M. Carey Lea — Allotropic Farms of Silver. 477 



chemists engaged in it. Almost all the views advanced have 

 been successively disproved by each subsequent publication. 

 It follows that what has obtained a place in the text books is 

 almost wholly incorrect. 



The earliest experimental work was Faraday's, but his 

 product has been proved to be a mixture.* The next was the 

 well known paper of Wohler published in 1839. It is not my 

 purpose here to enter npon a criticism of this memoir. If 

 this illustrious chemist succeeded in obtaining by the means 

 employed a true citrate of silver hemioxide, — as would appear 

 from his analyses — no chemist since his time seems to have 

 done so. The next publication to Wohler's was that of Yon 

 Bibra, who nsed "Wohler's method and, whilst affirming that he 

 obtained a similar citrate, found an entirely different constitu- 

 tion for the corresponding chloride. For instead of obtaining 

 a hemichloride Ag 2 Cl, he gives, as the result of 15 concordant 

 analyses, the constitution of his product as Ag 4 Cl 3 .f A citrate, 

 to yield such a chloride, (if such a chloride exists,) by the 

 simple action of hydrochloric acid, could scarcely have the con- 

 stitution assigned to it by both Wohler and Y. Bibra. 



In 1882, Pillitz published two papers.;}; He commences by 

 disputing the probability of the existence of Ag 4 on grounds 

 of valency ; namely as implying that oxygen may be quadri- 

 valent. Although it is very doubtful that any one has up to 

 the present time obtained Ag 4 0, the argument seems futile as 

 are many arguments deduced from supposed laws of valency. 

 Similar reasoning would make Ag 2 Cl impossible, which sub- 

 stance undoubtedly exists, and it would also deny the existence 

 of K 2 C1 which stands upon such authority as that of Rose, 

 Kirchhoff, and Bunsen.§ Pillitz carefully examined the so- 

 called hemioxide precipitated by alkaline solutions of antimony 

 and tin and could find no trace of Ag 4 in any of them. He 

 did not examine Wohler's products. 



The first person to deny categorically the existence of 

 Wohler's series of herai-compounds of silver appears to have 

 been Dr. Spencer Newbury. In two interesting papers, | he 

 describes a repetition of Wohler's methods and declares it to 

 be impossible to obtain products of constant composition. The 

 red solution taken by Wohler to be argentous citrate, Dr. 



*G. H. Bailey and G. J. Foster, Ohem. Soc. 1887, 416. Bericht D. Oh. G., xx, 

 Ref. 360. 



f Erdmann, J. prakt. Chim. 1875, 120, 39. Von Bibra precedes his paper with 

 a brief summary of the conclusions reached by previous chemists on the subject 

 of the action of light and chemical reducing agents on silver compounds. The 

 collection is interesting as showing to what inconsistent and even contrary opin- 

 ions careful observers have come on these reactions. 



± Zeitschrift fur Ann. Chemie, xxi, p. 27, p. 496. 



§ Gmelin Kraut, ii, 1, 72. 



|f Am. Chem. Jour, vi, 407 ; viii, 196. 



