94 Mr. Crum on Indigo. [Feb. 



salt, and carrying down no additional sulphuric acid of its own. 

 It is only in the amount of combined water, then, that any alte- 

 ration can possibly have taken place, and to ascertain whether 

 in this case there has been an abstraction, or an addition of 

 water to the indigo, it were only necessary to convert a given 

 weight of that substance into cerulin, and to weigh the product, 

 as M. de Saussure has done in the case of starch su°ar. There 

 are difficulties, however, which prevent such an experiment 

 from being performed with any precision ; principally the large 

 quantity of sulphuric acid necessarily mixed with the product, 

 and the solubility of the substance in water. I have contented 

 myself with analyzing, by means of peroxide of copper, the 

 ceruleo-sulphate of potash, after having ascertained as nearly as 

 possible, by incineration, how much saline matter it contained. 

 But, as this cannot be done with absolute precision, owing to the 

 dissipation of a small portion of acid along with the vegetable 

 matter, my results, particularly with regard to the hydrogen, 

 were by no means so uniform as those which I obtained when 

 operating upon indigo. All that is really necessary in this case, 

 is to determine the quantity of carbon, which may be done with 

 very little risk of error. The deficiency, after adding to the 

 carbon the proportion of azote, and of free hydrogen found in 

 indigo, is water. 



Accordingly, I first of all dried a quantity of the blue sub- 

 stance at the heat of 212° in vacuo, and having weighed it as 

 rapidly as possible, I exposed it to the air during a whole night, 

 and noted the increase of weight from moisture. One portion of 

 this was burnt to ashes. Another portion was ground with per- 

 oxide of copper, and the carbonic acid it produced was received 

 over mercury. In this manner I found that 1 grain of pure 

 cerulin yielded 4*5 cubic inches dry carbonic acid gas, which is 

 equivalent to 05718 grain of carbon, reckoning 0*12708 grain as 

 the quantity of carbon in a cubic inch of gas. The composition 

 Of the substance consequently is : 



Carbon 57-18 



Azote 8-79 



Oxygen 29-32 



Hydrogen 4-71 



100-00 



This approaches so near to indigo + 4 water, that there can be 

 little doubt such is its constitution. 



1 atom azote 1*75 or 8-43 



6 atoms oxygen 6-00 28-92 



8 atoms hydrogen 1-00 4-82 



16 atoms carbon 12-00 57-83 



20-75 100-00 



