126 Mr. J. Dalton on the Nature and Projjeitics (>/' Indigo. 



We now proceed to the consideration of the best means of 

 fixing a comparative value upon the different samples of the 

 indigo of commerce. After numerous trials I find the method 

 first suggested byDescroisille to judge of the strength of oxy- 

 muriatic acid to be preferred. The objects indeed are dif- 

 ferent, but the operations are analogous : he made use of a 

 given solution of indigo to ascertain the comparative strengths 

 of various solutions containing oxymuriatic acid ; on the other 

 hand, I propose to use a solution of oxymuiiatic acid of known 

 strength, to compare the relative quantities of pure indigo in 

 different samples. 



In the first volume of the Annals of Philosophy (1813) I 

 pomted out a safe and easy method of estimating the quantity 

 of oxymuriatic acid, in solutions of oxymuriate of lime, not by 

 solutions of indigo, which must be variable from the quality 

 of the indigo, but by solutions of proto-sulphate of iron, 

 which can always be obtained of the same strength. I say safe 

 and easy method, notwithstanding we are gravely told by one 

 professor of chemistry that he had tried the method and 

 was nearly killed by it; and another states that he had at- 

 tempted to follow my method, but did not succeed. Any person 

 who is tolerably skilled in chemical manipulations, and has 

 the two liquids, sulphate of iron of a known strength, and 

 oxymuriate of lime of a known specific gravity, before him, 

 may determine the strength of the oxymuriate in the space of 

 five minutes. In this time I found the strength of the oxy- 

 muriate of lime used on the present occasion. Having by me 

 a solution of proto-sulphate of iron containing eight per cent 

 oxide, I took 50 grain measures of it and poured them into a 

 wine glass, then 100 of the oxymuriate, stirring the mixture, 

 after which no smell was perceived; 100 more were poured 

 in, still no smell ; then dropped in ten grains at a time by a 

 dropping tube, stirring the mixture each time; the fifth 10 

 grains produced a slight but transient smell ; the sixth a 

 strong and permanent smell. Hence 250 were required to 

 saturate 50 of the sulphate. The oxide (four grains) being- 

 divided by nine, gives •44-4 for the weight of oxygen in 250 

 oxymuriate, or '17 parts of a grain of oxygen are imparted by 

 each hundred of the liquid. 



In the essay above referred to I mentioned another method 

 of investigating the value of oxymuriate of lime solution ; 

 but, owing to the then prevailing erroneous notion on the 

 proportion of the elements of nitric acid, no satisfactory ap- 

 plication could be made of it. I now find that oxymuriate of 

 lime converts nitrous gas into nitric acid immediately ; and 

 hence this operation may be used with great elegance and 



precision 



