April 19, 1900] 



NATURE 



599 



continues, "it seems an extraordinary circumstance, and to m'e 

 impossible to credit, that the nodules, the flakes and the imple- 

 ments should, notwithstanding the enormous rainfall . . . which 

 ploughed out the side valleys opening on the Nile, be found 

 lying, cT'cn in a single instance, in undisturbed association at 

 the present day." The same criticism is passed upon the flint 

 instruments also brought by Seton-Karr from Somaliland (of 

 which the Liverpool ISIuseum possesses a series), which have 

 been described by Sir John Evans, before the Royal Society, as 

 in form absolutely identical with some from the valley of the 

 Somme and other places, and proving " the unity of race be- 

 tween the inhabitants of Asia, Africa and Europe in palaeolithic 

 times." Additional flints were later found " scattered all over the 

 country, covering the ground sometimes for the space of half an 

 acre," and there was discovered also an "unfinished spear-head 

 on the ground surrounded by a mass of flakes and chips." This 

 remarkable distribution over the country, Dr. Forbes remarks, 

 " where no remains apparently exist of the deposits out of 

 which they have been washed, seems difiicult to reconcile with 

 the usual process of denudation acting through the enormous 

 period which has elapsed since the palaeolithic age of Europe," 

 and he disbelieves that "a nodule of stone surrounded by the 

 flakes chipped from it tens or hundreds of thousands of years 

 ago, could have remained undisturbed when the deposits by 

 which it was covered have entirely disappeared " ; he dissents 

 also from the opinion 

 that identity of form 

 in the stone imple- 

 ments is sufficient 

 evidence of unity of 

 race or of close con- 

 tact between the races 

 who made them. He 

 is of opinion, there- 

 fore, that none of the 

 surface so-called pal- 

 aeolithic implements 

 from Egypt and Som- 

 aliland " have yet 

 been clearly proved 

 to belong to that 

 period, while the pro- 

 bability is that the 

 bulk of them are of 



much later date." The only flint implements. Dr. Forbes adds, 

 believed to be authentically palceolithic are the flakes and very 

 rude scraper-like flints found by General Pitt- Rivers in the 

 stratified indurated gravelly debris from a Wady near the Tombs 

 of the Kings. 



ON THE MECHANISM OF GELATION, AND 



ON THE STABILITY OF HYDROSOLS?- 

 p ELATINE-WATER-ALCOHOL and agar-water are col- 

 ^-^ loidal mixtures which form a gel on cooling and a sol on 

 warming. In both cases the formation of the gel is due to 

 the separation of the fluid mixture into two partially miscible 

 fluids or phases. When a certain critical temperature is 

 reached, one of the phases separates out as a cloud of droplets. 

 With a further fall of temperature either this internal phase or 

 the external phase becomes a solid solution, and forms a 

 framework in the spaces of which the still fluid solution 

 is lodged. Thus two distinct types of gel occur. In the one 

 the structure is a solid mass, in which are embedded spherical 

 spaces filled with fluid. In the other it is an open sponge- 

 work of adherent solid spheres with fluid filling the meshes. 

 The former is firm and elastic, the latter is brittle and 

 undergoes spontaneous shrinkage. In the ternary mixture the 

 gel has the former structure when the gelatine content of the 

 mixture is high ; the latter when it is low. 



The hydrogel of agar is built of a solid solution of water 

 in agar, which forms a framework holding a fluid solution of 

 agar in water. The concentration of each of these two co- 

 existent solutions is dependent upon temperature, but the 

 values vary according to whether the system is cooled down 

 or warmed up to a given temperature. The system therefore 



1 Abstract of two papers read before the Royal Society, on January 25, 

 i.yW. B. Hardy. 



manifests a striking hysteresis. From the point of view of 

 the phase rule the hydrogel of agar is a system of two com- 

 ponents in three phases — a fluid, a solid, and a vapour phase. 

 The composition of the phases should therefore be fixed by 

 fixing either the temperature or the pressure. Fixing the 

 temperature, however, does not fix the composition, and this 

 is probably due to two things: (l) the fact that the surface 

 which separates the fluid and solid phases is curved, and (2) 

 the fact that that surface is freely permeable by the mobile 

 molecules of water, but is relatively impermeable to the im- 

 mobile molecule of agar. The system obviously has two 

 pressures which determine equilibrium, a lower hydrostatic 

 pressure on the convex side of the curved surface, and a 

 higher on the concave side. 



Hydrosols, such as those of gold, silver or hydrosulphides, 

 are systems in which equilibrium is between a solid phase 

 dispersed as minute particles, and a fluid phase which is a true 

 solution of the substance of the solid phase. The behaviour 

 of the particles in an electric field shows that each one is 

 surrounded by a double electric layer, which can be destroyed 

 by the addition of electrolytes, or, in some cases, by the 

 removal of all electrolytes. When this is done aggregation or 

 coagulation follows. The stability of these hydrosols, therefore, 

 is due to a contact difference of potential between the solid and 

 the fluid phases. 



Fig. 2.— Flint knife from Wady el Sheikh. 



The addition of an electrolyte may bring about coagulation 

 either by altering the potential of the fluid phase, so as to 

 make it agree with that of the solid phase, or by furnishing 

 "nuclei" about which the particles of solid aggregate. When 

 the particles carry a negative charge, acids act by decreasing 

 the positive charge of the fluid ; when the particles carry a 

 positive charge, alkalies act by decreasing the negative charge 

 of the fluid. In these cases the coagulating power of the acid 

 or alkali is directly measured by its chemical activity when 

 dissolved in water. The relation is expressed by the formula 



K — na{v + v) 



when K is the specific molecular coagulative power of a sub- 

 stance as measured by the volume occupied by one gram mol., 

 when it just suffices to coagulate the hydrosol. 



The coagulating action of a .salt is due to only one ion, which 

 is always of the opposite electrical sign to the colloid particles. 

 The valency of the active ion exerts a remarkable influence upon 

 its coagulative power, the relation being approximately 



r : I" : V" = K : K2 : K^. 



Therefore, to express the coagulating powers of salts, a factor 

 which is approximately squared or cubed by a change from 

 monovalent to di- or tri-valent ions must be added to the 

 formula given above. 



K = «o(u-f-j')A\ 



Thomson has pointed out that double electric layers must 

 be separated by a region of finite thickness, in which the com- 

 ponents are in a state of uncompleted chemical combination. 

 The solid and fluid phases in these hydrosols, therefore, are 

 separated by a layer which possesses considerable chemical 

 energy, and which is of very great extent, and this may account 

 for their marked catalytic or ferment-like properties. 



NO. 1590, VOL. 61] 



