Vol. XXIII. No. 7.] 



T>: 



OPULAR SCIENCE NEWS. 



101 



Practical Cljonjistrv aijd tlje ^Irts. 



A CHINESE DERRICK. 



TiiK complicated arrangement of beams, 

 cords, and knots represented in the accompa- 

 nying illustration, is taken from an original 

 drawing, showing how the Chinese manage 

 to lift to their places the large stones used in 

 building. This primitive arrangement evi- 

 dently serves its purpose fairly well, but the 

 waste of time and labor in erecting and using 

 it would not be tolerated by western nations. 

 With this clumsy tackle it requires four hours 

 to elevate the block of stone a distance of five 

 feet. In China, however, nothing is cheaper 

 than human labor, and the element of time is 

 of little account ; so they make up by the 

 combined strength of large numbers of work- 

 men, for the lack of the speedv and powerful 

 machinerv used in more enlightened coun- 

 tries. 



The illustration is not only a good example 

 of Chinese drawing, but it is of value from 

 an archaeological point of view, and may, 

 possibly, contain a hint as to the way in 

 which the pyramids and other great works of 

 antiquity were erected. The Chinese arc the 

 most conservative of all nations, and the in- 

 dustrial processes now in use have survived 

 from a remote antiquity. It is quite probable 

 that some such rude arrangement as the one 

 represented above, aided by the strength of 

 hundreds if not thousands of slaves, was used 

 n raising the stones of the pyramids, and the 

 great columns of the ancient Egyptian tem- 

 ples, to their present position. 



The drawing was made by a Chinese artist 

 for a correspondent of La Nature^ from 

 which journal we reproduce it. 



ON ALLOTROPIC FORMS OF SILVER. 



BY M. CAREY LEA. 



[The following is an abstract of Mr. Lea's original 

 paper in the June number of the American Journal 

 of Science, to which all who are interested in this 

 remarkable discovery are referred for further partic- 

 ulars.] 



Silver is capable of existing in allotropic forms 

 possessing qualities diifering greatly from those of 

 normal silver. There are three such forms, or 

 rather three modifications of one form, differing 

 from each other in many respects, but all more 

 nearly related to each other than any one of them to 

 normal silver. One of these forms is soluble in 

 water, pas.'^ing readily to an insoluble form, and tliis 

 last may, by the simple presence of a neutral sub- 

 stance exercising no chemical action upon it, recover 

 its solubility. Another form closely resembles gold in 

 color and lustre. 



Whether metallic silver shall be reduced from its 

 compounds in Its normal or in an allotropic form, 

 depends upon the reducing agent applied, so that it 

 caimot be said with any certainty whether it exists 

 in its compounds in its ordinary normal form, or in 

 an allotropic condition : the latter alternative seems 

 at least equally probable. 



The forms of allotropic silver which I have ob- 

 tained may be classified as follows : 



A. Soluble, deep red in solution, mat lilac, blue, 

 or green whilst moist, brilliant bluish green metallic 

 when dry. 



B. Insoluble, derived from A, dark reddish 

 brown whilst moist, when dry somewhat resem- 

 bling A. 



C. Gold silver, dark bronze whilst wet, when dry 

 exactly resembles metallic gold in burnished lumps. 

 Of this form there is a variety which is copper- 

 colored. Insoluble in water, appears to have no 

 corresponding soluble form. 



All these forms, if taken in a pasty condition and 

 spread evenly over paper with a fine brush, take on 

 spontaneously in drying a lustre as high as that of 

 metallic leaf. C when so treated would be taken for 

 gold leaf. But this property is much better seen by 

 brushing the pasty substance over glass. When 

 dry, an absolutely perfect mirror is obtained. The 

 particles next the glass, seen through the glass, are 

 as perfectly continuous as those of a mercurial 

 amalgam, and the mirror is as good. A and H form 

 bluish-green mirrors, C, gold or copper-colored 

 mirrors. 



All these allotropic forms of silver are easily re- 

 duced to an impalpable powder. One is surprised 

 to see what isapparently solid burnished metal break 

 easily to pieces, and by moderate trituration yield a 

 fine powder. 



A. Soluble Allotropic Silver. 



A solution of ferrous citrate added to one of a sil- 

 ver salt produces instantly a deep red liquid. (Fer- 

 rous tartrate gives the same reaction, but is less ad- 

 vantageous.) These red solutions may either 

 exhibit tolerable permanency or may decolorize, 

 letting fall a black precipitate. It is not necessary 

 to prepare the ferrous salt in an isolated form ; a 

 mixture of ferrous sulphate and sodic citrate answers 

 perfectly. 



When, however, concentrated solutions are used 

 with a large excess of ferrous sulphate and a still 

 larger one of alkaline citrate, the liquid turns almost 

 completely black. It should be stirred very 



thoroughly for several minutes, to make sure that 

 the whole of the precipitated silver citrate is acted 

 upon by the iron. After standing for ten or fifteen 

 minutes, the liquid may be decanted and will leave a 

 large quantity of a heavy precipitate of a fine lilac- 

 blue color. It is best to adhere closely to certain 

 proportions. Of a ten per cent, solution of silver 

 nitrate, Joo c. c. may be placed in a precipitating 

 jar. In another vessel are mixed 200 c. c. of a thirty 

 per cent, solution of pure ferrous sulphate and 280 

 c. c. of a forty per cent, solution of sodic citrate. 

 (The same quantity of ferrous sulphate or of sodic 

 citrate in a larger quantity of water will occasion 

 much loss of the silver product.) I think some ad- 

 vantage is gained by neutralizing the ferrous solu- 

 tion, which has a strong acid reaction, with solution 

 of sodium hydroxide: as much may be added as will 

 not cause a permanent precipitate. To the quanti- 

 ties already given, about 50 c. c. of 10 per cent, 

 soda solution. The reaction takes place equally 

 well without the soda, but I think the product is a 

 little more stable with it. The mixed solution is to 

 be added at once to the silver solution. 



The beautiful lilac shade of the precipitate is 

 rather ephemeral. It remains for some time if the 

 precipitate is left under the mother water, but when 

 thrown upon a filter, it is scarcely uncovered before 

 the lilac shade disappears and the precipitate takes a 

 deep blue color, without losing its solubility. It 

 may be washed either on a filter or by decantation, 

 with any saline solution in which it is insoluble and 

 which does not affect it too much. On the whole, 

 ammonic nitrate does best, but sodic nitrate, citrate, 

 or sulphate may be used, or the corresponding 

 ammonia salts. Although in pure water the pre- 

 cipitate instantly dissolves with an intense blood 

 red color, the presence of five or ten per cent, of any 

 of these salts renders it perfectly insoluble. I have 

 usually proceeded by adding to the precipitate (after 

 decanting the mother water as completely as maybe 

 and removing as much more as possible with a 

 pipette), a moderate amount of water; for the above 

 quantities about 150 c. c. Much less would dissolve 

 the precipitate but for the salts present : this much 

 will dissolve the greater part but not the whole, 

 which is not necessary. A little of a saturated solu- 

 tion of ammonic nitrate is to be added — just enough 

 to effect complete precipitation. 



The substance contained on an average 97.27 per 

 cent, of silver. 



The content of silver in the various products was 

 very carefully, and I believe I may say quite accu- 

 rately, determined: it was extremely high, always 

 above 97 per cent. As already remarked, this vir- 

 tually excludes the presence of all elements except 

 hydrogen and possibly oxygen. These elements 

 were carefully searched for, but their presence could 

 not be detected. To suppose that we had to do 

 with a mixture in which some compound of silver 

 was mixed with metallic silver was not possible, for 

 as the whole was soluble we should still have to ad- 

 mit the solubility of silver. 



We have consequently to deal with a substance 

 containing over 97 per cent, of silver, and neither 

 hydrogen nor oxygen in combination with it. the 

 remaining 2 or 3 per cent, fully accounted for by 

 ferric oxide and citric acid determined as present as 

 accidental impurity; the substance itself readily 

 amalgamating with mercury by simple friction, 

 nevertheless abundantly soluble in water. If I had 

 been able to find any other explanation for these 

 facts without admitting the solubility of silver, I 

 should have adopted it. But none presented itself. 



B. Insoluble Form of the Foregoing. 



The solution of the blue product just described is 

 influenced in a remarkable way by the addition of 



