406 Allen, Crenshaw, and Merwin — 



them were regular. Of course, ideal conditions of formation 

 would include precipitation at constant temperature and 

 constant acidity, but even though these can not as yet be com- 

 pletely controlled, we have been able to determine the direc- 

 tion and regularity of the influence of each factor on the crystal 

 form of the product. To keep the acidity constant would be 

 a well-nigh impossible task, for it will be borne in mind that 

 the formation of every molecule of sulphide is necessarily 

 accompanied by the generation of a molecule of acid, and to 

 keep the volume so great that the acidity would remain sensi- 

 bly constant while enough sulphide is obtained for experimental 

 purposes would be practically out of the question. 



(a) Method of plotting. — It is obvious that if the acid exerts 

 a specific influence on the composition of the product, the latter 

 should be changing from moment to moment so long as precip- 

 itation continues. Two different products formed from the 

 same initial acid could only contain the same percentage of the 

 two crystalline forms when the weight of each product was the 

 same. If the latter varied by a chance variation in the quantity 

 of hydrogen sulphide, or by a variation in the time of reaction, 

 the composition of the sulphide would also change. The 

 abscissa used in the figure is therefore the average acidity 

 (fig. 1). The results are tabulated in Table IV. 



\b) Significance of the curves. — The curves show clearly that 

 the greater the acidity up to a certain point, the greater is the 

 percentage of marcasite formed at either temperature ; and the 

 higher the temperature for a given acidity the greater is the 

 percentage of pyrite formed. 



The sensibility of the product to the influence of acid is 

 extraordinary ; even a difference of 0*1 per cent in the average 

 acid concentration makes a difference of about 2*5 per cent of 

 pyrite. The decrease in the acidity with falling temperature 

 required in the formation of pure marcasite is also notable, 

 and, when taken together with the results at the lowest 

 temperature, 25°, this fact throws interesting light on the 

 natural genesis of the mineral. The highest percentage of 

 marcasite we have been able to get at 300° from sulphuric 

 acid solution is about 80 per cent; at 200° the quantity 

 increases to 95 percent — the same percentage as was obtained 

 at 25° and at 300° from hydrochloric acid solutions. 



(c) Irregularity at high acid concentrations. — The plot 

 shows further that when a product is formed in an acid of 

 more than about 225 per cent average acidity, there is a 

 sudden and somewhat irregular increase in the percentage of 

 pyrite. We supposed at first that marcasite would be found 

 to change when heated in acids of this concentration and 

 higher, and experiment seemed to confirm this ; for pure 



, 



