592 



NATURE 



\pct. 16, 1879 



Fire-bellied Toad (Boml'htaloi- ignais), a Natterjack Toad (Buf> 

 calamita), Kuropean, presented by Mr. H. A. Macpherson ; ^ix 

 Green Tree Frogs (I/yla arborca), European, presented by Mr. 

 A. Leipner, F.Z.S. ; a Chacma Baboon (Cynocephalus porcarius) 

 from South Africa, deposited ; two Kendall's Guinea Fowls 

 (l^itmida reiidalli) from West Africa, a Javan Peafowl [Pavo 

 spicifer) from Java, three Royal Pythons {Python rcgitis) from 

 West Africa, received in exchange ; two Saffron Finches (Sycalis 

 /lavcola), bred in the Gardens. 



SOME RECENT EXPERIMENTS ON THE 

 CRYSTALLISATION OF SUPERSATURATED 

 SALINE SOLUTIONS'- 



"T^HE history of the various views held by different experi- 

 -^ menters on the nuclear action of substances in exciting the 

 sudden crystallisation of a supersaturated solution, has been 

 already so well described by Mr. Charles Tomlinson, Prof. 

 Liversidge, Prof. Grenfell, and others in their several papers 

 upon the subject, that it is unnecessary to enter into any detailed 

 description of their opinions ; suffice it to say that they may be 

 divided into two classes : the first holding the opinion that the 

 crystallisation may be excited by the presence of certain fatty, 

 oily, greasy, or other matters in the form of films ; the second 

 that the initial cause of the crystallisation must be sought for in 

 the entrance of a particle of the same salt as that which is in 

 solution. 



In 1866 M. Gernez indicated that in the case of certain salts 

 the sudden crystallisation might be brought about by the agency 

 of some other salt perfectly isomorphous with the one in solu- 

 tion; thus a supersaturated solution of magnesium sulphate 

 invariably crystallised when crystals of zinc or nickel sulphates 

 containing seven molecules of water in their composition were 

 introduced. Having been for a long time interested in the 

 crystallisation of such solutions, I determined to carry out a 

 large munber of experiments upon carefully purified substances 

 which observations have resulted in a confirmation of the views 

 held by Gernez that truly isomorphous substances are active to 

 solutions of each other. 



The two methods employed in the experiments may be briefly 

 stated as follows : — (i) The supersaturated solutions of the salts 

 to be examined were placed in small wide-mouthed flasks ; and 

 the solutions of the salts employed as nuclei were introduced 

 into very thin glass bulbs plugged with cotton wool, and sus- 

 pended through a second plug of cotton in the neck of the flask, 

 in a manner such as is indicated in Fig. i. (2) A large number 

 of the experiments were also performed by using, instead of the 

 bulb tube for the introduction of the salt employed as nucleus, a 

 tube bent as a siphon, thus f\J, and like the bulb tube passing 

 through cotton wool plugs in the neclis of the flasks ; the whole 

 arrangement when complete being as represented in Fig. 2. To 

 perform an experiment the solution in the bulb or in the siphon 

 tubes was crystallised, and after a short time these tubes were 

 gently lowered into the 'solutions in the flasks and the results 

 observed. When the bulb tubes were used they were gently 

 broken against the bottom of the flask, the contained crystals 

 being thus brought in contact with the solution. To show that 

 the disturbance produced by this breaking had of itself no 

 exciting action on the solutions in the flasks, corresponding ex- 

 l^eriments were made with bulbs containing clean pieces of glass, 

 Nvhen no crystallising action took place, showing that the mere 

 disruption of the solution did not cause crystallisation. The 

 Lirgest number of experiments were, however, carried out by the 

 ^iphon tube method, which is perfectly applicable after a little 

 jiractice in the introduction of the tubes. The substances em- 

 ployed were in all cases carefully examined to ensure their 

 purity. 



With magnesium sulphate (MgS04.7HjO) as a supersatu- 

 rated solution, it was found that the other sulphates of the same 

 group of metals, namely, those of zinc, nickel, cobalt, and iron, 

 were immediately active in causing the crystallisation of the 

 magnesium solution when they possessed an identical composition 

 with it, as represented by the general formula (M"S04.7H._iO), 

 M representing the different metals. When these salts con- 

 tained only six proportions of water crystallisation is sometimes 

 induced, but in this case it is of an entirely different nature, the 

 deposition being slow and gradual. In connection with this 



' Abstract of a paper read before the Chemical Society on March 6, 1879. 



group of salts interesting results were obtained with the double 

 salt potassio-magnesic sulphate (MgKj(SO J.j . 6H„0), crystals 

 of which, although possessing the elements of magnesium 

 sulphate, were found inactive to a solution of that body. In 

 the case of supersaturated solutions of sodiiun sulphate 

 (Na2SO4.IoH.jO), the bodies sodium selenate (NajSe04.loHjO), 

 and sodium chromate (NajCrOj.ioHjO), each analogous in 

 form and constitution to the sulphates but containing the 

 elements selenium and chromium instead of sulphur, were found 

 capable of exciting crystallisation in solutions of the sulphate. 



Experiments were also performed upon supersaturated solu- 

 tions of potash alum (AlKCSOiJjiaHaO), with crystals of iron 

 and chromium potash alums, bodies agreeing in form and consti- 

 tution with common alum, but containing chromium or iron in 

 place of the metal aluminium. These bodies were found in- 

 variably active in exciting the crystallisation of the common alum 

 solution. As alum crystallises in beautiful octahedral crystals 

 some experiments were made to see how far the mere shape of 

 the crystal could render it active in exciting the crystallisation ; 

 and for this purpose cubes of copper pyrites and octahedra <i 

 magnetite, both belonging to the same crystalline system as alura 

 but having a different chemical structure, were employed. When 

 these substances in a perfectly clear condition were placed in the 

 alum solutions no sudden crystallisation was produced, showing 

 that mere form alone is inactive in exciting this kind of crystal- 

 lisation. Crystals of hydric disodic arsenate (NajHAsOj. l2Hj,0) 

 were also found active in the case of the isomorphous hydric 

 disodic phosphate, containing phosphorus instead of arsenic, biit 

 otherwise analogous in form and composition. In connection 



P,g.l. 



Fig.2. 



with this latter salt it is worth noting that when the hydric 

 disodic phosphate is heated to convert it into sodium pyrophoi.- 

 phate (Na^PjO;), this substance is no longer active to a supe.- 

 saturated solution of the disodic salt. At the same time corre- 

 sponding experiments were performed on the supersaturate 1 

 solutions of magnesium sulphate, alum, &.C., with substances : f 

 dissimilar form and chemical constitution; these bodies, hov.- 

 ever, invariably yielded negative results, being incapable .1 

 exciting the sudden crystallisation of the solutions. 



Experiments carried out upon mixtiu-es of similarly consi.- 

 tuted and dissimilar bodies, yielded some interesting result'. 

 The substances employed were mixtures of the similarly cons; - 

 tuted s-dlphates of magnesium and nickel, and tie dissimiira" 

 sulphates of magnesium and sodium ; experiments were .ilso 

 made with sulphates of nickel and sodium. In the case of these 

 mixtures it was observed that two results might occur : — (A) The 

 nucleus might remain growing in the solution without causing 

 immediate crystallisation ; or (b). Crystallisation might be in- 

 duced at once on the addition of the nucleus ; the deposition ::f 

 the salts, however, differs according to the nature of the sub- 

 stances employed. 



In the case of the dissimilar bodies, magnesium or nickel 

 sulphates with sodium sulphate, it was found that a nucleus of 

 either sulphate when gradual crystallisation took place, slowly 

 increased by a deposition of the substance of the same nature .-.s 

 the nucleus added ; and that even with the rapid crystaUlsa'.ion 

 of dissimilar bodies the salt deposited consisted essentially of ;he 

 substance of the same nature as the nucleus added. 



