83 REPORT 1871. 



Compound molecules exist in tte solid, liquid, and gaseous condition, providing 

 that the temperature necessary to convert tliem into these physical modifications 

 is not above the temperature at which their components are dissociated. Thus 

 we can easily conceive that a substance A may be of sufficient structural stability 

 to pass through all the increasing vibratory action of heat without dissociation of 

 its component molecules, until it has passed through the solid, liquid, and far into 

 the vaporous condition ; whilst a substance B has what the author calls a 

 thermanalvtic point, or the point where the equilibrium is broken. If it lies 

 below 100'' C., we have dissociation in the liquid condition among compounds so- 

 luble in water. 



A well-kno-mi natural gi-oup of bases had been studied as regards these phe- 

 nomena, viz. the trioxides, alumina, chromic and ferric oxides, and it has been 

 found that all the compound molecules of these bases were more or less disso- 

 ciated on heating their solutions. 



The feiTic compounds are the most easily affected. The solutions of these 

 compounds, if pure, are almost colourless; the usual slight tinge being in most 

 cases produced by the ba.sic action of the water. By the cautious addition of 

 dilute acid, almost colom-less solutions will be procured. On the application of 

 heat this solution becomes gradually darker and darker, until it becomes a dark 

 reddish-brown fluid. If the water bears any considerable proportion to the salt, 

 a basic p-ecipitate falls before it has reached the boiling-point. The relative 

 amount of the water is of the utmost importance in these phenomena, because its 

 basic action lowers the thermanah-tic point. The result of the dissociative in- 

 fluence of heat when a precipitate' is not produced, is the repartitioning of the 

 elements by which a basic and an acid salt are produced in the same flmd simul- 

 taneously. If these experiments are carried on under pressure, or in the presence 

 of a great excess of water, the dissociative influence is so great from the increased 

 range of temperature, that anhydrous oxide of iron can be produced in the pre- 

 sence of water. 



The compounds of chromium are capable of dissociation in a similar manner, 

 and the change of colour produced by heat upon these solutions is due to basic 

 condition, and not to the state of hydration of the salt as generally stated. 



The aluminic molecules obey exactly the same rule ; i)ut as the thermanalytic 

 point is much higher, and as there is no chromatic change to niarlj the dissociative 

 influence of heat, it is difficult to discern the phenomenon. Under the influence 

 of solutions boiling at an increased pressure of 11 or 12 atmospheres alumina was 

 procm-ed. The same results may be obtained by increasing the basic condition of 

 the solution by a large volume of water. As the pressure raises the boiling-point 

 of the water until we reach the thermanalytic point of the molecule, so the basic 

 action of the water upon the stvlous grou'p lowers the thermanalytic point until 

 we get it within the range of 100=' C. If 500,000 to 600,000 times the weight of 

 water is used to the amount of salt, a precipitate is produced at lOO' C. This pre- 

 cipitate is best seeu by passing a beam of electric light through the flask. _ Most 

 of the precipitates niay be observed by the eye, but not all ; they redissolve 

 on cooling. 



'o' 



On the hehMviow)' of Supersaturated Saline Solutions wlien exposed to the open 

 air. By Chaeles Tojilixsox, F.Il.S. 



It is known that when a vessel containing a supersaturated saline solution is 

 opened in a room, it immediately ciystallizes provided the temperature be not too 

 high. Mr. Tomlinson shows that supersaturated solutions of Glauber's salt (and 

 also of Epsom salts and of alum ) may be exposed to the open air of the country for 

 many hours, and even be taken out of the flasks in clean metal spoons, without 

 crystallizing. From a large number of experiments conducted under various con- 

 ditions, the follo\s-ing conclusions are drawn : — 



1. That a highly supersaturated solution of sodic sulphate may be exposed 

 to the open air of the country in an imcovered flask, and in cloudy weather, 

 for from twelve to twenty hours, without any formation of the ordinary ten- 

 watered crystals, 



