272 Experiments on Indigo. 



And in a second experiment, conducted with greater care to 

 exclude external air, the result was 86 per cent, of carbonic 

 acid, and the residue contained too feeble a portion of oxygen 

 to explode with the electric spark on the addition of a requisite 

 proportion of hydrogen. 



In no case, therefore, does the extricated gas in practice 

 appear to be pure carbonic acid ; but from the prevalence of 

 the latter increasing with the precautions taken to exclude 

 common air, it may in fact be the only gas strictly due to the 

 fermentation properly so called, the remainder of common air 

 having been suspended in contact with the leaves, and being 

 deprived of a portion of its oxygen by the free carbon, or by 

 the liquid in its passage to the surface. No carburetted hydro- 

 gen or other combustible gas was found among the gaseous 

 products. 



The indigo manufacturer does not wait until the extrication 

 of gas is concluded, but withdraws the liquid from the steeping- 

 vat as soon as he considers it to be sufficiently fermented ; 

 judging either from the smell, from the greenish tint of the 

 liquor on the surface, or from the formation of an iridescent 

 scum on the bubbles of gas. In fact, when the liquid, which 

 is of itself of a bright yellow colour, begins to assume a greenish 

 tint, it is evidently caused by an incipient precipitation of the 

 blue colouring matter; and it would be attended with a loss of 

 produce, to allow this precipitation to take place in the steep- 

 ing- vat among the leaves and branches of the plant. 



"The length of the fermentation depends upon the tempera- 

 ture, the weather, the wind, the water employed, and the ripe- 

 ness of the plant ; it may last in common cases from seven to 

 fifteen hours. It is generally longer when the temperature is 

 high, the weather cloudy, but not rainy ; the wind eastward 

 and moderate, the plant ripe and fresh." 



Upon these several points, constant experience leaves little 

 for the experimentalist to advance ; but it may be remarked, 

 that almost all the criteria of a good fermentation, as far as the 

 weather is concerned, concur in one simple principle, — the 

 prevention of the water of the vat from being cooled ; for the 

 west wind, being dry, cools it by evaporation ; strong wind 

 does the same; clear sky cools it by radiation ; and rain, by 

 the low temperature of rain-water. Thermometers placed in 



