492 EEPOET— 1882. 



It seems probable, therefore, that the double salts formed from these monobasic 

 acids, although they form good supersaturated solutions, are not so firmly united 

 together as to withstand the disturbing influence of certain of their constituents ; 

 but that the disruption produced by them is not sufficient to cause the decomposi- 

 tion of the body, and consequently the double salt is deposited. In the case last 

 mentioned, also, of the double cyanide and chloride, both salts are deposited as the 

 final result of the crystallisation. 



In experimenting with mercuric iodide, this substance was introduced by 

 means of a pipette-shaped tube, and the iodide strongly steamed by boiling the flask 

 before it was allowed to cool. 



In the case of the alum and double phosphate, however, which may be taken 

 as examples of very definite double salts, neither of the components have any 

 action on the solution. In connection with the alum experiments some interesting 

 results were obtained from the double sulphate of zinc and copper, known as 

 'Lefort's salt,' the composition of which, according to that author, is Zn3Cu(S0j)^, 

 II^O, and which can be crystallised without decomposition. 



Quantities of this salt were dissolved in half their weight of water, zinc 

 sulphate (ZnS0^,7H.,0) and copper sulphate (CuSOj,5H20) being employed as 

 the nuclei. 



In these cases both the constituents were active in causing crystallisation, 

 that from the zinc sulphate nucleus being more rapid than that from the copper 

 sulphate nucleus. An examination of the crystals deposited showed that they 

 were crystals of the double salt, and their deposition presented some very peculiar 

 phenomena, referred to at length in the paper. 



In the last group of salts employed, viz., the tartrates and citrates, it will be 

 observed that in the case of two of them one of the constituents w^as active and 

 one remained inert. It now became important to examine, if possible, the com- 

 position of the crystals which were gradually deposited from the nucleus. For 

 this piu^Dose experiments were carefully performed in flasks containing the 

 Rochelle salt, to which the nuclei of neutral sodium tartrate were added by means 

 of the syphon-tubes. The crystals gradually and slowly formed from the point 

 of the syphon-tube, and were allowed to grow till a considerable cluster had 

 formed, and a great part of the salt had been thus removed from the solution in 

 the flask. The deposit adhering to the syphon-tube was withdrawn from the 

 mother-liquor removed from the flask, washed with ice-cold water, pressed between 

 blotting paper, air-dried, and analysed. On analysis the deposit gave numbers 

 closely agreeing with the composition of Rochelle salt. 



Now it is to be observed that the crystals forming the deposit here in no wise 

 resemble those of Hochelle salt, but closely resemble those of sodiimi tartrate, 

 giving us evidence of the probable dimorphism of Rochelle salt. 



At present it is difficult to see why the sodiimi tartrate should be active and 

 the potassium salt inactive ; but it may be remarked that the solubility of the 

 sodium salt is less than that of the potassium salt, which may perhaps account for 

 its activity. A corresponding result has been observed with the citrate ol 

 potassium and sodium, in which the sodium citrate has always proved active, 

 whilst the potassium salt is inactive. Here also the solubility of the potassium 

 salt is greater than that of the corresponding sodium salt. 



In the case, however, of the double magnesium and sodiimi citrates, the 

 results were dififerent, both of the constituent salts proving inactive to the double 

 salt. 



From these experiments it will be seen that the double salts of monobasic 

 acid apparently sufler disruption more easily than the salts from acid of higher 

 basicity, like sulphuric and phosphoric acids, but that in the case of the first 

 of these two latter acids, salts may exist, like Lefort's salt, which is acted upon by 

 the constituents. 



It will be observed, however, that such salts indicate more of molecular than 

 of atomic grouping in their constitution, whereas in the alimi and double 

 phosphate we have a firmer union of the salts. This is also, the author thinks, to 

 be observed in the salts from the organic acids ; by its mode of formation and its 



I 



