow 

 appearance ; aftc)- the dross has subsided, the clear liquid may 

 be ))()ured offj and a little more water put to the sediment, 

 with a few drops of oxynuniate of lime, and a drop of dilute 

 sulphuric acid; if more yellow liquid is produced, it arises 

 from particles of indigo winch have escaped the action of the 

 oxymuriate before, and must be added to the Test. 



The value of the indigo 1 consider in j)ro[)ortion to the 

 quantity of real oxynuu'iate of lime necessary to destroy its 

 colour. The vahie also may be well cstimaled by the quantity 



and 



